Endochrine System Flashcards
Reverse.Prompt
- Located in the brain
- Secretes melatonin that regulates the sleep/wake cycle Melatonin production changes by season.
Pineal gland
(circadian rhythm) May also regulate sexual development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 P.M. 6 A.M. b. Winter a. Experimental © The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer c. Summer Figure 16.21
Reverse.Prompt
16.2 Hypothalamus and Pituitary Gland
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain the role of the hypothalamus in the endocrine system.
List the hormones produced by the anterior and posterior pituitary glands and provide a function for each.
Summarize the conditions produced by excessive and inadequate levels of the major hormones.
Page 334The hypothalamus acts as the link between the nervous and endocrine systems. It regulates the internal environment through communications with the autonomic nervous system. For example, it helps control body temperature and water-salt balance. The hypothalamus also controls the glandular secretions of the pituitary gland. The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalklike structure. The pituitary has two portions: the posterior and the anterior pituitary. Although the anterior and posterior pituitary glands are connected, they operate as separate physiological glands.
Posterior Pituitary
Neurons in the hypothalamus called neurosecretory cells produce the hormones antidiuretic hormone (ADH) and oxytocin (Fig. 16.6). These hormones pass through axons into the posterior pituitary, where they are stored in axon endings.
Figure 16.6 Hormones produced by the hypothalamus and posterior pituitary. The hypothalamus produces two hormones, ADH and oxytocin, stored and secreted by the posterior pituitary.
Hormonal Communication
Page 335Certain neurons in the hypothalamus are sensitive to the water-salt balance of the blood. When these cells determine that the blood is too concentrated, ADH is released from the posterior pituitary. On reaching the kidneys, ADH causes more water to be reabsorbed into kidney capillaries, decreasing urine volume. As the blood becomes dilute, ADH is no longer released. This is an example of control by negative feedback, because the effect of the hormone (to dilute blood) acts to shut down the release of the hormone. Negative feedback maintains stable conditions and homeostasis.
Inability to produce ADH causes diabetes insipidus. A person with this type of diabetes produces copious amounts of urine. Excessive urination results in severe dehydration and loss of important ions from the blood. The condition can be corrected by the administration of ADH.
Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing. The more the uterus contracts during labor, the more nerve signals reach the hypothalamus, causing oxytocin to be released. Similarly, as a baby suckles while being breastfed, nerve signals from breast tissue reach the hypothalamus. As a result, oxytocin is produced by the hypothalamus and released from the posterior pituitary. The hormone causes the woman’s breast milk to be released. The sound of a baby crying may also stimulate the release of oxytocin and milk letdown, much to the chagrin of women who are nursing. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback. The stimulus continues to bring about an effect that ever increases in intensity. Positive feedback terminates due to some external event. Therefore, positive feedback mechanisms are rarely used to maintain homeostasis; that role is typically associated with negative feedback mechanisms.
SCIENCE IN YOUR LIFE
How is labor induced if a woman’s pregnancy extends past her due date?
After medication to prepare the birth canal for delivery, oxytocin (Pitocin) is used to induce labor. Pitocin is a synthetic version of the oxytocin released by the posterior pituitary. During labor, oxytocin may also be given to increase the strength of contractions. Stronger contractions speed the labor process if necessary (e.g., if the woman’s uterus is contracting poorly or if the health of the mother or child is at risk during delivery). Oxytocin is routinely used following delivery to minimize postpartum bleeding by ensuring that strong uterine contractions continue.
Use of oxytocin must be monitored carefully, because it may cause excessive uterine contractions. Should this occur, the uterus could tear itself. Further, reduced blood supply to the fetus caused by very strong contractions may be fatal to the baby. Though it reduces the duration of labor, inducing labor with oxytocin can be very painful for the mother. Whenever possible, physicians prefer gentler and more natural methods to induce labor and/or strengthen contractions.
Anterior Pituitary
A portal system, consisting of two capillary systems connected by a vein, lies between the hypothalamus and the anterior pituitary. The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing and hypothalamic-inhibiting hormones, which pass from the hypothalamus to the anterior pituitary by way of the portal system (Fig. 16.7). Examples are thyroid-releasing hormone (TRH) and thyroid-inhibiting hormone (TIH). The TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone, and the TIH inhibits the pituitary from secreting thyroid-stimulating hormone.
Figure 16.7 Hormones produced by the anterior pituitary. The hypothalamus controls the secretions of the anterior pituitary, and the anterior pituitary controls the secretions of the thyroid, adrenal cortex, and gonads, which are also endocrine glands.
Four of the seven hormones produced by the anterior pituitary have an effect on other glands. Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce the thyroid hormones. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce cortisol. The gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—stimulate the gonads (the testes in males and the ovaries in females) to produce gametes and sex hormones. In each instance, the blood level of the last hormone in the sequence exerts negative feedback control over the secretion of the first two hormones (Fig. 16.8).
Figure 16.8 Negative feedback mechanisms in the endocrine system. Feedback mechanisms (red arrows) provide means of controlling the amount of hormones produced (blue arrows) by the hypothalamus and pituitary glands.
The other three hormones produced by the anterior pituitary do not affect other endocrine glands. Prolactin is produced in quantity only after childbirth. It causes the mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism.
Melanocyte-stimulating hormone causes skin-color changes in many fishes, amphibians, and reptiles having melanophores, skin cells that produce color variations. The concentration of this hormone in humans is very low.
Growth hormone (GH), or somatotropic hormone, promotes skeletal and muscular growth. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism as opposed to glucose metabolism. The production of insulin-like growth factor 1 (IGF-1) by the liver is stimulated by growth hormone as well. IGF-1 is often measured as a means of determining GH level. Growth and development are also stimulated by IGF-1, and it may well be the means by which GH influences growth and development.
Effects of Growth Hormone
Growth hormone is produced by the anterior pituitary. The quantity is greatest during childhood and adolescence, when most body growth is occurring. If too little GH is produced during childhood, the individual has pituitary dwarfism, characterized by perfect proportions but small stature. The Bioethics feature “Growth Hormones and Pituitary Dwarfism” in this section discusses how a synthetic growth hormone Page 336can be used to treat some forms of dwarfism. If too much GH is secreted, gigantism may result (Fig. 16.9). Individuals with gigantism often have additional health problems, primarily because GH has a secondary effect on the blood sugar level, promoting an illness called diabetes mellitus (see Section 16.5).
Figure 16.9 Growth hormone influences height. Irregularities in growth hormone can lead to gigantism.
©Xinhua News/Associated Press
BIOLOGY TODAY Bioethics
Growth Hormones and Pituitary Dwarfism
Without treatment, children with a deficiency of growth hormone (GH) experience pituitary dwarfism: slow growth, short stature, and in some cases failure to begin puberty. Prior to the advent of biotechnology in the 1980s, treating these children was incredibly difficult and expensive. The GH needed to treat deficiencies had to be obtained from cadaver pituitaries. Although the treatment was generally very successful, the use of cadaveric GH caused Creutzfeldt–Jakob disease (a neurological disease similar to “mad cow” disease) in a small number of treated individuals.
Thanks to biotechnology, technologists are now able to synthesize human GH (HGH) using bacteria. These bacteria have had the gene for HGH inserted into their genetic information. The altered bacteria are then grown in laboratories and make unlimited amounts of GH. Children with insufficient GH can be treated more safely and inexpensively with this GH. Recombinant HGH can also be used to treat other disorders, such as the chromosomal deficiency known as Turner syndrome (discussed in Section 19.6). It may even be possible to slow or reverse the aging process with HGH treatments.
There is some controversy surrounding treating short children without HGH deficiency for essentially cosmetic reasons. Unfortunately, Americans are obsessed with height. Shorter children are often bullied and teased by their peers. Some data suggest that shorter individuals are discriminated against at their jobs. Their salaries are often lower than those of their taller counterparts with equivalent education and experience. Many people of short stature report having greater self-esteem problems than individuals of average to above-average height. Treatment with HGH could be the solution to these problems.
Although the supply of HGH is seemingly unlimited, the cost of treatments is still quite high (though much cheaper than cadaveric GH), with annual treatments costing up to $25,000. In most cases, insurance companies will not cover these costs. Of greater concern, however, are the potential side effects of supplemental HGH therapy, which are not well understood. Moreover, it is not clear whether HGH treatment will result in a significant increase in the final height of short children.
Questions to Consider
Now that HGH is easier to obtain, what potential abuses would you predict?
Do you think insurance companies should be expected to pay for HGH treatment if a child shows no hormone deficiency and is simply short?
On occasion, GH is overproduced in the adult and a condition called acromegaly results. Long bone growth is no longer possible in adults, so only the feet, hands, and face (particularly the chin, nose, and eyebrow ridges) can respond, and these portions of the body become overly large (Fig. 16.10).Page 337
Figure 16.10 Overproduction of growth hormone in adults leads to acromegaly. Acromegaly is caused by overproduction of GH in the adult. It is characterized by enlargement of the bones in the face, fingers, and toes as a person ages.
(both hands): ©Bart’s Medical Library/Medical Images; (man): ©Yasser Al-Zayyat/AFP/Getty Images
CHECK YOUR PROGRESS 16.2
Explain how the endocrine system and nervous system communicate with one another.
Answer
Through neurotransmitters and hormones—for example, the nervous system sends input to the adrenal medullae, so that a fight-or-flight response can be triggered when needed. Meanwhile, several hormones secreted by the endocrine system regulate the hypothalamus and/or anterior pituitary.
List the hormones produced by the posterior pituitary and provide a function for each.
Answer
Posterior pituitary does not produce any hormones, but it stores and releases ADH and oxytocin produced in the hypothalamus. ADH conserves water, and oxytocin stimulates uterine contractions and milk letdown.
List the hormones produced by the anterior pituitary and provide a function for each.
Answer
TSH stimulates the thyroid to produce T3 and T4; ACTH stimulates the adrenal cortex to produce glucocorticoids; gonadotropic hormones FSH and LH stimulate the gonads to produce gametes and sex hormones; PRL causes breast development and milk production; MSH causes skin color changes; GH promotes skeletal and muscular growth.
CONNECTING THE CONCEPTS
For more information on the hormones presented in this section, refer to the following discussions:
Section 12.2 examines the influence of growth hormone on bone growth.
Section 17.2 describes the role of pituitary hormones in the production of sperm cells in males.
Section 17.4 describes the role of pituitary hormones in the female ovarian cycle.
16.3 Thyroid and Parathyroid Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the hormones produced by the thyroid and parathyroid glands and provide a function for each.
Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis.
Summarize the diseases and conditions associated with the thyroid and parathyroid glands.
The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland
The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four.
Effects of Thyroid Hormones
To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency.
Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue.
(a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source
While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used.
Mechanism of Thyroxine Action
If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance.
In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent.
Calcitonin
Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited.
Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal.
Parathyroid Glands
Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops.
Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death.
Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones.
When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area.
CHECK YOUR PROGRESS 16.3
Explain how the hormones of the thyroid gland influence the metabolic rate.
Answer
T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy.
Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels.
Answer
When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal.
Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body.
Answer
Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones.
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CONNECTING THE CONCEPTS
For more information on the importance of calcium, refer to the following discussions:
Section 12.5 explains the role of the bones in maintaining calcium homeostasis.
Section 13.2 examines how calcium ions are involved in muscle contraction.
Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.
Reverse.Prompt
Second Messengers
The Action of Steroid Hormones
Figure 16.5 Action of a steroid hormone. A steroid hormone passes directly through the target cell’s plasma membrane before binding to a receptor in the nucleus or cytoplasm. The hormone–receptor complex binds to DNA, and gene expression follows.
- adrenal cortex, t
- he ovaries, and the
- testes produce steroid hormones.
- Thyroid hormones belong to a class of molecules called the amines. Amines act in a manner similar to the steroid hormones, even though they have a different structure. Steroid hormones do not bind to plasma membrane receptors. Because they are hydrophobic (see Section 2.5), steroids are able to enter the cell in the same manner as lipids (Fig. 16.5).
Once inside, a steroid hormone binds to a receptor, usually in the nucleus but sometimes in the cytoplasm. Inside the nucleus, the hormone–receptor complex binds with DNA and activates certain genes. Messenger RNA (mRNA) moves to the ribosomes in the cytoplasm, and protein (e.g., enzyme) synthesis follows (see Section 22.2). To continue our analogy, a steroid hormone is like a courier who has a pass to enter the factory (the cell). Once inside, it makes contact with the plant manager (DNA), who sees to it that the factory (cell) is ready to produce a product.
Tutorial: Action of a Steroid Hormone
An example of a steroid hormone is aldosterone, which is produced by the adrenal glands. Aldosterone targets the kidneys, where it helps regulate the water-salt balance of the blood. In general, steroid hormones act more slowly than peptide hormones, because it takes more time to synthesize new proteins than to activate enzymes already present in cells. Their action, however, typically lasts longer.
Mechanism of Steroid Hormone Action
CHECK YOUR PROGRESS 16.1
State the role of a hormone.
Answer
A hormone is a chemical signal that affects the metabolism of a target cell.
Compare and contrast the nervous and endocrine systems with regard to function and the types of signals used.
Answer
The nervous and endocrine systems both regulate the activities of other systems in the body. The nervous system responds rapidly to stimuli, using neurotransmitters as signals, whereas endocrine system responses using hormones are slower but longer lasting.
Summarize the differences between a peptide hormone and a steroid hormone.
Answer
Reverse.Prompt
Hormones Are Chemical Signals
Like other chemical signals, hormones are a means of communication between cells, between body parts, and even between individuals. They affect the metabolism of cells that have receptors to receive them (Fig. 16.3).
Figure 16.3 Hormones target specific cells. Most hormones are distributed by the bloodstream to target cells. Target cells have receptors for the hormones, and a hormone combines with a receptor like a key fits a lock.
Page 331The importance of these receptors can be demonstrated by examining a condition called androgen insensitivity syndrome. Individuals with this syndrome have both X and Y sex chromosomes. Because they possess a Y chromosome, they produce the sex hormone testosterone (see Section 16.6), even though the testes usually remain in the abdominal cavity. However, the body cells lack receptors for testosterone, and therefore do not respond to the hormone. Therefore, the individuals appear to be normal females, although genetically they are males.
Like testosterone, most hormones act at a distance between body parts. They travel in the bloodstream from the gland that produced them to their target cells. Also considered to be hormones are the secretions produced by neurosecretory cells in the hypothalamus of the brain. They travel in the capillary network that runs between the hypothalamus and the pituitary gland. Some of these secretions stimulate the pituitary to secrete its hormones, and others prevent it from doing so.
Not all hormones act between body parts. As we will see, prostaglandins are a good example of local hormones. After prostaglandins are produced, they are not carried elsewhere in the bloodstream. Instead, they affect neighboring cells, sometimes promoting pain and inflammation. Also, growth factors are local hormones that promote cell division and mitosis.
Chemical signals that influence the behavior of other individuals are called pheromones. Nonhuman animals rely heavily on pheromones for communication—to mark one’s territory and to attract a mate. Humans produce pheromones, too. Researchers have isolated a pheromone released by men that reduces premenstrual nervousness and tension in women. Women who live in the same household often have menstrual cycles in synchrony. This is likely caused by the armpit secretions of a woman who is menstruating, affecting the menstrual cycles of other women in the household.
The Action of Hormones
Hormones have a wide range of effects on cells. Some of these effects induce a target cell to increase its uptake of particular substances (such as glucose) or ions (such as calcium). Other effects bring about an alteration of the target cell’s structure in some way. A few hormones simply influence cell metabolism. Growth hormone is a peptide that influences cell metabolism leading to a change in the structure of bone. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins, and modified amino acids. Growth hormone is a protein produced and secreted by the anterior pituitary. All steroid hormones have a similar structure of four carbon rings because they are all derived from cholesterol (see Fig. 2.20).
The Action of Peptide Hormones
Most endocrine glands secrete peptide hormones. The actions of peptide hormones can vary depending on the type of target cell. Peptide hormones do not have the ability to cross the plasma membrane, and therefore must interact with a receptor on the surface of the membrane.
As an example, we will explore what happens when the hormone epinephrine binds to a plasma membrane receptor of a muscle cell (Fig. 16.4). In muscle cells, the reception of epinephrine leads to the breakdown of glycogen to glucose, which provides energy for ATP production. The immediate result of binding is the formation of cyclic adenosine monophosphate (cAMP). Cyclic AMP contains one phosphate group attached to adenosine at two locations, producing a circular, or cyclic, molecule. Cyclic AMP activates a protein kinase enzyme in the cell. This enzyme, in turn, activates another enzyme, and so forth. The series of enzymatic reactions that follows cAMP formation is called an enzyme cascade. Each enzyme can be used over and over at every step of the cascade, so more enzymes are involved. Finally, many molecules of glycogen are broken down to glucose, which enters the bloodstream.
Figure 16.4 Action of a peptide hormone. A peptide hormone (first messenger) binds to a receptor in the plasma membrane. Thereafter, cyclic AMP (second messenger) forms and activates an enzyme cascade.
Tutorial: Action of a Peptide Hormone
Typical of a peptide hormone, epinephrine never enters the cell. Therefore, the hormone is called the first messenger; cAMP, which sets the metabolic machinery in motion, is called the second messenger. To explain this terminology, let’s imagine that the adrenal medulla, which produces epinephrine, is like a company’s home office that sends out a courier (the hormone epinephrine is the first messenger) to a factory (the cell). The courier doesn’t have a pass to enter the factory, so when he arrives at the factory, he tells a supervisor through the intercom that the home office wants the factory to produce a particular product. The supervisor (cAMP, the second messenger) enters a command in the computer that instructs the machinery (the enzymatic pathway) to make the product.
Second Messengers
The Action of Steroid Hormones
Only the adrenal cortex, the ovaries, and the testes produce steroid hormones. Thyroid hormones belong to a class of molecules called the amines. Amines act in a manner similar to the steroid hormones, even though they have a different structure. Steroid hormones do not bind to plasma membrane receptors. Because they are hydrophobic (see Section 2.5), steroids are able to enter the cell in the same manner as lipids (Fig. 16.5).
Figure 16.5 Action of a steroid hormone. A steroid hormone passes directly through the target cell’s plasma membrane before binding to a receptor in the nucleus or cytoplasm. The hormone–receptor complex binds to DNA, and gene expression follows.
Page 332Once inside, a steroid hormone binds to a receptor, usually in the nucleus but sometimes in the cytoplasm. Inside the nucleus, the hormone–receptor complex binds with DNA and activates certain genes. Messenger RNA (mRNA) moves to the ribosomes in the cytoplasm, and protein (e.g., enzyme) synthesis follows (see Section 22.2). To continue our analogy, a steroid hormone is like a courier who has a pass to enter the factory (the cell). Once inside, it makes contact with the plant manager (DNA), who sees to it that the factory (cell) is ready to produce a product.
Tutorial: Action of a Steroid Hormone
An example of a steroid hormone is aldosterone, which is produced by the adrenal glands. Aldosterone targets the kidneys, where it helps regulate the water-salt balance of the blood. In general, steroid hormones act more slowly than peptide hormones, because it takes more time to synthesize new proteins than to activate enzymes already present in cells. Their action, however, typically lasts longer.
Mechanism of Steroid Hormone Action
CHECK YOUR PROGRESS 16.1
State the role of a hormone.
Answer
A hormone is a chemical signal that affects the metabolism of a target cell.
Compare and contrast the nervous and endocrine systems with regard to function and the types of signals used.
Answer
The nervous and endocrine systems both regulate the activities of other systems in the body. The nervous system responds rapidly to stimuli, using neurotransmitters as signals, whereas endocrine system responses using hormones are slower but longer lasting.
Summarize the differences between a peptide hormone and a steroid hormone.
Answer
Peptide hormones contain amino acids, whereas steroid hormones are derived from cholesterol. Peptide hormones act by binding to surface receptors on target cells, activating an enzyme cascade via a second messenger. Steroid hormones interact with receptors inside cells, usually in the nucleus, and the hormone–receptor complex that is formed binds to DNA, activating certain genes.
Explain why second messenger systems are needed for peptide hormones.
Answer
Most peptide hormones cannot pass through the plasma membrane and thus work by interacting with surface receptors, which in turn use second messengers to alter cell metabolism.
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CONNECTING THE CONCEPTS
For more information on the interactions in this section, refer to the following discussions:
Sections 2.5 and 2.6 summarize the roles of steroids and proteins in the body.
Section 3.3 explores the structure of the plasma membrane and the proteins associated with it.
Section 14.2 describes the location and function of the hypothalamus, which integrates the nervous and endocrine systems.
Reverse.Prompt
CHAPTER 16
Endocrine System
©Oscar Gimeno Baldo/Alamy
CHAPTER OUTLINE
- 1Endocrine Glands
- 2Hypothalamus and Pituitary Gland
- 3Thyroid and Parathyroid Glands
- 4Adrenal Glands
- 5Pancreas
- 6Other Endocrine Glands
- 7Hormones and Homeostasis
BEFORE YOU BEGIN
Before beginning this chapter, take a few moments to review the following discussions:
Section 2.5 What is the structure of a steroid?
Section 4.8 How are negative feedback mechanisms involved in homeostasis?
Section 14.2What is the role of the hypothalamus in the nervous system?
Diabetes
For some time, Hanna had been feeling very sluggish and had been losing weight. At first, she attributed this to her very active lifestyle. Between school, work, and her social activities, Hanna had very little time for sleep. However, she was beginning to notice she was always thirsty and was urinating much more frequently than usual. Concerned about her health, Hanna visited the local health clinic, where she discussed her health history and symptoms with the physician. The doctor mentioned that her symptoms were consistent with many disorders, including viral infections and diabetes. As a quick test, the doctor ordered a urinalysis to see if there was glucose in her urine, which would indicate that Hanna’s symptoms were caused by diabetes mellitus, a disease that affects over 30.3 million Americans. The results of the urinalysis indicated that there were small amounts of glucose in Hanna’s urine, a sign that Hanna’s body may not be adequately maintaining its blood glucose levels. The doctor scheduled Hanna for a blood glucose test the following morning and instructed her to not eat or drink anything for 8 hours prior to the test.
During a blood glucose test, a small vial of blood is drawn and the amount of glucose in the blood is measured. Normally, after 8 hours of fasting, the blood glucose level should be between 70 and 100 mg per deciliter (mg/dl) of blood. Hanna’s value was slightly above this, but it was not high enough for the doctor to conclude that diabetes was the cause of Hanna’s symptoms. The next test was an oral glucose tolerance test (OGTT). In this test, Hanna drank a solution containing 100 grams (g) of glucose. Then, over the next 3 hours, five additional vials of blood were drawn and tested for glucose levels. In a normal individual participating in this test, blood glucose levels rise rapidly and then fall to below 140 mg/dl within 2 hours. In Hanna’s case, the response was much slower, and her 2-hour blood glucose level was 150 mg/dl. The physician told Hanna that the cause of her symptoms was most likely type 2 diabetes mellitus, a disease of the endocrine system, an organ system that is responsible for the long-term homeostasis of the body.
As you read through the chapter, think about the following questions:
What hormones control the level of glucose in the blood?
What is the difference between type 1 and type 2 diabetes?
How do feedback mechanisms help control blood glucose levels?
6.1 Endocrine Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Distinguish between the mode of action of a neurotransmitter and that of a hormone.
Distinguish between endocrine and exocrine glands.
Identify the organs and glands of the endocrine system.
Compare the actions of peptide and steroid hormones.
The organs of the endocrine system (Fig. 16.1) are responsible for the production of chemical signals, called hormones, that are involved in the regulation of the other organs in the body. The endocrine system works very closely with the nervous system to maintain homeostasis in the body.
Figure 16.1 The endocrine system. This diagram indicates the major endocrine glands of the body. Other organs also produce hormones, such as the kidneys, the gastrointestinal tract, and the heart, but this is not the primary function of these organs.
There is a difference in function between an endocrine gland and an exocrine gland. Exocrine glands have ducts and secrete their products into these ducts. The glands’ products are carried to the interior of other organs or outside the body. The accessory glands of the digestive system (see Section 9.4) are good examples Page 330of the exocrine glands. For example, the salivary glands send saliva into the mouth by way of the salivary ducts. In contrast, endocrine glands secrete their products into the bloodstream, which delivers them throughout the body. Only certain cells, called target cells, can respond to a specific hormone. A target cell for a particular hormone has a receptor protein for that hormone. The hormone and the receptor protein bind together like a key that fits a lock. The target cell then responds to that hormone.
Comparison of the Endocrine and Nervous Systems
The nervous system and endocrine system both use chemical signals when they respond to changes that might affect homeostasis. However, they have different means of delivering these signals (Fig. 16.2). As discussed in Section 14.1, the nervous system is composed of neurons. In this system, sensory receptors detect changes in the internal and external environments. The central nervous system (CNS) then integrates the information and responds by stimulating muscles and glands. Communication depends on nerve signals, conducted in axons, and neurotransmitters, which cross synapses. Axon conduction occurs rapidly, as does the diffusion of a neurotransmitter across the short distance of a synapse. In other words, the nervous system is organized to respond rapidly to stimuli. This is particularly useful if the stimulus is an external event that endangers our safety—we can move quickly to avoid being hurt.
Figure 16.2 Action of neurotransmitters and hormones. a. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen. b. Nerve impulses passing along an axon cause the release of a neurotransmitter. The neurotransmitter, a chemical signal, causes the wall of an arteriole to constrict. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen.
The endocrine system functions differently than the nervous system. The endocrine system is largely composed of glands (see Fig. 16.1). These glands secrete hormones, which are carried by the bloodstream to target cells throughout the body. It takes time to deliver hormones, and it takes time for cells to respond. The effect initiated by the endocrine system is longer lasting. In other words, the endocrine system is organized for a slow but prolonged response.
Both the nervous system and the endocrine system make use of negative feedback mechanisms (see Section 4.8). If the blood pressure falls, sensory receptors signal a control center in the brain. This center sends out nerve signals to the arterial walls, so that they constrict, and blood pressure rises. Now the sensory receptors are no longer stimulated, and the feedback mechanism is inactivated. Similarly, a rise in blood glucose level causes the pancreas to release insulin. This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body (see Fig. 16.2). When the blood glucose level falls, the pancreas no longer secretes insulin.
Hormones Are Chemical Signals
Like other chemical signals, hormones are a means of communication between cells, between body parts, and even between individuals. They affect the metabolism of cells that have receptors to receive them (Fig. 16.3).
Figure 16.3 Hormones target specific cells. Most hormones are distributed by the bloodstream to target cells. Target cells have receptors for the hormones, and a hormone combines with a receptor like a key fits a lock.
Page 331The importance of these receptors can be demonstrated by examining a condition called androgen insensitivity syndrome. Individuals with this syndrome have both X and Y sex chromosomes. Because they possess a Y chromosome, they produce the sex hormone testosterone (see Section 16.6), even though the testes usually remain in the abdominal cavity. However, the body cells lack receptors for testosterone, and therefore do not respond to the hormone. Therefore, the individuals appear to be normal females, although genetically they are males.
Table 16.1 summarizes the hormones of the endocrine system and provides the functions and targets of these hormones in the body.
Table 16.1Principal Endocrine Glands and the Hormones They Produce
Table Summary: Table lists the names of different endocrine glands in column 1, with pituitary gland, adrenal gland, and gonads sub-divided into its parts. Other information related to each type of gland is listed in columns 2 through 4.
Endocrine GlandHormone ReleasedTarget Tissues/OrgansChief Functions
HypothalamusHypothalamic-releasingAnterior pituitaryRegulates anterior pituitary hormones
Pituitary gland
Posterior pituitaryAntidiuretic (ADH)KidneysStimulates water reabsorption by kidneys
OxytocinUterus, mammary glandsStimulates uterine muscle contraction, release of milk by mammary glands
Anterior pituitaryThyroid-stimulating (TSH)ThyroidStimulates thyroid
Adrenocorticotropic (ACTH)Adrenal cortexStimulates adrenal cortex
Gonadotropic (FSH, LH)GonadsEgg and sperm production, sex hormone production
Prolactin (PRL)Mammary glandsMilk production
Growth (GH)Soft tissues, bonesCell division, protein synthesis, bone growth
Melanocyte-stimulating (MSH)Melanocytes in skinUnknown function in humans; regulates skin color in lower vertebrates
ThyroidThyroxine (T4) and triiodothyronine (T3)All tissuesIncrease metabolic rate, regulate growth and development
CalcitoninBones, kidneys, intestineLowers blood calcium level
ParathyroidsParathyroid (PTH)Bones, kidneys, intestineRaises blood calcium level
Adrenal gland
Adrenal cortexGlucocorticoids (cortisol)All tissuesRaise blood glucose level, stimulate breakdown of protein
Mineralocorticoids (aldosterone)KidneysReabsorb sodium and excrete potassium
Sex hormonesGonads, skin, muscles, bonesStimulate reproductive organs and bring about sex characteristics
Adrenal medullaEpinephrine and norepinephrineCardiac and other musclesAre released in emergency situations, raise blood glucose level
PancreasInsulinLiver, muscles, adipose tissueLowers blood glucose level, promotes glycogen formation
GlucagonLiver, muscles, adipose tissueRaises blood glucose level
Gonads
TestesAndrogens (testosterone)Gonads, skin, muscles, bonesStimulate male sex characteristics
OvariesEstrogens, progesterone, small amounts of testosteroneGonads, skin, muscles, bonesStimulate female sex characteristics
ThymusThymosinsT lymphocytesStimulate production and maturation of T lymphocytes
Pineal glandMelatoninBrainControls circadian rhythms, possibly involved in maturation of sexual organs
Like testosterone, most hormones act at a distance between body parts. They travel in the bloodstream from the gland that produced them to their target cells. Also considered to be hormones are the secretions produced by neurosecretory cells in the hypothalamus of the brain. They travel in the capillary network that runs between the hypothalamus and the pituitary gland. Some of these secretions stimulate the pituitary to secrete its hormones, and others prevent it from doing so.
Not all hormones act between body parts. As we will see, prostaglandins are a good example of local hormones. After prostaglandins are produced, they are not carried elsewhere in the bloodstream. Instead, they affect neighboring cells, sometimes promoting pain and inflammation. Also, growth factors are local hormones that promote cell division and mitosis.
Chemical signals that influence the behavior of other individuals are called pheromones. Nonhuman animals rely heavily on pheromones for communication—to mark one’s territory and to attract a mate. Humans produce pheromones, too. Researchers have isolated a pheromone released by men that reduces premenstrual nervousness and tension in women. Women who live in the same household often have menstrual cycles in synchrony. This is likely caused by the armpit secretions of a woman who is menstruating, affecting the menstrual cycles of other women in the household.
The Action of Hormones
Hormones have a wide range of effects on cells. Some of these effects induce a target cell to increase its uptake of particular substances (such as glucose) or ions (such as calcium). Other effects bring about an alteration of the target cell’s structure in some way. A few hormones simply influence cell metabolism. Growth hormone is a peptide that influences cell metabolism leading to a change in the structure of bone. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins, and modified amino acids. Growth hormone is a protein produced and secreted by the anterior pituitary. All steroid hormones have a similar structure of four carbon rings because they are all derived from cholesterol (see Fig. 2.20).
The Action of Peptide Hormones
Most endocrine glands secrete peptide hormones. The actions of peptide hormones can vary depending on the type of target cell. Peptide hormones do not have the ability to cross the plasma membrane, and therefore must interact with a receptor on the surface of the membrane.
As an example, we will explore what happens when the hormone epinephrine binds to a plasma membrane receptor of a muscle cell (Fig. 16.4). In muscle cells, the reception of epinephrine leads to the breakdown of glycogen to glucose, which provides energy for ATP production. The immediate result of binding is the formation of cyclic adenosine monophosphate (cAMP). Cyclic AMP contains one phosphate group attached to adenosine at two locations, producing a circular, or cyclic, molecule. Cyclic AMP activates a protein kinase enzyme in the cell. This enzyme, in turn, activates another enzyme, and so forth. The series of enzymatic reactions that follows cAMP formation is called an enzyme cascade. Each enzyme can be used over and over at every step of the cascade, so more enzymes are involved. Finally, many molecules of glycogen are broken down to glucose, which enters the bloodstream.
Figure 16.4 Action of a peptide hormone. A peptide hormone (first messenger) binds to a receptor in the plasma membrane. Thereafter, cyclic AMP (second messenger) forms and activates an enzyme cascade.
Tutorial: Action of a Peptide Hormone
Typical of a peptide hormone, epinephrine never enters the cell. Therefore, the hormone is called the first messenger; cAMP, which sets the metabolic machinery in motion, is called the second messenger. To explain this terminology, let’s imagine that the adrenal medulla, which produces epinephrine, is like a company’s home office that sends out a courier (the hormone epinephrine is the first messenger) to a factory (the cell). The courier doesn’t have a pass to enter the factory, so when he arrives at the factory, he tells a supervisor through the intercom that the home office wants the factory to produce a particular product. The supervisor (cAMP, the second messenger) enters a command in the computer that instructs the machinery (the enzymatic pathway) to make the product.
Second Messengers
The Action of Steroid Hormones
Only the adrenal cortex, the ovaries, and the testes produce steroid hormones. Thyroid hormones belong to a class of molecules called the amines. Amines act in a manner similar to the steroid hormones, even though they have a different structure. Steroid hormones do not bind to plasma membrane receptors. Because they are hydrophobic (see Section 2.5), steroids are able to enter the cell in the same manner as lipids (Fig. 16.5).
Figure 16.5 Action of a steroid hormone. A steroid hormone passes directly through the target cell’s plasma membrane before binding to a receptor in the nucleus or cytoplasm. The hormone–receptor complex binds to DNA, and gene expression follows.
Page 332Once inside, a steroid hormone binds to a receptor, usually in the nucleus but sometimes in the cytoplasm. Inside the nucleus, the hormone–receptor complex binds with DNA and activates certain genes. Messenger RNA (mRNA) moves to the ribosomes in the cytoplasm, and protein (e.g., enzyme) synthesis follows (see Section 22.2). To continue our analogy, a steroid hormone is like a courier who has a pass to enter the factory (the cell). Once inside, it makes contact with the plant manager (DNA), who sees to it that the factory (cell) is ready to produce a product.
Tutorial: Action of a Steroid Hormone
An example of a steroid hormone is aldosterone, which is produced by the adrenal glands. Aldosterone targets the kidneys, where it helps regulate the water-salt balance of the blood. In general, steroid hormones act more slowly than peptide hormones, because it takes more time to synthesize new proteins than to activate enzymes already present in cells. Their action, however, typically lasts longer.
Mechanism of Steroid Hormone Action
CHECK YOUR PROGRESS 16.1
State the role of a hormone.
Answer
A hormone is a chemical signal that affects the metabolism of a target cell.
Compare and contrast the nervous and endocrine systems with regard to function and the types of signals used.
Answer
The nervous and endocrine systems both regulate the activities of other systems in the body. The nervous system responds rapidly to stimuli, using neurotransmitters as signals, whereas endocrine system responses using hormones are slower but longer lasting.
Summarize the differences between a peptide hormone and a steroid hormone.
Answer
Peptide hormones contain amino acids, whereas steroid hormones are derived from cholesterol. Peptide hormones act by binding to surface receptors on target cells, activating an enzyme cascade via a second messenger. Steroid hormones interact with receptors inside cells, usually in the nucleus, and the hormone–receptor complex that is formed binds to DNA, activating certain genes.
Explain why second messenger systems are needed for peptide hormones.
Answer
Most peptide hormones cannot pass through the plasma membrane and thus work by interacting with surface receptors, which in turn use second messengers to alter cell metabolism.
Page 333
CONNECTING THE CONCEPTS
For more information on the interactions in this section, refer to the following discussions:
Sections 2.5 and 2.6 summarize the roles of steroids and proteins in the body.
Section 3.3 explores the structure of the plasma membrane and the proteins associated with it.
Section 14.2 describes the location and function of the hypothalamus, which integrates the nervous and endocrine systems.
Reverse.Prompt
- Responding to External Changes
- The nervous system is particularly able to respond to changes in the external environment.
- Some responses are automatic, as you can verify by trying this:
- The eyes and other organs that have sensory receptors provide us with valuable information about the external environment. The central nervous system, on the receiving end of millions of bits of information, integrates information, compares it with previously stored memories, and “decides” on the proper course of action. The nervous system often responds to changes in the external environment through body movement. It gives us the ability to stay in as moderate an environment as possible. Otherwise, we test the ability of the nervous system to maintain homeostasis despite extreme conditions.
See Figure 16.23 The nervous system and endocrine system interact to control homeostasis. The nervous and endocrine systems work together to regulate and control the other systems.
Reverse.Prompt
16.2 Hypothalamus and Pituitary Gland
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain the role of the hypothalamus in the endocrine system.
List the hormones produced by the anterior and posterior pituitary glands and provide a function for each.
Summarize the conditions produced by excessive and inadequate levels of the major hormones.
Reverse.Prompt
Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels.
When blood calcium is high:
- , the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium.
- When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the
- kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium.
- These effects continue until the blood calcium levels return to normal.
Reverse.Prompt
Diabetes Mellitus
An estimated 30.3 million Americans (9.4% of the population) have diabetes mellitus, often referred to simply as diabetes. Of these, an estimated 7.2 million are undiagnosed. Diabetes is characterized by the body’s inability to maintain blood glucose homeostasis (Fig. 16.19), resulting in an excess of glucose in the blood. This creates a number of problems with homeostasis. As the blood glucose level rises, glucose, along with water, is excreted in the urine. The term mellitus, from Greek, refers to “honey” or “sweetness.” As a result, diabetics urinate frequently and are always thirsty. Other symptoms of diabetes include fatigue, constant hunger, and weight loss.
The high blood glucose levels often cause an increase in blood pressure due to osmosis and as a result can damage the small capillaries of the kidneys, eyes, and other areas of the circulatory system. Diabetics often experience vision problems due to diabetic retinopathy and swelling in the lens of the eye. If untreated, diabetics often develop serious and even fatal complications. Sores that don’t heal develop into severe infections. Blood vessel damage causes kidney failure, nerve destruction, heart attack, or stroke.
Blood Sugar Regulation in Diabetics
Types of Diabetes
There are two types of diabetes. Type 1 diabetes is sometimes called juvenile diabetes or insulin-dependent diabetes mellitus (IDDM). Type 2 diabetes is also known as adult-onset diabetes, or non-insulin-dependent diabetes mellitus (NIDDM). Although the causes of these forms of diabetes are different, they can occur in children or adults.
Type 1 Diabetes
In type 1 diabetes, the pancreas is not producing enough insulin. This condition is believed to be brought on by exposure to an environmental agent, most likely a virus, whose presence causes cytotoxic T cells to destroy the pancreatic islets as Page 345part of an autoimmune response (see Section 7.5). The body turns to the metabolism of fat, which leads to the buildup of ketones in the blood, called ketoacidosis, which increases the acidity of the blood and can lead to coma and death.
Individuals with type 1 diabetes must have daily insulin injections. These injections control the diabetic symptoms but still can cause inconveniences, because the blood sugar level may swing between hypoglycemia (low blood glucose) and hyperglycemia (high blood glucose). Without testing the blood glucose level, it is difficult to be certain which of these is present, because the symptoms can be similar. These symptoms include perspiration, pale skin, shallow breathing, and anxiety. Whenever these symptoms appear, immediate attention is required to bring the blood glucose back within the normal range. If the problem is hypoglycemia, the treatment is one or two glucose tablets, hard candy, or orange juice. If the problem is hyperglycemia, the treatment is insulin. Better control of blood glucose levels can often be achieved with an insulin pump, a small device worn outside the body that is connected to a plastic catheter inserted under the skin.
Because diabetes is such a common problem, many researchers are working to develop more effective methods for treating diabetes. Recently, progress has been made in the development of an artificial pancreas, which would act as an automated system to provide insulin based on real-time changes in blood sugar levels. It is also possible to transplant a working pancreas, or even fetal pancreatic islet cells, into patients with type 1 diabetes. Another possibility is xenotransplantation, in which insulin-producing islet cells of another species, such as pigs, are placed inside a capsule that allows insulin to exit but prevents the immune system from attacking the foreign cells. Finally, researchers are close to testing a vaccine that could block the immune system’s attack on the islet cells, perhaps by inducing the T cells capable of suppressing these responses.
Type 2 Diabetes
Most adult diabetics have type 2 diabetes. Often, the patient is overweight or obese, and adipose tissue produces a substance that impairs insulin receptor function. However, complex genetic factors can be involved, as shown by the tendency for type 2 diabetes to occur more often in certain families or even ethnic groups. For example, the condition is 77% more common in African Americans than in non-Hispanic whites.
Normally, the binding of insulin to its plasma membrane receptor causes the number of protein carriers for glucose to increase, causing more glucose to enter the cell. In the type 2 diabetic, insulin still binds to its receptor, but the number of glucose carriers does not increase. Therefore, the cell is said to be insulin resistant.
It is possible to prevent or at least control type 2 diabetes by adhering to a low-fat, low-sugar diet and exercising regularly. If this fails, oral drugs are available that stimulate the pancreas to secrete more insulin and enhance the metabolism of glucose in the liver and muscle cells. Millions of Americans may have type 2 diabetes without being aware of it, yet the effects of untreated type 2 diabetes are as serious as those of type 1 diabetes.
Reverse.Prompt
- The is divided into three regions These are the
- zona glomerulosa,
- the zona fasciculata, and the
- zona reticularis.
- In contrast to the adrenal medulla, the hormones produced by the adrenal cortex provide a long-term response to stress
- The two major types of hormones produced by the adrenal cortex
- are the glucocorticoids and the
- mineralocorticoids.
- The adrenal cortex also secretes a small amount of sex hormones
adrenal cortex
(Fig. 16.14b).
Page 342
Reverse.Prompt
The hypothalamus acts as the link between the nervous and endocrine systems.
- It regulates the internal environment through communications with the autonomic nervous system.
- For example, it helps control body temperature and water-salt balance.
- The hypothalamus also controls the glandular secretions of the pituitary gland. The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalklike structure.
- The pituitary has two portions:
- the posterior and the anterior pituitary.
- Although the anterior and posterior pituitary glands are connected,
- they operate as separate physiological glands.
Reverse.Prompt
CHECK YOUR PROGRESS 16.3
Explain how the hormones of the thyroid gland influence the metabolic rate.
Answer
T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy.
Reverse.Prompt
See Figure 16.11
Adrenal glands
- Glands that sit on top of the kidneys
- 2 parts of each gland
- Adrenal medulla: controlled by the nervous system
- Adrenal cortex: portions are controlled by ACTH from the anterior pituitary
Reverse.Prompt
Page 333
CONNECTING THE CONCEPTS
For more information on the interactions in this section, refer to the following discussions:
Sections 2.5 and 2.6 summarize the roles of steroids and proteins in the body.
Section 3.3 explores the structure of the plasma membrane and the proteins associated with it.
Section 14.2 describes the location and function of the hypothalamus, which integrates the nervous and endocrine systems.
Reverse.Prompt
16.5 Pancreas
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain how the pancreas functions as both an endocrine and an exocrine gland.
Describe how the pancreatic hormones help maintain blood glucose homeostasis.
Distinguish between type 1 and type 2 diabetes mellitus.
Reverse.Prompt
Prostaglandins are local hormones that affect neighboring cells and thus are not carried in the bloodstream.
Pheromones are chemical signals that influence the behavior of other individuals
Hormones Hormones
Reverse.Prompt
16.6 Other Endocrine Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the hormones produced by the sex organs, thymus, and pineal gland and provide a function for each.
List the hormones produced by glands and organs outside of the endocrine system.
The male testes and female ovaries, collectively called the gonads, produce hormones and therefore are considered endocrine glands. In addition, the thymus and pineal gland, as well as some other tissues in the body, have endocrine functions.
Testes and Ovaries
The activity of the testes and ovaries is controlled by the hypothalamus and pituitary. The testes (sing., testis) are located in the scrotum, and the ovaries are located in the pelvic cavity. The testes produce androgens (male sex hormones), such as testosterone. The ovaries produce estrogen and progesterone, the female sex hormones. These hormones feed back to control the hypothalamic secretion of gonadotropin-releasing hormone (GnRH). The pituitary gland secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the gonadotropic hormones (Fig. 16.21), is controlled by feedback from the sex hormones, too. The activities of FSH and LH are discussed in more detail in Section 17.4. The ovaries also produce a small amount of testosterone.
Figure 16.21 The hormones produced by the testes and the ovaries. The testes and ovaries secrete the sex hormones. The testes secrete testosterone, and the ovaries secrete estrogens and progesterone. In each sex, secretion of GnRH from the hypothalamus and secretion of FSH and LH from the pituitary are controlled by their respective hormones.
Under the influence of the gonadotropic hormones, the testes begin to release increased amounts of testosterone at the time of puberty. Testosterone stimulates the growth of the penis and the testes. Testosterone also brings about and maintains the male secondary sex characteristics that develop during puberty. These include the growth of facial, axillary (underarm), and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to lower. Testosterone also stimulates oil and sweat glands in the skin. It is largely responsible for acne and body odor. Another side effect of testosterone is baldness. Although females, like males, inherit genes for baldness, baldness is seen more often in males because of the presence of testosterone. Testosterone is partially responsible for the muscular strength of males, and this is why some athletes take supplemental amounts of anabolic steroids, which are either testosterone or related chemicals.
The female sex hormones, estrogens (often referred to in the singular) and progesterone, have many effects on the body. In particular, estrogen secreted at the time of puberty stimulates the growth of the uterus and the vagina. Estrogen is necessary for egg maturation and is largely responsible for the secondary sex characteristics in females. These include female body hair and fat distribution. In general, females have a more rounded appearance than males because of a greater accumulation of fat beneath the skin. Also, the pelvic girdle is wider in females than in males, resulting in a larger pelvic cavity. Both estrogen and progesterone are required for breast development and for regulation of the uterine cycle. This includes monthly menstruation (discharge of blood and mucosal tissues from the uterus).Page 348
Thymus
The lobular thymus lies just beneath the sternum (see Fig. 16.1). This organ reaches its largest size and is most active during childhood. With aging, the organ gets smaller and becomes fatty. Lymphocytes that originate in the bone marrow and then pass through the thymus are transformed into T lymphocytes. The lobules of the thymus are lined by epithelial cells that secrete hormones called thymosins. These hormones aid in the differentiation of lymphocytes packed inside the lobules. Although thymosins ordinarily work in the thymus, research is investigating ways that they could be used in AIDS or cancer patients to enhance T-lymphocyte function.
Pineal Gland
The pineal gland, located in the brain (see Fig. 16.1), produces the hormone melatonin, primarily at night. Melatonin is involved in our daily sleep-wake cycle. Normally, we grow sleepy at night when melatonin levels increase and awaken once daylight returns and melatonin levels are low (Fig. 16.22). Daily 24-hour cycles such as this are called circadian rhythms. These rhythms are controlled by a biological clock located in the hypothalamus.
Figure 16.22 Melatonin production changes by season. Melatonin production is greatest at night when we are sleeping. a. Light suppresses melatonin production. Melatonin is secreted for a longer time in the (b) winter than in the (c) summer.
(photo): ©Evelyn Jo Johnson
Animal research suggests that melatonin also regulates sexual development. In keeping with these findings, it has been noted that children whose pineal glands have been destroyed due to brain tumors experience early puberty.
Hormones from Other Organs or Tissues
Some organs not usually considered endocrine glands secrete hormones. We have already mentioned that the kidneys secrete renin and that the heart produces atrial natriuretic hormone (see Section 16.4); recall also that the stomach and the small intestine produce peptide hormones that regulate digestive secretions. A number of other types of tissues produce hormones.
Erythropoietin
In response to a low oxygen blood level, the kidneys secrete erythropoietin (EPO). Erythropoietin stimulates red blood cell formation in the red bone marrow. A greater number of red blood cells results in increased blood oxygen. A number of different types of organs and cells also produce peptide growth factors, which stimulate cell division and mitosis. Growth factors can be considered hormones because they act on cell types with specific receptors to receive them. Some are released into the blood; others diffuse to nearby cells.
Leptin
Leptin is a protein hormone produced by adipose tissue. Leptin acts on the hypothalamus, where it signals satiety, or fullness. Strange to say, the blood of obese individuals may be rich in leptin. It is possible that the leptin they produce is ineffective because of a genetic mutation or because their hypothalamic cells lack a suitable number of receptors for leptin.Page 349
Prostaglandins
Prostaglandins are potent chemical signals produced in cells from arachidonate, a fatty acid. Prostaglandins are not distributed in the blood. They act locally, quite close to where they were produced. They are often produced by a tissue where damage has occurred, resulting in the sensation of pain (see Section 15.2). In the uterus, prostaglandins cause muscles to contract. Therefore, they are implicated in the pain and discomfort of menstruation in some women. Also, prostaglandins mediate the effects of pyrogens, chemicals believed to reset the temperature regulatory center in the brain. Aspirin reduces body temperature and controls pain because of its effect on prostaglandins.
Certain prostaglandins reduce gastric secretion and have been used to treat gastric reflux. Others lower blood pressure and have been used to treat hypertension. Still others inhibit platelet aggregation and have been used to prevent thrombosis. However, different prostaglandins have contrary effects, and it has been very difficult to standardize their use. Therefore, prostaglandin therapy is still considered experimental.
CHECK YOUR PROGRESS 16.6
Summarize the role of testosterone and estrogen in the body.
Answer
Estrogen maintains the secondary sexual characteristics in the female, along with regulating the monthly uterine cycle. Testosterone maintains the secondary sexual characteristics in males.
Explain the relationship between melatonin and the sleep-wake cycle.
Answer
Levels of melatonin increase at night, leading to sleep. They decrease by morning, when we awaken.
Describe the response of the body to low levels of oxygen in the blood.
Answer
The kidneys will secrete erythropoietin, which stimulates red blood cell production.
CONNECTING THE CONCEPTS
For more information on the hormones presented in this section, refer to the following discussions:
Section 6.2 illustrates the role of erythropoietin in the manufacture of new red blood cells.
Section 9.4 examines the role of the digestive hormones.
Chapter 17 explores the role of the male and female sex hormones.
16.7 Hormones and Homeostasis
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize how the endocrine and nervous systems respond to external changes in the body.
Summarize how the endocrine and nervous systems respond to internal changes in the body.
The nervous and endocrine systems exert control over the other systems and thereby maintain homeostasis (Fig. 16.23).
Figure 16.23 The nervous system and endocrine system interact to control homeostasis. The nervous and endocrine systems work together to regulate and control the other systems.
Responding to External Changes
The nervous system is particularly able to respond to changes in the external environment. Some responses are automatic, as you can verify by trying this: Hold a piece of clear plastic in front of your face. Get someone to gently toss a soft object, such as a wadded-up piece of paper, at the plastic. Can you prevent yourself from blinking? This reflex protects your eyes.
The eyes and other organs that have sensory receptors provide us with valuable information about the external environment. The central nervous system, on the receiving end of millions of bits of information, integrates information, compares it with previously stored memories, and “decides” on the proper course of action. The nervous system often responds to changes in the external environment through body movement. It gives us the ability to stay in as moderate an environment as possible. Otherwise, we test the ability of the nervous system to maintain homeostasis despite extreme conditions.
Responding to Internal Changes
The governance of internal organs usually requires that the nervous and endocrine systems work together. This usually occurs below the level of consciousness. Subconscious control often depends on reflex actions that involve the hypothalamus and the medulla oblongata. Let’s take blood pressure as an example. After a 3-mile run, you decide to sit down under a tree to rest a bit. When you stand up to push off again, you feel faint. The feeling quickly passes, because the medulla oblongata responds to input from the baroreceptors in the aortic arch and carotid arteries. The sympathetic system immediately acts to increase heart rate and constrict the blood vessels, so your blood pressure rises. Sweating may have upset the water-salt balance of your blood. If so, the hormone aldosterone from the adrenal cortex will act on the kidney tubules to conserve sodium ions (Na+), and water reabsorption will follow. The hypothalamus can also help by sending antidiuretic hormone (ADH) to the posterior pituitary gland, which releases it into the blood. ADH actively promotes water reabsorption by the kidney tubules.
Recall from Section 14.5 that certain drugs, such as alcohol, can affect ADH secretion. When you consume alcohol, it is quickly absorbed across the stomach lining into the bloodstream, where it travels to the hypothalamus and inhibits ADH secretion. When ADH levels fall, the kidney tubules absorb less water. The result is increased production of dilute urine. Excessive water loss, or dehydration, is a disturbance of homeostasis. This is why drinking alcohol when you are exercising or perspiring heavily on a hot day is not a good idea. Instead of keeping you hydrated, an alcoholic beverage, such as beer, has the opposite effect.
Controlling the Reproductive System
Few systems intrigue us more than the reproductive system, which couldn’t function without nervous and endocrine control. The hypothalamus controls the anterior pituitary, which controls the release of hormones from the testes and the ovaries and the production of their gametes. The nervous system directly controls the muscular contractions of the ducts that propel the sperm. Page 350Contractions of the uterine tubes, which move a developing embryo to the uterus, where development continues, are stimulated by the nervous system, too. Without the positive feedback cycle involving oxytocin produced by the hypothalamus, birth might not occur.
The Neuroendocrine System
The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system. As we have seen, the hypothalamus certainly bridges the regulatory activities of the nervous and endocrine systems. In addition to producing the hormones released by the posterior pituitary, the hypothalamus produces hormones that control the anterior pituitary. The nerves of the autonomic system, which control other organs, are acted upon directly by the hypothalamus. The hypothalamus truly belongs to both the nervous and endocrine systems. Indeed, it is often and appropriately referred to as a neuroendocrine organ.Page 351
CHECK YOUR PROGRESS 16.7
Summarize the role of the endocrine system in maintaining homeostasis.
Answer
The nervous system can detect inputs from sensory receptors both internally and externally. The endocrine system, which works with the nervous system, is a mechanism for responding to these stimuli to maintain homeostasis.
Explain how the body restores its water-salt balance after it has lost water and salt through sweating.
Answer
Aldosterone from the adrenal cortex acts on kidney tubules to conserve sodium, and water reabsorption will follow. ADH from the anterior pituitary also increases water reabsorption by the kidneys.
Explain why the nervous and endocrine systems are integrated with one another.
Answer
The nervous and endocrine systems are integrated with each other because of the role of the hypothalamus in each system.
CONNECTING THE CONCEPTS
For more information on the organ systems presented in this section, refer to the following discussions:
Section 5.3 examines the factors that regulate heart rate.
Section 11.4 explains the role of aldosterone and ADH on the function of the kidney.
Section 14.2 explores the roles of the hypothalamus and medulla oblongata in the CNS.
CONCLUSION
For diabetics, the prospects of controlling their blood glucose levels and living a healthy life are better today than in the past. Prior to the development of recombinant DNA technology, which now allows human insulin to be produced in large quantities, insulin was derived from the pancreases of pigs or cows. This required laborious purification, and because the animal insulins were not identical to the human form, sometimes immunological reactions occurred. Increasingly, insulin pumps are being used to treat diabetes. An insulin pump is a device a little bigger than a cell phone, which can deliver precise amounts of insulin under the skin using a small plastic catheter. The insulin pump more accurately mimics the pancreas’s natural release of the correct amount of insulin needed by the body. Studies have shown that insulin pumps are more effective than traditional injections of insulin in controlling blood sugar levels. In the near future, it may be possible to implant a device—sometimes called an “artificial pancreas”—into patients with diabetes. Such a device would not only monitor the blood sugar level but also provide appropriate doses of insulin.
Reverse.Prompt
- This is secreted by the A cells of the pancreas, usually between eating, when the
- Trigger: blood glucose level is low
- major target tissues: are the liver and adipose tissue; How it raises glucose level:
- Glucagon stimulates the liver to break down glycogen to glucose.
- It also promotes the use of fat and protein in preference to glucose as energy sources.
- Adipose tissue cells break down fat to glycerol and fatty acids. T
- The liver takes these up and uses them as substrates for glucose formation. I.
Glucagon
How does it raise the blood glucose level?
What cells secrete it?
- Usually adult-onset and most common type
- Tends to occur in obese, sedentary people
- Cells do not respond to insulin
- Usually diet and exercise are important for controlling this and may even prevent this
Type 2 Diabetes
Reverse.Prompt
The thyroid gland
Location and Structure
- is a large gland located in the neck,
- where it is attached to the trachea just below the larynx
- (see Fig. 16.1).
- The parathyroid glands are embedded in the posterior surface of the thyroid gland.
- 6.
Reverse.Prompt
Hormonal Communication
- Certain neurons in the hypothalamus are sensitive to the water-salt balance of the blood. When these cells determine that the blood is too concentrated, ADH is released from the posterior pituitary.
- On reaching the kidneys, ADH causes more water to be reabsorbed into kidney capillaries, decreasing urine volume.
- As the blood becomes dilute, ADH is no longer released. This is an example of control by negative feedback, because the effect of the hormone (to dilute blood) acts to shut down the release of the hormone. Negative feedback maintains stable conditions and homeostasis.
- Inability to produce ADH causes diabetes insipidus. A person with this type of diabetes produces copious amounts of urine. Excessive urination results in severe dehydration and loss of important ions from the blood. The condition can be corrected by the administration of ADH.
-
Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing.
- The more the uterus contracts during labor, the more nerve signals reach the hypothalamus, causing oxytocin to be released.
- Similarly, as a baby suckles while being breastfed, nerve signals from breast tissue reach the hypothalamus.
- As a result, oxytocin is produced by the hypothalamus and released from the posterior pituitary. The hormone causes the woman’s breast milk to be released. The sound of a baby crying may also stimulate the release of oxytocin and milk letdown, much to the chagrin of women who are nursing. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback. The stimulus continues to bring about an effect that ever increases in intensity. Positive feedback terminates due to some external event. Therefore, positive feedback mechanisms are rarely used to maintain homeostasis; that role is typically associated with negative feedback mechanisms.
6.
Reverse.Prompt
Hormonal Communication
Page 335Certain neurons in the hypothalamus are sensitive to the water-salt balance of the blood. When these cells determine that the blood is too concentrated, ADH is released from the posterior pituitary. On reaching the kidneys, ADH causes more water to be reabsorbed into kidney capillaries, decreasing urine volume. As the blood becomes dilute, ADH is no longer released. This is an example of control by negative feedback, because the effect of the hormone (to dilute blood) acts to shut down the release of the hormone. Negative feedback maintains stable conditions and homeostasis.
Inability to produce ADH causes diabetes insipidus. A person with this type of diabetes produces copious amounts of urine. Excessive urination results in severe dehydration and loss of important ions from the blood. The condition can be corrected by the administration of ADH.
Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing. The more the uterus contracts during labor, the more nerve signals reach the hypothalamus, causing oxytocin to be released. Similarly, as a baby suckles while being breastfed, nerve signals from breast tissue reach the hypothalamus. As a result, oxytocin is produced by the hypothalamus and released from the posterior pituitary. The hormone causes the woman’s breast milk to be released. The sound of a baby crying may also stimulate the release of oxytocin and milk letdown, much to the chagrin of women who are nursing. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback. The stimulus continues to bring about an effect that ever increases in intensity. Positive feedback terminates due to some external event. Therefore, positive feedback mechanisms are rarely used to maintain homeostasis; that role is typically associated with negative feedback mechanisms.
Select all mechanisms by which glucocorticoids raise the blood glucose level.
Multiple select question.
metabolism of fatty acids
usage of amino acids to build proteins
Reason:
This is the opposite of what glucocorticoids cause, which is the breakdown of proteins and the conversion of amino acids into glucose.
production of glycogen
Reason:
This would decrease the blood glucose level.
conversion of amino acids to glucose
metabolism of fatty acids
conversion of amino acids to glucose
Reverse.Prompt
Peptide hormones
- contain amino acids
- act by binding to surface receptors on target cells, activating an enzyme cascade via a second messenger**. cAMP
- Most cannot pass through the plasma membrane and thus work by interacting with surface receptors, which in turn use second messengers to alter cell metabolism.
Steroid hormones
- are derived from cholesterol.
- Steroid hormones interact with receptors inside cells, usually in the nucleus, and the hormone–receptor complex that is formed binds to DNA, activating certain genes.
Peptide Hormones v. Steroid Hormones
Reverse.Prompt
Compare and contrast exocrine and endocrine glands. What are the different hormones and their functions? What are their effects? What are steroid and peptide/non-steroidal hormones? What are their methods of action? What are the triggers for hormone release? Name the major glands and their functions in the endocrine system. What happens when hormones are not secreted properly– what are some common endocrine diseases? How do the endocrine and nervous systems work with the rest of the systems in the body to maintain homeostasis?
Reverse.Prompt
- regulate ion (electrolyte) balances in the body and
- are primarily produced by the zona glomerulosa in the adrenal cortex.
- Aldosterone is the most important of the mineralocorticoids.
- Aldosterone primarily targets the kidney, where it promotes renal absorption of sodium ions (Na+) and renal excretion of potassium ions (K+).
- The secretion of mineralocorticoids is not controlled by the anterior pituitary: Steps:
- When the blood sodium level and pressure are low, the kidneys secrete renin
- Renin is an enzyme that converts the plasma protein angiotensinogen to angiotensin I.
- Angiotensin I is changed to angiotensin II by a converting enzyme found in lung capillaries.
- Angiotensin II stimulates the adrenal cortex to release aldosterone.
- The effect of this system, called the renin-angiotensin-aldosterone system, is to raise blood pressure in two ways.
- Angiotensin II constricts the arterioles, and
- aldosterone causes the kidneys to reabsorb sodium ions (Na+).
- When the blood sodium level rises, water is reabsorbed, in part because the hypothalamus secretes ADH.
- Reabsorption means that water enters kidney capillaries and, thus, the blood. Then, blood pressure increases to normal.
- Regulation of blood pressure is under hormonal control.
7.
Mineralocorticoids
(Fig. 16.15).
(see Section 16.2)
Figure 16.15
Bottom: When the blood sodium level is low, a low blood pressure causes the kidneys to secrete renin. Renin leads to the secretion of aldosterone from the adrenal cortex. Aldosterone causes the kidneys to reabsorb sodium ions +(Na+), and water follows, so that blood volume and pressure return to normal. Top: When a high blood sodium level accompanies a high blood volume, the heart secretes atrial natriuretic hormone (ANH). ANH causes the kidneys to excrete sodium ions +(Na+), and water follows. The blood volume and pressure return to normal.
Reverse.Prompt
What is diabetes? General symptoms include frequent urination. unusual hunger and/or thirst. unexplained change in weight. blurred vision. sores that heal slowly or not at all. excessive fatigue. Long-term effects are blindness, loss of limbs, nerve deterioration, kidney and cardiovascular disease.
16.12
Type 1 Usually early-onset Autoimmune disorder that tends to run in families Pancreatic cells are attacked and cannot produce insulin Need insulin injections Type 2 Usually adult-onset and most common type Tends to occur in obese, sedentary people Cells do not respond to insulin Usually diet and exercise are important for controlling this and may even prevent this
Gonads found in males Produce androgens (e.g., testosterone) Stimulates growth of the penis and testes Responsible for male sex characteristics such as facial, underarm, and pubic hair Prompts the larynx and vocal cords to enlarge, resulting in a lower voice Promotes muscular strength
Gonads found in males Produce androgens (e.g., testosterone) Stimulates growth of the penis and testes Responsible for male sex characteristics such as facial, underarm, and pubic hair Prompts the larynx and vocal cords to enlarge, resulting in a lower voice Promotes muscular strength
Figure 16.20
Gonads found in males Produce androgens (e.g., testosterone) Stimulates growth of the penis and testes Responsible for male sex characteristics such as facial, underarm, and pubic hair Prompts the larynx and vocal cords to enlarge, resulting in a lower voice Promotes muscular strength
- Pineal gland Located in the brain Secretes melatonin that regulates the sleep/wake cycle (circadian rhythm) May also regulate sexual development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 P.M. 6 A.M. b. Winter a. Experimental © The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer c. Summer Figure 16.21 Melatonin production changes by season. 16.6 Other Endocrine Glands 22 Erythropoietin is secreted by the kidneys to increase red blood cell production. Leptin is produced by fat cells, and acts on the hypothalamus to give a feeling of being satiated. Hormones from other tissues 16.6 Other Endocrine Glands Hormones from other tissues Prostaglandins Groups of potent chemicals that are not carried in the bloodstream, but work locally on neighboring cells Some cause smooth muscle contraction Major impact on reproductive organs Many other roles in the body Aspirin and ibuprofen block the synthesis of these
Homeostasis
The nervous and endocrine systems are important in maintaining homeostasis. The hypothalamus bridges regulatory functions of both systems. The nervous system is able to respond to changes in the external environment. H
Reverse.Prompt
Regulates internal environment through the autonomic nervous system Helps control heartbeat Helps control body temperature Helps control water balance Controls glandular secretions 1
Reverse.Prompt
Diabetes
16.12
- Type 1 Usually early-onset
- Autoimmune disorder that tends to run in families
- Pancreatic cells are attacked and cannot produce insulin
- Need insulin injections
- Type 2 Usually adult-onset and most common type
- Tends to occur in obese, sedentary people
- Cells do not respond to insulin
- Usually diet and exercise are important for controlling this and may even prevent this
Reverse.Prompt
Under the influence of the gonadotropic hormones,
- the testes begin to release increased amounts of testosterone at the time of puberty. Testosterone stimulates the growth of the penis and the testes. Testosterone also brings about and maintains the male secondary sex characteristics that develop during puberty. These include the growth of facial, axillary (underarm), and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to lower. Testosterone also stimulates oil and sweat glands in the skin. It is largely responsible for acne and body odor. Another side effect of testosterone is baldness. Although females, like males, inherit genes for baldness, baldness is seen more often in males because of the presence of testosterone. Testosterone is partially responsible for the muscular strength of males, and this is why some athletes take supplemental amounts of anabolic steroids, which are either testosterone or related chemicals.
Testes and testosterone
Reverse.Prompt
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Distinguish between the mode of action of a neurotransmitter and that of a hormone.
Distinguish between endocrine and exocrine glands.
Identify the organs and glands of the endocrine system.
Compare the actions of peptide and steroid hormones.
The organs of the endocrine system (Fig. 16.1) are responsible for the production of chemical signals, called hormones, that are involved in the regulation of the other organs in the body. The endocrine system works very closely with the nervous system to maintain homeostasis in the body.
Figure 16.1 The endocrine system. This diagram indicates the major endocrine glands of the body. Other organs also produce hormones, such as the kidneys, the gastrointestinal tract, and the heart, but this is not the primary function of these organs.
There is a difference in function between an endocrine gland and an exocrine gland. Exocrine glands have ducts and secrete their products into these ducts. The glands’ products are carried to the interior of other organs or outside the body. The accessory glands of the digestive system (see Section 9.4) are good examples Page 330of the exocrine glands. For example, the salivary glands send saliva into the mouth by way of the salivary ducts. In contrast, endocrine glands secrete their products into the bloodstream, which delivers them throughout the body. Only certain cells, called target cells, can respond to a specific hormone. A target cell for a particular hormone has a receptor protein for that hormone. The hormone and the receptor protein bind together like a key that fits a lock. The target cell then responds to that hormone.
Comparison of the Endocrine and Nervous Systems
The nervous system and endocrine system both use chemical signals when they respond to changes that might affect homeostasis. However, they have different means of delivering these signals (Fig. 16.2). As discussed in Section 14.1, the nervous system is composed of neurons. In this system, sensory receptors detect changes in the internal and external environments. The central nervous system (CNS) then integrates the information and responds by stimulating muscles and glands. Communication depends on nerve signals, conducted in axons, and neurotransmitters, which cross synapses. Axon conduction occurs rapidly, as does the diffusion of a neurotransmitter across the short distance of a synapse. In other words, the nervous system is organized to respond rapidly to stimuli. This is particularly useful if the stimulus is an external event that endangers our safety—we can move quickly to avoid being hurt.
Figure 16.2 Action of neurotransmitters and hormones. a. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen. b. Nerve impulses passing along an axon cause the release of a neurotransmitter. The neurotransmitter, a chemical signal, causes the wall of an arteriole to constrict. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen.
The endocrine system functions differently than the nervous system. The endocrine system is largely composed of glands (see Fig. 16.1). These glands secrete hormones, which are carried by the bloodstream to target cells throughout the body. It takes time to deliver hormones, and it takes time for cells to respond. The effect initiated by the endocrine system is longer lasting. In other words, the endocrine system is organized for a slow but prolonged response.
Both the nervous system and the endocrine system make use of negative feedback mechanisms (see Section 4.8). If the blood pressure falls, sensory receptors signal a control center in the brain. This center sends out nerve signals to the arterial walls, so that they constrict, and blood pressure rises. Now the sensory receptors are no longer stimulated, and the feedback mechanism is inactivated. Similarly, a rise in blood glucose level causes the pancreas to release insulin. This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body (see Fig. 16.2). When the blood glucose level falls, the pancreas no longer secretes insulin.
Hormones Are Chemical Signals
Like other chemical signals, hormones are a means of communication between cells, between body parts, and even between individuals. They affect the metabolism of cells that have receptors to receive them (Fig. 16.3).
Figure 16.3 Hormones target specific cells. Most hormones are distributed by the bloodstream to target cells. Target cells have receptors for the hormones, and a hormone combines with a receptor like a key fits a lock.
Page 331The importance of these receptors can be demonstrated by examining a condition called androgen insensitivity syndrome. Individuals with this syndrome have both X and Y sex chromosomes. Because they possess a Y chromosome, they produce the sex hormone testosterone (see Section 16.6), even though the testes usually remain in the abdominal cavity. However, the body cells lack receptors for testosterone, and therefore do not respond to the hormone. Therefore, the individuals appear to be normal females, although genetically they are males.
Table 16.1 summarizes the hormones of the endocrine system and provides the functions and targets of these hormones in the body.
Table 16.1Principal Endocrine Glands and the Hormones They Produce
Table Summary: Table lists the names of different endocrine glands in column 1, with pituitary gland, adrenal gland, and gonads sub-divided into its parts. Other information related to each type of gland is listed in columns 2 through 4.
Endocrine GlandHormone ReleasedTarget Tissues/OrgansChief Functions
HypothalamusHypothalamic-releasingAnterior pituitaryRegulates anterior pituitary hormones
Pituitary gland
Posterior pituitaryAntidiuretic (ADH)KidneysStimulates water reabsorption by kidneys
OxytocinUterus, mammary glandsStimulates uterine muscle contraction, release of milk by mammary glands
Anterior pituitaryThyroid-stimulating (TSH)ThyroidStimulates thyroid
Adrenocorticotropic (ACTH)Adrenal cortexStimulates adrenal cortex
Gonadotropic (FSH, LH)GonadsEgg and sperm production, sex hormone production
Prolactin (PRL)Mammary glandsMilk production
Growth (GH)Soft tissues, bonesCell division, protein synthesis, bone growth
Melanocyte-stimulating (MSH)Melanocytes in skinUnknown function in humans; regulates skin color in lower vertebrates
ThyroidThyroxine (T4) and triiodothyronine (T3)All tissuesIncrease metabolic rate, regulate growth and development
CalcitoninBones, kidneys, intestineLowers blood calcium level
ParathyroidsParathyroid (PTH)Bones, kidneys, intestineRaises blood calcium level
Adrenal gland
Adrenal cortexGlucocorticoids (cortisol)All tissuesRaise blood glucose level, stimulate breakdown of protein
Mineralocorticoids (aldosterone)KidneysReabsorb sodium and excrete potassium
Sex hormonesGonads, skin, muscles, bonesStimulate reproductive organs and bring about sex characteristics
Adrenal medullaEpinephrine and norepinephrineCardiac and other musclesAre released in emergency situations, raise blood glucose level
PancreasInsulinLiver, muscles, adipose tissueLowers blood glucose level, promotes glycogen formation
GlucagonLiver, muscles, adipose tissueRaises blood glucose level
Gonads
TestesAndrogens (testosterone)Gonads, skin, muscles, bonesStimulate male sex characteristics
OvariesEstrogens, progesterone, small amounts of testosteroneGonads, skin, muscles, bonesStimulate female sex characteristics
ThymusThymosinsT lymphocytesStimulate production and maturation of T lymphocytes
Pineal glandMelatoninBrainControls circadian rhythms, possibly involved in maturation of sexual organs
Like testosterone, most hormones act at a distance between body parts. They travel in the bloodstream from the gland that produced them to their target cells. Also considered to be hormones are the secretions produced by neurosecretory cells in the hypothalamus of the brain. They travel in the capillary network that runs between the hypothalamus and the pituitary gland. Some of these secretions stimulate the pituitary to secrete its hormones, and others prevent it from doing so.
Not all hormones act between body parts. As we will see, prostaglandins are a good example of local hormones. After prostaglandins are produced, they are not carried elsewhere in the bloodstream. Instead, they affect neighboring cells, sometimes promoting pain and inflammation. Also, growth factors are local hormones that promote cell division and mitosis.
Chemical signals that influence the behavior of other individuals are called pheromones. Nonhuman animals rely heavily on pheromones for communication—to mark one’s territory and to attract a mate. Humans produce pheromones, too. Researchers have isolated a pheromone released by men that reduces premenstrual nervousness and tension in women. Women who live in the same household often have menstrual cycles in synchrony. This is likely caused by the armpit secretions of a woman who is menstruating, affecting the menstrual cycles of other women in the household.
The Action of Hormones
Hormones have a wide range of effects on cells. Some of these effects induce a target cell to increase its uptake of particular substances (such as glucose) or ions (such as calcium). Other effects bring about an alteration of the target cell’s structure in some way. A few hormones simply influence cell metabolism. Growth hormone is a peptide that influences cell metabolism leading to a change in the structure of bone. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins, and modified amino acids. Growth hormone is a protein produced and secreted by the anterior pituitary. All steroid hormones have a similar structure of four carbon rings because they are all derived from cholesterol (see Fig. 2.20).
The Action of Peptide Hormones
Most endocrine glands secrete peptide hormones. The actions of peptide hormones can vary depending on the type of target cell. Peptide hormones do not have the ability to cross the plasma membrane, and therefore must interact with a receptor on the surface of the membrane.
As an example, we will explore what happens when the hormone epinephrine binds to a plasma membrane receptor of a muscle cell (Fig. 16.4). In muscle cells, the reception of epinephrine leads to the breakdown of glycogen to glucose, which provides energy for ATP production. The immediate result of binding is the formation of cyclic adenosine monophosphate (cAMP). Cyclic AMP contains one phosphate group attached to adenosine at two locations, producing a circular, or cyclic, molecule. Cyclic AMP activates a protein kinase enzyme in the cell. This enzyme, in turn, activates another enzyme, and so forth. The series of enzymatic reactions that follows cAMP formation is called an enzyme cascade. Each enzyme can be used over and over at every step of the cascade, so more enzymes are involved. Finally, many molecules of glycogen are broken down to glucose, which enters the bloodstream.
Figure 16.4 Action of a peptide hormone. A peptide hormone (first messenger) binds to a receptor in the plasma membrane. Thereafter, cyclic AMP (second messenger) forms and activates an enzyme cascade.
Tutorial: Action of a Peptide Hormone
Typical of a peptide hormone, epinephrine never enters the cell. Therefore, the hormone is called the first messenger; cAMP, which sets the metabolic machinery in motion, is called the second messenger. To explain this terminology, let’s imagine that the adrenal medulla, which produces epinephrine, is like a company’s home office that sends out a courier (the hormone epinephrine is the first messenger) to a factory (the cell). The courier doesn’t have a pass to enter the factory, so when he arrives at the factory, he tells a supervisor through the intercom that the home office wants the factory to produce a particular product. The supervisor (cAMP, the second messenger) enters a command in the computer that instructs the machinery (the enzymatic pathway) to make the product.
Second Messengers
The Action of Steroid Hormones
Only the adrenal cortex, the ovaries, and the testes produce steroid hormones. Thyroid hormones belong to a class of molecules called the amines. Amines act in a manner similar to the steroid hormones, even though they have a different structure. Steroid hormones do not bind to plasma membrane receptors. Because they are hydrophobic (see Section 2.5), steroids are able to enter the cell in the same manner as lipids (Fig. 16.5).
Figure 16.5 Action of a steroid hormone. A steroid hormone passes directly through the target cell’s plasma membrane before binding to a receptor in the nucleus or cytoplasm. The hormone–receptor complex binds to DNA, and gene expression follows.
Page 332Once inside, a steroid hormone binds to a receptor, usually in the nucleus but sometimes in the cytoplasm. Inside the nucleus, the hormone–receptor complex binds with DNA and activates certain genes. Messenger RNA (mRNA) moves to the ribosomes in the cytoplasm, and protein (e.g., enzyme) synthesis follows (see Section 22.2). To continue our analogy, a steroid hormone is like a courier who has a pass to enter the factory (the cell). Once inside, it makes contact with the plant manager (DNA), who sees to it that the factory (cell) is ready to produce a product.
Tutorial: Action of a Steroid Hormone
An example of a steroid hormone is aldosterone, which is produced by the adrenal glands. Aldosterone targets the kidneys, where it helps regulate the water-salt balance of the blood. In general, steroid hormones act more slowly than peptide hormones, because it takes more time to synthesize new proteins than to activate enzymes already present in cells. Their action, however, typically lasts longer.
Mechanism of Steroid Hormone Action
CHECK YOUR PROGRESS 16.1
State the role of a hormone.
Answer
A hormone is a chemical signal that affects the metabolism of a target cell.
Compare and contrast the nervous and endocrine systems with regard to function and the types of signals used.
Answer
The nervous and endocrine systems both regulate the activities of other systems in the body. The nervous system responds rapidly to stimuli, using neurotransmitters as signals, whereas endocrine system responses using hormones are slower but longer lasting.
Summarize the differences between a peptide hormone and a steroid hormone.
Answer
Peptide hormones contain amino acids, whereas steroid hormones are derived from cholesterol. Peptide hormones act by binding to surface receptors on target cells, activating an enzyme cascade via a second messenger. Steroid hormones interact with receptors inside cells, usually in the nucleus, and the hormone–receptor complex that is formed binds to DNA, activating certain genes.
Explain why second messenger systems are needed for peptide hormones.
Answer
Most peptide hormones cannot pass through the plasma membrane and thus work by interacting with surface receptors, which in turn use second messengers to alter cell metabolism.
Page 333
CONNECTING THE CONCEPTS
For more information on the interactions in this section, refer to the following discussions:
Sections 2.5 and 2.6 summarize the roles of steroids and proteins in the body.
Section 3.3 explores the structure of the plasma membrane and the proteins associated with it.
Section 14.2 describes the location and function of the hypothalamus, which integrates the nervous and endocrine systems.
Reverse.Prompt
Comparison of the Endocrine and Nervous Systems
Look at Figure 16.2 Figure 16.2 Action of neurotransmitters and hormones. a. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen. b. Nerve impulses passing along an axon cause the release of a neurotransmitter. The neurotransmitter, a chemical signal, causes the wall of an arteriole to constrict. The hormone insulin, a chemical signal, travels in the cardiovascular system from the pancreas to the liver, where it causes liver cells to store glucose as glycogen.
- The nervous system and endocrine system both use chemical signals when they respond to changes that might affect homeostasis.
- However, they have different means of delivering these signals (Fig. 16.2).
- As discussed in Section 14.1, the nervous system is composed of neurons.
- In this system, sensory receptors detect changes in the internal and external environments.
- The central nervous system (CNS) then integrates the information and responds by stimulating muscles and glands.
- Communication depends on nerve signals, conducted in axons, and neurotransmitters, which cross synapses. Axon conduction occurs rapidly, as does the diffusion of a neurotransmitter across the short distance of a synapse.
- In other words, the nervous system is organized to respond rapidly to stimuli. This is particularly useful if the stimulus is an external event that endangers our safety—we can move quickly to avoid being hurt.
Reverse.Prompt
- There is a difference in function between an endocrine gland and an exocrine gland.
- Exocrine glands
- have ducts and secrete their products into these ducts.
- The glands’ products are carried to the interior of other organs or outside the body.
- The accessory glands of the digestive system (see Section 9.4) are good examples of the exocrine glands.
- For example, the salivary glands send saliva into the mouth by way of the salivary ducts.
- In contrast, endocrine glands secrete their products into the bloodstream, which delivers them throughout the body. Only certain cells, called target cells, can respond to a specific hormone.
- A target cell for a particular hormone has a receptor protein for that hormone. The hormone and the receptor protein bind together like a key that fits a lock. The target cell then responds to that hormone.
- Page 330
Exochrine vs Endochrine glands
Reverse.Prompt
Hormone effects •
Changes in plasma membrane permeability or electrical state •
Synthesis of proteins, such as enzymes •
Activation or inactivation of enzymes •
Stimulation of mitos
Reverse.Prompt
BIOLOGY TODAY Science
Identifying Insulin as a Chemical Messenger
In 1920, physician Frederick Banting decided to try to isolate insulin in order to identify it as a chemical messenger. Previous investigators had been unable to do this, because the enzymes in the digestive juices destroyed insulin (a protein) during the isolation procedure. Banting hit upon the idea of ligating (tying off) the pancreatic duct, which he knew from previous research would lead to the degeneration only of the cells that produce digestive juices and not of the pancreatic islets (islets of Langerhans), where insulin is made. His professor, J. J. Macleod, made a laboratory available to him at the University of Toronto and assigned a graduate student, Charles Best, to assist him.
Banting and Best had limited funds and spent that summer working, sleeping, and eating in the lab. By the end of the summer, they had obtained pancreatic extracts that did lower the blood glucose level in diabetic dogs. Macleod then brought in biochemists, who purified the extract. Insulin therapy for the first human patient began in 1922, and large-scale production of purified insulin from pigs and cattle followed. The basic experimental system used by Banting and Best is shown in Table 16A.
Table 16AExperimental System Used by Banting and Best
Table Summary:
ProcedureResults
- Identify the source of the chemical.Pancreatic islets are source.
- Identify the effect to be studied.Presence of pancreatic secretions in body lowers blood glucose.
- Isolate the chemical.Insulin isolated from pancreatic secretions.
- Show that the chemical has the desired effect.Insulin lowers blood glucose.
For their discovery, Banting and Macleod received the Nobel Prize in Physiology or Medicine in 1923. The amino acid sequence of insulin was determined in 1953. Insulin is now synthesized using recombinant DNA technology, using bacteria, such as Escherichia coli, to produce the hormone. The availability of recombinant insulin (Fig. 16A), sometimes called synthetic insulin, has played a major role in improving the health of diabetics around the world.
Figure 16A Synthetic insulin. Insulin is now a product of recombinant engineering and biotechnology.
©McGraw-Hill Education/Jill Braaten, photographer
Questions to Consider
What are some advantages, and potential disadvantages, of producing a medicine destined to be injected into humans (such as insulin) in a bacterium such as E. coli?
Some people oppose the use of animals for medical research. Do you think insulin would have eventually been discovered without animal experimentation? Why or why not?
SCIENCE IN YOUR LIFE
What is gestational diabetes, and what causes it?
Women who were not diabetic prior to pregnancy but have high blood glucose during pregnancy have gestational diabetes. Gestational diabetes affects a small percentage of pregnant women. This form of diabetes is caused by insulin resistance—body insulin concentration is normal, but cells fail to respond normally. Gestational diabetes and insulin resistance generally develop later in the pregnancy. Carefully planned meals and exercise often control this form of diabetes, but insulin injections may be necessary.
If the woman is not treated, additional glucose crosses the placenta, causing high blood glucose in the fetus. The extra energy in the fetus is stored as fat, resulting in macrosomia, or a “fat” baby. Delivery of a very large baby can be dangerous for both the infant and the mother; thus, cesarean section is often necessary. Complications after birth are common for these babies. Further, there is a greater risk that the child will become obese and develop type 2 diabetes mellitus later in life.
Gestational diabetes usually goes away after the birth of the child. However, once a woman has experienced gestational diabetes, she has a greater chance of developing it again during future pregnancies. These women also tend to develop type 2 diabetes later in life.
Page 346
Testing for Diabetes
The oral glucose tolerance test assists in the diagnosis of diabetes mellitus. After the patient is given 100 g of glucose, the blood glucose concentration is measured at intervals. In a diabetic, the blood glucose level rises greatly and remains elevated for several hours (Fig. 16.20), and glucose appears in the urine. In a nondiabetic person, the blood glucose level rises somewhat and then returns to normal after about 2 hours.
Figure 16.20 The results of a glucose tolerance test for diabetes. Following the administration of 100 g of glucose, the blood glucose level rises dramatically in the diabetic, and glucose appears in the urine. Also, the blood glucose level at 2 hours is equal to more than 200 mg/dl.
SCIENCE IN YOUR LIFE
Are there alternatives to injections for insulin?
Until recently, diabetics had to rely mostly on insulin pumps or injections to receive insulin. However, in 2014 the FDA approved an insulin inhaler as a method of delivering insulin. The inhaler provides a dry powder to the lungs, where it is absorbed into the bloodstream. The inhaler is not designed to replace injections but, instead, to provide a small dose of insulin around mealtimes.
CHECK YOUR PROGRESS 16.5
Distinguish between the exocrine and endocrine functions of the pancreas.
Answer
Exocrine—produces and secretes enzymes and digestive juices, which are delivered to the small intestine via the pancreatic duct. Endocrine—produces and secretes insulin, glucagon, and somatostatin into the blood.
Describe how the pancreatic hormones interact to regulate blood glucose levels.
Answer
When blood glucose levels are high, insulin is released, which aids in storing excess glucose. When blood glucose levels are low, glucagon is released. This breaks down glycogen to glucose, which is delivered to the bloodstream.
Explain the difference in the relationship of the pancreas to type 1 and type 2 diabetes.
Answer
In type 1 diabetes, the pancreas does not produce insulin. In type 2 diabetes, cells become insulin resistant, and thus the pancreas may secrete more insulin than normal.
CONNECTING THE CONCEPTS
For additional information on the various forms of diabetes, refer to the following discussions:
Section 4.8 examines how feedback mechanisms are involved in maintaining blood glucose homeostasis.
Section 9.6 explains the body mass index (BMI), an indicator that is used to determine obesity and the subsequent risk of diabetes.
Section 11.3 examines the influence of diabetes on the urinary system.
Reverse.Prompt
16.2 Hypothalamus and Pituitary Gland
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain the role of the hypothalamus in the endocrine system.
List the hormones produced by the anterior and posterior pituitary glands and provide a function for each.
Summarize the conditions produced by excessive and inadequate levels of the major hormones.
Page 334The hypothalamus acts as the link between the nervous and endocrine systems. It regulates the internal environment through communications with the autonomic nervous system. For example, it helps control body temperature and water-salt balance. The hypothalamus also controls the glandular secretions of the pituitary gland. The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalklike structure. The pituitary has two portions: the posterior and the anterior pituitary. Although the anterior and posterior pituitary glands are connected, they operate as separate physiological glands.
Posterior Pituitary
Neurons in the hypothalamus called neurosecretory cells produce the hormones antidiuretic hormone (ADH) and oxytocin (Fig. 16.6). These hormones pass through axons into the posterior pituitary, where they are stored in axon endings.
Figure 16.6 Hormones produced by the hypothalamus and posterior pituitary. The hypothalamus produces two hormones, ADH and oxytocin, stored and secreted by the posterior pituitary.
Hormonal Communication
Page 335Certain neurons in the hypothalamus are sensitive to the water-salt balance of the blood. When these cells determine that the blood is too concentrated, ADH is released from the posterior pituitary. On reaching the kidneys, ADH causes more water to be reabsorbed into kidney capillaries, decreasing urine volume. As the blood becomes dilute, ADH is no longer released. This is an example of control by negative feedback, because the effect of the hormone (to dilute blood) acts to shut down the release of the hormone. Negative feedback maintains stable conditions and homeostasis.
Inability to produce ADH causes diabetes insipidus. A person with this type of diabetes produces copious amounts of urine. Excessive urination results in severe dehydration and loss of important ions from the blood. The condition can be corrected by the administration of ADH.
Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing. The more the uterus contracts during labor, the more nerve signals reach the hypothalamus, causing oxytocin to be released. Similarly, as a baby suckles while being breastfed, nerve signals from breast tissue reach the hypothalamus. As a result, oxytocin is produced by the hypothalamus and released from the posterior pituitary. The hormone causes the woman’s breast milk to be released. The sound of a baby crying may also stimulate the release of oxytocin and milk letdown, much to the chagrin of women who are nursing. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback. The stimulus continues to bring about an effect that ever increases in intensity. Positive feedback terminates due to some external event. Therefore, positive feedback mechanisms are rarely used to maintain homeostasis; that role is typically associated with negative feedback mechanisms.
SCIENCE IN YOUR LIFE
How is labor induced if a woman’s pregnancy extends past her due date?
After medication to prepare the birth canal for delivery, oxytocin (Pitocin) is used to induce labor. Pitocin is a synthetic version of the oxytocin released by the posterior pituitary. During labor, oxytocin may also be given to increase the strength of contractions. Stronger contractions speed the labor process if necessary (e.g., if the woman’s uterus is contracting poorly or if the health of the mother or child is at risk during delivery). Oxytocin is routinely used following delivery to minimize postpartum bleeding by ensuring that strong uterine contractions continue.
Use of oxytocin must be monitored carefully, because it may cause excessive uterine contractions. Should this occur, the uterus could tear itself. Further, reduced blood supply to the fetus caused by very strong contractions may be fatal to the baby. Though it reduces the duration of labor, inducing labor with oxytocin can be very painful for the mother. Whenever possible, physicians prefer gentler and more natural methods to induce labor and/or strengthen contractions.
Anterior Pituitary
A portal system, consisting of two capillary systems connected by a vein, lies between the hypothalamus and the anterior pituitary. The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing and hypothalamic-inhibiting hormones, which pass from the hypothalamus to the anterior pituitary by way of the portal system (Fig. 16.7). Examples are thyroid-releasing hormone (TRH) and thyroid-inhibiting hormone (TIH). The TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone, and the TIH inhibits the pituitary from secreting thyroid-stimulating hormone.
Figure 16.7 Hormones produced by the anterior pituitary. The hypothalamus controls the secretions of the anterior pituitary, and the anterior pituitary controls the secretions of the thyroid, adrenal cortex, and gonads, which are also endocrine glands.
Four of the seven hormones produced by the anterior pituitary have an effect on other glands. Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce the thyroid hormones. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce cortisol. The gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—stimulate the gonads (the testes in males and the ovaries in females) to produce gametes and sex hormones. In each instance, the blood level of the last hormone in the sequence exerts negative feedback control over the secretion of the first two hormones (Fig. 16.8).
Figure 16.8 Negative feedback mechanisms in the endocrine system. Feedback mechanisms (red arrows) provide means of controlling the amount of hormones produced (blue arrows) by the hypothalamus and pituitary glands.
The other three hormones produced by the anterior pituitary do not affect other endocrine glands. Prolactin is produced in quantity only after childbirth. It causes the mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism.
Melanocyte-stimulating hormone causes skin-color changes in many fishes, amphibians, and reptiles having melanophores, skin cells that produce color variations. The concentration of this hormone in humans is very low.
Growth hormone (GH), or somatotropic hormone, promotes skeletal and muscular growth. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism as opposed to glucose metabolism. The production of insulin-like growth factor 1 (IGF-1) by the liver is stimulated by growth hormone as well. IGF-1 is often measured as a means of determining GH level. Growth and development are also stimulated by IGF-1, and it may well be the means by which GH influences growth and development.
Effects of Growth Hormone
Growth hormone is produced by the anterior pituitary. The quantity is greatest during childhood and adolescence, when most body growth is occurring. If too little GH is produced during childhood, the individual has pituitary dwarfism, characterized by perfect proportions but small stature. The Bioethics feature “Growth Hormones and Pituitary Dwarfism” in this section discusses how a synthetic growth hormone Page 336can be used to treat some forms of dwarfism. If too much GH is secreted, gigantism may result (Fig. 16.9). Individuals with gigantism often have additional health problems, primarily because GH has a secondary effect on the blood sugar level, promoting an illness called diabetes mellitus (see Section 16.5).
Figure 16.9 Growth hormone influences height. Irregularities in growth hormone can lead to gigantism.
©Xinhua News/Associated Press
BIOLOGY TODAY Bioethics
Growth Hormones and Pituitary Dwarfism
Without treatment, children with a deficiency of growth hormone (GH) experience pituitary dwarfism: slow growth, short stature, and in some cases failure to begin puberty. Prior to the advent of biotechnology in the 1980s, treating these children was incredibly difficult and expensive. The GH needed to treat deficiencies had to be obtained from cadaver pituitaries. Although the treatment was generally very successful, the use of cadaveric GH caused Creutzfeldt–Jakob disease (a neurological disease similar to “mad cow” disease) in a small number of treated individuals.
Thanks to biotechnology, technologists are now able to synthesize human GH (HGH) using bacteria. These bacteria have had the gene for HGH inserted into their genetic information. The altered bacteria are then grown in laboratories and make unlimited amounts of GH. Children with insufficient GH can be treated more safely and inexpensively with this GH. Recombinant HGH can also be used to treat other disorders, such as the chromosomal deficiency known as Turner syndrome (discussed in Section 19.6). It may even be possible to slow or reverse the aging process with HGH treatments.
There is some controversy surrounding treating short children without HGH deficiency for essentially cosmetic reasons. Unfortunately, Americans are obsessed with height. Shorter children are often bullied and teased by their peers. Some data suggest that shorter individuals are discriminated against at their jobs. Their salaries are often lower than those of their taller counterparts with equivalent education and experience. Many people of short stature report having greater self-esteem problems than individuals of average to above-average height. Treatment with HGH could be the solution to these problems.
Although the supply of HGH is seemingly unlimited, the cost of treatments is still quite high (though much cheaper than cadaveric GH), with annual treatments costing up to $25,000. In most cases, insurance companies will not cover these costs. Of greater concern, however, are the potential side effects of supplemental HGH therapy, which are not well understood. Moreover, it is not clear whether HGH treatment will result in a significant increase in the final height of short children.
Questions to Consider
Now that HGH is easier to obtain, what potential abuses would you predict?
Do you think insurance companies should be expected to pay for HGH treatment if a child shows no hormone deficiency and is simply short?
On occasion, GH is overproduced in the adult and a condition called acromegaly results. Long bone growth is no longer possible in adults, so only the feet, hands, and face (particularly the chin, nose, and eyebrow ridges) can respond, and these portions of the body become overly large (Fig. 16.10).Page 337
Figure 16.10 Overproduction of growth hormone in adults leads to acromegaly. Acromegaly is caused by overproduction of GH in the adult. It is characterized by enlargement of the bones in the face, fingers, and toes as a person ages.
(both hands): ©Bart’s Medical Library/Medical Images; (man): ©Yasser Al-Zayyat/AFP/Getty Images
CHECK YOUR PROGRESS 16.2
Explain how the endocrine system and nervous system communicate with one another.
Answer
Through neurotransmitters and hormones—for example, the nervous system sends input to the adrenal medullae, so that a fight-or-flight response can be triggered when needed. Meanwhile, several hormones secreted by the endocrine system regulate the hypothalamus and/or anterior pituitary.
List the hormones produced by the posterior pituitary and provide a function for each.
Answer
Posterior pituitary does not produce any hormones, but it stores and releases ADH and oxytocin produced in the hypothalamus. ADH conserves water, and oxytocin stimulates uterine contractions and milk letdown.
List the hormones produced by the anterior pituitary and provide a function for each.
Answer
TSH stimulates the thyroid to produce T3 and T4; ACTH stimulates the adrenal cortex to produce glucocorticoids; gonadotropic hormones FSH and LH stimulate the gonads to produce gametes and sex hormones; PRL causes breast development and milk production; MSH causes skin color changes; GH promotes skeletal and muscular growth.
CONNECTING THE CONCEPTS
For more information on the hormones presented in this section, refer to the following discussions:
Section 12.2 examines the influence of growth hormone on bone growth.
Section 17.2 describes the role of pituitary hormones in the production of sperm cells in males.
Section 17.4 describes the role of pituitary hormones in the female ovarian cycle.
16.3 Thyroid and Parathyroid Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the hormones produced by the thyroid and parathyroid glands and provide a function for each.
Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis.
Summarize the diseases and conditions associated with the thyroid and parathyroid glands.
The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland
The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four.
Figure 16.20
Gonads found in males Produce androgens (e.g., testosterone) Stimulates growth of the penis and testes Responsible for male sex characteristics such as facial, underarm, and pubic hair Prompts the larynx and vocal cords to enlarge, resulting in a lower voice Promotes muscular strength
Reverse.Prompt
Like testosterone, most hormones act at a distance between body parts. They travel in the bloodstream from the gland that produced them to their target cells. Also considered to be hormones are the secretions produced by neurosecretory cells in the hypothalamus of the brain. They travel in the capillary network that runs between the hypothalamus and the pituitary gland. Some of these secretions stimulate the pituitary to secrete its hormones, and others prevent it from doing so.
Not all hormones act between body parts. As we will see, prostaglandins are a good example of local hormones. After prostaglandins are produced, they are not carried elsewhere in the bloodstream. Instead, they affect neighboring cells, sometimes promoting pain and inflammation. Also, growth factors are local hormones that promote cell division and mitosis.
Reverse.Prompt
- Both the nervous system and the endocrine system make use of negative feedback mechanisms (see Section 4.8).
- If the blood pressure falls, sensory receptors signal a control center in the brain.
- This center sends out nerve signals to the arterial walls, so that they constrict, and blood pressure rises.
- Now the sensory receptors are no longer stimulated, and the feedback mechanism is inactivated.
5.
Comparing Nervous and Endochrine Systems
(see Fig. 16.2).
Reverse.Prompt
Endochrine System Negative Feedback Loop
- blood glucose level rise causes
- the pancreas to release insulin.
- This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body
- When the blood glucose level falls, the pancreas no longer secretes insulin.
Reverse.Prompt
Outer portion of the adrenal glands Produces hormones that provide a long-term response to stress
Adrenal cortex 2 major types of hormones Glucocorticoids regulate carbohydrate, protein, and fat metabolism. suppress the body’s inflammatory response. e.g., cortisol and cortisone
Adrenal cortex Mineralocorticoids regulate salt and water balance. e.g., aldosterone (targets the kidney)
Adrenal cortex Mineralocorticoids regulate salt and water balance. e.g., aldosterone (targets the kidney) 16.4- Adrenal Glands can malfunction
Addison disease – hyposecretion of glucocorticoids by the adrenal cortex, characterized by bronzing of the skin
Cushing syndrome – hypersecretion of glucocorticoids by the adrenal cortex, characterized by weight gain in the trunk of the body but not the arms and legs Adrenal glands can malfunction Figure 16.16 Cushing syndrome. 16.4 Adrenal Glands Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display
.
Pancreas
Fish-shaped organ behind the stomach Composed of 2 tissues Exocrine function: produces and secretes digestive juice
Endocrine function (by islets of Langerhans): produces and secretes hormones 1. Insulin – secreted when blood glucose is high; stimulates the uptake of glucose by cells (muscle and liver) 2. Glucagon – secreted when blood glucose is low; stimulates the breakdown of glycogen in the liver
Figure 16.18
It is the inability to control blood glucose levels. There are two types: type 1 and type 2. 25.8 million people in the US have diabetes. What is diabetes
What is diabetes? General symptoms include frequent urination. unusual hunger and/or thirst. unexplained change in weight. blurred vision. sores that heal slowly or not at all. excessive fatigue. Long-term effects are blindness, loss of limbs, nerve deterioration, kidney and cardiovascular disease.
Reverse.Prompt
Melanocyte-stimulating hormone causes skin-color changes in many fishes, amphibians, and reptiles having melanophores, skin cells that produce color variations. The concentration of this hormone in humans is very low.
Growth hormone (GH), or somatotropic hormone, promotes skeletal and muscular growth. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism as opposed to glucose metabolism. The production of insulin-like growth factor 1 (IGF-1) by the liver is stimulated by growth hormone as well. IGF-1 is often measured as a means of determining GH level. Growth and development are also stimulated by IGF-1, and it may well be the means by which GH influences growth and development.
16.3 Thyroid and Parathyroid Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the hormones produced by the thyroid and parathyroid glands and provide a function for each.
Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis.
Summarize the diseases and conditions associated with the thyroid and parathyroid glands.
The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland
The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four.
Effects of Thyroid Hormones
To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency.
Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue.
(a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source
While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used.
Mechanism of Thyroxine Action
If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance.
In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent.
Calcitonin
Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited.
Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal.
Parathyroid Glands
Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops.
Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death.
Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones.
When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area.
CHECK YOUR PROGRESS 16.3
Explain how the hormones of the thyroid gland influence the metabolic rate.
Answer
T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy.
Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels.
Answer
When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal.
Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body.
Answer
Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones.
Page 340
CONNECTING THE CONCEPTS
For more information on the importance of calcium, refer to the following discussions:
Section 12.5 explains the role of the bones in maintaining calcium homeostasis.
Section 13.2 examines how calcium ions are involved in muscle contraction.
Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.
Reverse.Prompt
Figure 16.9
Thyroid
- A large gland located below the larynx
- Iodine is needed in the diet to allow the thyroid gland to produce thyroid hormone
- Thyroid hormone works like a steroid hormone and can directly enter the target cell even though it is a non-steroidal hormone
- It produces:
- Thyroid hormone (TH): regulates metabolism
- Triiodothyronine (T3 ), thyroxine (T4 )
- Calcitonin: helps lower blood Ca2+ levels by stimulating the deposition of calcium in the bones 4
Reverse.Prompt
What are the triggers for hormone release?
Ca+ high: Calcitonin
Ca+ low: PTSH
Reverse.Prompt
16.12
Adrenal Medulla
Inner portion of the adrenal glands Hypothalamus initiates stimulation of hormone secretion in the adrenal medulla Produces hormones that allow a short-term response to stress (“fight or flight” response) Epinephrine (adrenaline) Norepinephrine (noradrenaline) A
Reverse.Prompt
Stores antidiuretic hormone (ADH) and oxytocin that are produced by the hypothalamus ADH regulates water balance by reabsorbing water into the bloodstream. Oxytocin causes uterine contractions during childbirth and allows milk to be released during nursing. 2
Posterior Pituitary Gland
Reverse.Prompt
What are steroid and peptide/non-steroidal hormones? What are their methods of action?
Reverse.Prompt
Excessive levels of glucocorticoids result in Cushing syndrome (Fig. 16.17). This disorder can be caused by tumors that affect either the pituitary gland, resulting in excess ACTH secretion, or the adrenal cortex itself. The most common cause, however, is the administration of glucocorticoids to treat other conditions (e.g., to suppress chronic inflammation). Regardless of the source, excess glucocorticoids cause muscle protein to be metabolized and subcutaneous fat to be deposited in the midsection. Excess production of adrenal male sex hormones in women may result in masculinization, including an increase in body hair, deepening of the voice, and beard growth. Depending on the cause, treatment of Cushing syndrome may involve a careful reduction in the amount of cortisone being taken, the use of cortisol-inhibiting drugs, or surgery to remove any existing pituitary or adrenal tumor.
a.
b.
Figure 16.17 Cushing syndrome. a. The tumors associated with adrenal hyperplasia often contribute to Cushing syndrome. b. A man showing some of the common characteristics of Cushing syndrome.
(a): ©McGraw-Hill Education; (b): ©Biophoto Associates/Science Source
CHECK YOUR PROGRESS 16.4
List the hormones produced by the adrenal glands, and indicate whether they are produced by the adrenal cortex or the adrenal medulla.
Answer
Adrenal medulla—epinephrine and norepinephrine; adrenal cortex—glucocorticoids, mineralocorticoids, and small amounts of sex steroids.
Summarize the involvement of the adrenal glands during a stress response.
Answer
Short-term stress response—heart rate and blood pressure increase, blood glucose rises, muscles become energized; long-term stress response—increased breakdown of protein and fats instead of glucose, reduction of inflammation, sodium ions and water are reabsorbed by the kidneys, causing blood volume and pressure to increase.
Contrast the roles of glucocorticoids and mineralocorticoids in the body.
Answer
are secreted by the adrenal cortex under the control of ACTH. They regulate carbohydrate, protein, and fat metabolism. Mineralocorticoids regulate electrolyte balances in the body. The secretion of mineralocorticoids is under the regulation of the angiotensin-aldosterone system.
CONNECTING THE CONCEPTS
For more information on the hormones produced by the adrenal glands, refer to the following discussions:
Section 5.3 describes how epinephrine and norepinephrine influence the heart rate.
Section 11.4 examines how aldosterone is involved in maintaining the water-salt balance of the body fluids.
Reverse.Prompt
- Unlike most other endocrine organs, not under pituitary control, but instead responds directly to changes in blood glucose levels.
-
Insulin is secreted by the B cells when the blood glucose level is high, which usually occurs just after eating.
- Insulin stimulates the uptake of glucose by cells, especially liver cells, muscle cells, and adipose tissue cells.
- In liver and muscle cells, glucose is then stored as glycogen.
- In muscle cells, the glucose supplies energy for muscle contraction.
- Glucose enters the metabolic pool in fat cells and thereby supplies glycerol for the formation of fat.
- In these various ways, insulin lowers the blood glucose level (Fig. 16.19, top).
7.
pancreas
B cells and lowering blood glucose level
Figure 16.19 Blood glucose homeostasis. Top: When the blood glucose level is high, the pancreas secretes insulin. Insulin promotes the storage of glucose as glycogen and the synthesis of proteins and fats. Therefore, insulin lowers the blood glucose level. Bottom: When the blood glucose level is low, the pancreas secretes glucagon. Glucagon acts opposite to insulin; therefore, glucagon raises the blood glucose level to normal.
Reverse.Prompt
Responding to Internal Changes
- the nervous and endocrine systems work together to govern internal systems:
The sympathetic system immediately acts to increase heart rate and constrict the blood vessels, so your blood pressure rises. Sweating may have upset the water-salt balance of your blood. If so, the hormone aldosterone from the adrenal cortex will act on the kidney tubules to conserve sodium ions (Na+), and water reabsorption will follow. The hypothalamus can also help by sending antidiuretic hormone (ADH) to the posterior pituitary gland, which releases it into the blood. ADH actively promotes water reabsorption by the kidney tubules.- This usually occurs below the level of consciousness.
- Subconscious control often depends on reflex actions that involve the
- hypothalamus
- medulla oblongata.
- Let’s take blood pressure as an example.
- After a 3-mile run, you decide to sit down under a tree to rest a bit. When you stand up to push off again, you feel faint.
- The feeling quickly passes, because the medulla oblongata responds to input from the baroreceptors in the aortic arch and carotid arteries.
- Recall from Section 14.5 that certain drugs, such as alcohol, can affect ADH secretion. When you consume alcohol, it is quickly absorbed across the stomach lining into the bloodstream, where it travels to the hypothalamus and inhibits ADH secretion.
- When ADH levels fall, the kidney tubules absorb less water.
- The result is increased production of dilute urine.
- Excessive water loss, or dehydration, is a disturbance of homeostasis.
- This is why drinking alcohol when you are exercising or perspiring heavily on a hot day is not a good idea. Instead of keeping you hydrated, an alcoholic beverage, such as beer, has the opposite effect.
Reverse.Prompt
16.2
Anterior Pituitary gland Controlled by hypothalamic-releasing and hypothalamic-inhibiting hormones 3
Reverse.Prompt
The endocrine system functions differently than the nervous system.
The endocrine system is largely composed of glands (see Fig. 16.1). These glands secrete hormones, which are carried by the bloodstream to target cells throughout the body. It takes time to deliver hormones, and it takes time for cells to respond. The effect initiated by the endocrine system is longer lasting. In other words, the endocrine system is organized for a slow but prolonged response.
Both the nervous system and the endocrine system make use of negative feedback mechanisms (see Section 4.8). If the blood pressure falls, sensory receptors signal a control center in the brain. This center sends out nerve signals to the arterial walls, so that they constrict, and blood pressure rises. Now the sensory receptors are no longer stimulated, and the feedback mechanism is inactivated. Similarly, a rise in blood glucose level causes the pancreas to release insulin. This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body (see Fig. 16.2). When the blood glucose level falls, the pancreas no longer secretes insulin.
Reverse.prompt.
Select all of these hormones that are secreted by the anterior pituitary gland.
growth hormone
thyroid-stimulating hormone
adrenocorticotropic hormone
Reverse.Prompt
- In response to a low oxygen blood level,
- the kidneys secrete this
- Erythropoietin stimulates red blood cell formation in the red bone marrow. A greater number of red blood cells results in increased blood oxygen.
- A number of different types of organs and cells also produce peptide growth factors, which stimulate cell division and mitosis.
- Growth factors can be considered hormones because they act on cell types with specific receptors to receive them. Some are released into the blood; others diffuse to nearby cells.
2.
Erythropoietin
Reverse.Prompt
The organs of the endocrine system (Fig. 16.1) are responsible for the production of chemical signals, called hormones, that are involved in the regulation of the other organs in the body.
Figure 16.1 The endocrine system. This diagram indicates the major endocrine glands of the body. Other organs also produce hormones, such as the kidneys, the gastrointestinal tract, and the heart, but this is not the primary function of these organs.
Reverse.Prompt
The Action of Hormones
Hormones have a wide range of effects on cells. Some of these effects induce a target cell to increase its uptake of particular substances (such as glucose) or ions (such as calcium). Other effects bring about an alteration of the target cell’s structure in some way. A few hormones simply influence cell metabolism. Growth hormone is a peptide that influences cell metabolism leading to a change in the structure of bone. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins, and modified amino acids. Growth hormone is a protein produced and secreted by the anterior pituitary. All steroid hormones have a similar structure of four carbon rings because they are all derived from cholesterol (see Fig. 2.20).
Reverse.Prompt
- The male testes and female ovaries, collectively called the gonads, produce hormones and therefore are considered endocrine glands.
- In addition, the thymus and pineal gland, as well as some other tissues in the body, have endocrine functions.
Gonads are considered endochrine glands
Reverse.Prompt
Chapter Review
SUMMARIZE
16.1Endocrine Glands
The endocrine system works with the nervous system to regulate the activities of the other body systems. Endocrine glands secrete hormones into the bloodstream. From there, they are distributed to target organs or tissues. This differs from exocrine glands, which secrete products into ducts.
Hormones, a type of chemical signal, usually act at a distance between body parts. Hormones are either peptides or steroids.
Pheromones are chemical signals that influence the behavior of another individual.
Reception of a peptide hormone at the plasma membrane activates an enzyme cascade inside the cell. Peptide hormones typically use second messenger systems, such as cyclic adenosine monophosphate (cAMP).
Steroid hormones combine with a receptor, and the complex attaches to and activates DNA. Protein synthesis follows.
16.2Hypothalamus and Pituitary Gland
The endocrine system is controlled by the hypothalamus, which regulates the secretions of the pituitary gland. Neurosecretory cells in the hypothalamus produce antidiuretic hormone (ADH) and oxytocin, which are stored in axon endings in the posterior pituitary until released.
The hypothalamus produces hypothalamic-releasing and hypothalamic-inhibiting hormones, which pass to the anterior pituitary by way of a portal system.
The anterior pituitary produces several types of hormones, including thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), gonadotropic hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, melanocyte-stimulating hormone, and growth hormone (GH). Some of these stimulate other hormonal glands to secrete hormones.
Endocrine disorders associated with growth hormones include pituitary dwarfism, gigantism, and acromegaly.
16.3Thyroid and Parathyroid Glands
The thyroid gland requires iodine to produce triiodothyronine (T3) and thyroxine (T4), which increase the metabolic rate.
If iodine is available in limited quantities, endemic goiter develops. Congenital hypothyroidism occurs if the thyroid does not develop correctly.
In adults, hypothyroidism leads to myxedema, while hyperthyroidism results in an exophthalmic goiter.
The thyroid gland produces calcitonin, which helps lower the blood calcium level.
The parathyroid glands secrete parathyroid hormone (PTH), which raises the blood calcium level.
16.4Adrenal Glands
The adrenal glands respond to stress.
Adrenal Medulla
The adrenal medulla immediately secretes epinephrine and norepinephrine. Heartbeat and blood pressure increase, blood glucose level rises, and muscles become energized.
Adrenal Cortex
The adrenal cortex produces the glucocorticoids (cortisol), the mineralocorticoids (aldosterone), and gonadocorticoids (dehydroepiandrosterone [DHEA]), androgens, and estradiol (estrogen). The glucocorticoids regulate carbohydrate, protein, and fat metabolism and suppress the inflammatory response. Mineralocorticoids are influenced by renin from the kidneys and regulate water and salt balance, leading to increases in blood volume and blood pressure.
Problems with the adrenal cortex may result in Addison disease or Cushing syndrome.
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16.5Pancreas
The pancreas contains both endocrine and exocrine cells. The pancreatic islets secrete the hormones insulin and glucagon.
Insulin lowers the blood glucose level.
Glucagon raises the blood glucose level.
Diabetes mellitus is due to the failure of the pancreas to produce insulin or the failure of the cells to take it up.
16.6Other Endocrine Glands
Other endocrine glands also produce hormones:
The testes and ovaries (gonads) produce the sex hormones. Male sex hormones are the androgens (testosterone); female sex hormones are the estrogens and progesterone. Anabolic steroids mimic the action of testosterone.
The thymus secretes thymosins, which stimulate T-lymphocyte production and maturation.
The pineal gland produces melatonin, which may be involved in circadian rhythms and the development of the reproductive organs.
Some organs and tissues also produce hormones:
Kidneys produce erythropoietin (EPO).
Adipose tissue produces leptin, which acts on the hypothalamus.
Prostaglandins are produced within cells and act locally.
16.7Hormones and Homeostasis
The nervous and endocrine systems exert control over the other systems and thereby maintain homeostasis.
The nervous system is able to respond to the external environment after receiving data from the sensory receptors. Sensory receptors are present in such organs as the eyes and ears.
The nervous and endocrine systems work together to govern the subconscious control of internal organs. This control often depends on reflex actions involving the hypothalamus and medulla oblongata.
The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system.
ASSESS
TESTING YOURSELF
Choose the best answer for each question.
16.1Endocrine Glands
Identify each of the endocrine organs in the figure.
Page 353Peptide hormones interact with which structures on the surface of a cell?
receptor proteins
second messenger systems
pheromones
neurotransmitters
digestive enzymes
A steroid hormone requires the use of a second messenger system to enter a cell.
true
false
16.2Hypothalamus and Pituitary Gland
Which of the following acts as the link between the nervous system and the endocrine system?
posterior pituitary gland
anterior pituitary gland
hypothalamus
parathyroid
Growth hormone (GH) is released by which endocrine gland?
posterior pituitary gland
anterior pituitary gland
hypothalamus
adrenal gland
Which of the following hormones is regulated by positive feedback mechanisms?
thyroid-stimulating hormone (TSH)
gonadotropic hormone
oxytocin
growth hormone
None of these are correct.
16.3Thyroid and Parathyroid Glands
Thyroid hormones directly regulate which aspect of human physiology?
circadian rhythm
sex hormone production
metabolic rate
stress response
None of these are correct.
Which of the following hormones increase(s) blood calcium levels?
calcitonin
parathyroid hormone
thyroxine (T4)
mineralocorticoids
16.4Adrenal Glands
Which of the following hormones is (are) not produced by the adrenal cortex?
glucocorticoids
mineralocorticoids
gonadocorticoids
norepinephrine
Which of the following is not correct regarding aldosterone?
It is produced by the adrenal cortex.
It is inhibited by the action of epinephrine.
Its release is regulated by renin from the kidneys.
It causes the kidneys to reabsorb sodium +(Na+) ions.
All of these are correct.
Which of the following hormones help(s) regulate the electrolyte balance of body fluids?
mineralocorticoids
glucocorticoids
androgens
epinephrine
None of these are correct.
16.5Pancreas
Which of the following correctly describes the hormone insulin?
It is produced by B cells in the pancreas.
It increases glucose uptake by liver and muscle cells.
It is a peptide hormone.
It lowers blood glucose levels.
All of these are correct.
The hormone antagonistic to insulin is
epinephrine.
parathyroid hormone.
glucagon.
cortisol.
progesterone.
The disease that is believed to be caused by an autoimmune response that destroys the pancreatic islets is called
Addison disease.
diabetes insipidus.
type 1 diabetes mellitus.
type 2 diabetes mellitus.
gestational diabetes.
16.6Other Endocrine Glands
The hormone produced by the pineal gland to regulate the circadian rhythm is called
estradiol.
renin.
leptin.
melatonin.
None of these are correct.
This hormone is involved with providing a feeling of fullness after a meal and thus has a role in weight regulation.
erythropoietin (EPO)
melatonin
cortisol
prostaglandin
leptinPage 354
16.7Hormones and Homeostasis
The nervous system is primarily responsible for responses to the ______ environment, while the endocrine system responds to the ______ environment.
external; internal
external; external
internal; external
internal; internal
Which of the following would not be a response of the endocrine system?
release of ADH to prevent water loss
use of cortisol to control the stress response
movement of your fingers away from a hot surface
regulation of blood glucose levels
production of sex hormones
ENGAGE
BioNOW
Want to know how this science is relevant to your life? Check out the BioNOW video below:
BioNOW: Quail Hormones
From your understanding of the endocrine system in humans, what endocrine glands and hormones are most likely involved in this response of the quails to the changes in their environment?
THINKING CRITICALLY
Blood tests are a way to diagnose any number of endocrine disorders because hormones are transported by the circulatory system. GH and IGF-1 can be checked to determine if deficiencies are the reason for a child’s slow growth. Blood levels of TSH, T3, and T4 provide information about thyroid function. Some tests, such as the glucose tolerance test from the chapter opener, do not directly measure the level of the glucose-regulating hormones (in this case, insulin). Instead, they indirectly monitor whether an endocrine gland is performing correctly by measuring specific compounds in the blood.
How is follicle-stimulating hormone similar to growth hormone with regard to how their target cells respond to their signals?
It is possible to diagnose hypothyroidism by high levels of TSH in the blood. Explain what would cause a high TSH level. (Hint: You may want to consider what happens to TSH when the activity of the thyroid is normal.)
Why would a diabetic urinate frequently and always be thirsty?
Many diets advertise that they are specifically designed for diabetics. How would these diets be different from a “normal” diet?
Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC
ANSWER KEY
Testing Yourself
Click here for the answers to the Testing Yourself questions.
Answer
Testing Yourself: 1. a. hypothalamus; b. pituitary gland; c. thyroid; d. adrenal gland; e. parathyroid glands; f. thymus; g. pancreas; h. testes; i. ovary; 2. a; 3. b; 4. c; 5. b; 6. c; 7. c; 8. b; 9. d; 10. b; 11. a; 12. e; 13. c; 14. c; 15. d; 16. e; 17. a; 18. c
BioNOW
Click here for the answer to the BioNOW question.
Answer
BioNOW: The quail responded to increased temperature and light levels by beginning breeding behaviors. Melatonin released by the pineal gland in response to light regulates sexual development in the birds. The hypothalamus becomes active in stimulating the release of GH, LH, and FSH, which are involved in sexual development and reproduction. With increased levels of testosterone and estrogens, secondary sex characteristics develop, and sperm and egg production is stimulated.
Thinking Critically
Click here for the answers to the Thinking Critically questions.
Answer
Thinking Critically: 1. Follicle-stimulating hormone and growth hormone are both protein hormones, thus they both bind to a receptor on the plasma membrane and activate a second messenger system (in this case, both use cAMP). 2. When thyroxine is produced, negative feedback occurs to stop TSH, but when T3 and T4 are low, the anterior pituitary produces more TSH than normal. 3. When blood glucose is too high, the excess glucose cannot be reabsorbed from the glomerular filtrate in the kidneys. By osmotic pressure, water follows the glucose into the filtrate and an excessive amount of urine is produced, resulting in dehydration and thirst. 4. The diet would regulate the intake of glucose by favoring foods with a lower glycemic index (see Chapter 9). Because type 2 diabetes is associated with obesity, any diet that reduces overall caloric intake might be helpful.
Reverse.Prompt
SCIENCE IN YOUR LIFE
How is labor induced if a woman’s pregnancy extends past her due date?
After medication to prepare the birth canal for delivery, oxytocin (Pitocin) is used to induce labor. Pitocin is a synthetic version of the oxytocin released by the posterior pituitary. During labor, oxytocin may also be given to increase the strength of contractions. Stronger contractions speed the labor process if necessary (e.g., if the woman’s uterus is contracting poorly or if the health of the mother or child is at risk during delivery). Oxytocin is routinely used following delivery to minimize postpartum bleeding by ensuring that strong uterine contractions continue.
Use of oxytocin must be monitored carefully, because it may cause excessive uterine contractions. Should this occur, the uterus could tear itself. Further, reduced blood supply to the fetus caused by very strong contractions may be fatal to the baby. Though it reduces the duration of labor, inducing labor with oxytocin can be very painful for the mother. Whenever possible, physicians prefer gentler and more natural methods to induce labor and/or strengthen contractions.
Reverse.Prompt
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Distinguish between the mode of action of a neurotransmitter and that of a hormone.
Distinguish between endocrine and exocrine glands.
Identify the organs and glands of the endocrine system.
Compare the actions of peptide and steroid hormones.
Reverse.Prompt
CHECK YOUR PROGRESS 16.7
Summarize the role of the endocrine system in maintaining homeostasis.
Answer
The nervous system can detect inputs from sensory receptors both internally and externally. The endocrine system, which works with the nervous system, is a mechanism for responding to these stimuli to maintain homeostasis.
Explain how the body restores its water-salt balance after it has lost water and salt through sweating.
Answer
Aldosterone from the adrenal cortex acts on kidney tubules to conserve sodium, and water reabsorption will follow. ADH from the anterior pituitary also increases water reabsorption by the kidneys.
Explain why the nervous and endocrine systems are integrated with one another.
Answer
The nervous and endocrine systems are integrated with each other because of the role of the hypothalamus in each system.
CONNECTING THE CONCEPTS
For more information on the organ systems presented in this section, refer to the following discussions:
Section 5.3 examines the factors that regulate heart rate.
Section 11.4 explains the role of aldosterone and ADH on the function of the kidney.
Section 14.2 explores the roles of the hypothalamus and medulla oblongata in the CNS.
CONCLUSION
For diabetics, the prospects of controlling their blood glucose levels and living a healthy life are better today than in the past. Prior to the development of recombinant DNA technology, which now allows human insulin to be produced in large quantities, insulin was derived from the pancreases of pigs or cows. This required laborious purification, and because the animal insulins were not identical to the human form, sometimes immunological reactions occurred. Increasingly, insulin pumps are being used to treat diabetes. An insulin pump is a device a little bigger than a cell phone, which can deliver precise amounts of insulin under the skin using a small plastic catheter. The insulin pump more accurately mimics the pancreas’s natural release of the correct amount of insulin needed by the body. Studies have shown that insulin pumps are more effective than traditional injections of insulin in controlling blood sugar levels. In the near future, it may be possible to implant a device—sometimes called an “artificial pancreas”—into patients with diabetes. Such a device would not only monitor the blood sugar level but also provide appropriate doses of insulin.
Reverse.Prompt
16.2 Hypothalamus and Pituitary Gland
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain the role of the hypothalamus in the endocrine system.
List the hormones produced by the anterior and posterior pituitary glands and provide a function for each.
Summarize the conditions produced by excessive and inadequate levels of the major hormones.
16.3 Thyroid and Parathyroid Glands
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the hormones produced by the thyroid and parathyroid glands and provide a function for each.
Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis.
Summarize the diseases and conditions associated with the thyroid and parathyroid glands.
The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland
The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four.
Effects of Thyroid Hormones
To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency.
Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue.
(a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source
While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used.
Mechanism of Thyroxine Action
If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance.
In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent.
Calcitonin
Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited.
Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal.
Parathyroid Glands
Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops.
Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death.
Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones.
When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area.
CHECK YOUR PROGRESS 16.3
Explain how the hormones of the thyroid gland influence the metabolic rate.
Answer
T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy.
Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels.
Answer
When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal.
Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body.
Answer
Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones.
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CONNECTING THE CONCEPTS
For more information on the importance of calcium, refer to the following discussions:
Section 12.5 explains the role of the bones in maintaining calcium homeostasis.
Section 13.2 examines how calcium ions are involved in muscle contraction.
Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.
Reverse.Prompt
Supplemental and not in book
- Stores antidiuretic hormone (ADH) and oxytocin that are produced by the hypothalamus
- ADH regulates water balance by reabsorbing water into the bloodstream.
- Oxytocin causes uterine contractions during childbirth and allows milk to be released during nursing
