Biology: Endocrinology

Question 1

Endocrine System

Answer 1

Acts as a means of internal communication, coordinating the activities of the organ systems.

Question 2

Endocrine vs. Exocrine Glands

Answer 2

Endocrine glands synthesize and secrete chemical substances called hormones directly into the circulatory system. (In contrast, exocrine glands, such as the gall bladder, secrete substances that are transported by ducts.)

Question 3

Glands and Organs that Synthesize or Secrete Hormones

Answer 3

Glands or organs that synthesize or secrete hormones include the pituitary, hypothalmus, thyroid, parathyroids, adrenals, pancreas, testes, ovaries, pineal, kidneys, gastrointestinal glands, heart, and thymus.

Question 4

Hormone Spectrum

Answer 4

Some hormones regulate a single type of cell or organ, whereas others have more widespread actions.

Question 5

Specificity of hormonal action

Answer 5

The specificity of hormonal action is usually determined by the presence of specific receptors on or in the target cells.

Question 6

Adrenal Glands

Answer 6

Situated on top of the kidneys and consist of the adrenal cortex and the adrenal medulla.

Question 7

ACTH

Answer 7

In response to stress, the adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary, stimulates the adrenal cortex to produce more than two dozen different steroid hormones, collectively known as adrenocortical steroids, or simply corticosteroids.

Corticosteroids exert their mechanism of action by determining which genes are transcribed in the nuclei of their target cells and at what particular rate.

The subsequent changes in the nature and given concentration of the enzymes will affect cellular metabolism.

Question 8

Glucocorticoids

Answer 8

Such as cortisol and cortisone.

They’re involved in glucose regulation and protein metabolism.

Glucocorticoids raise blood glucose levels by promoting protein breakdown and gluconeogenesis and decreasing protein synthesis.

Glucocorticoids increase the plasma glucose levels and are antagonistic to the effects of insulin.

Glucocorticoids release amino acids from skeletal muscle as well as lipids from adipose tissue. They also promote peripheral use of lipids and have anti-inflammatory effects.

Question 9

Mineralocorticoids

Answer 9

Particularly aldosterone.

They regulate plasma levels of sodium and potassium and, consequently, the total extracellular water volume.

Aldosterone causes active reabsorption of sodium and passive reabsorption of water in the nephron of the kidney. This results in an increase in both blood volume and blood pressure.

Excess production of aldosterone results in excess rendition of water with resulting hypertension (high blood pressure).

The mineralocorticoids are stimulated by angiotensin II and inhibited by ANP (atrial natriuretic peptide).

Question 10

Cortical Sex Hormones

Answer 10

The adrenal cortex secretes small quantities of androgens (male sex hormones) like androstenedione and dehydroepiandrosterone in both men and women.

Because in men most of the androgens are produced by the testes, the physiologic effect of the adrenal androgens is quite small.

In women, however, overproduction of the adrenal androgens may have masculinizing effects, such as excessive facial hair.

Question 11

Adrenal Medulla

Answer 11

Produces epinephrine (adrenaline) and norepinephrine (noradrenaline), both or which belong to a class of amino acid-derived compounds called catecholamines.

Question 12

Epinephrine

Answer 12

Epinephrine increases the conversion of glycogen to glucose in liver and muscle tissue, causing an increase in blood glucose levels and an increase in the basal metabolic rate.

Question 13

Epinephrine and Norepinephrine

Answer 13

Both epinephrine and norepinephrine increase the rate and strength of the heartbeat and dilate and constrict blood vessels in such a way as to increase the blood supply to skeletal muscles, the heart, and the brain, while decreasing the blood supply to the kidneys, skin, and digestive tract.

Both epinephrine and norepinephrine will also promote the release of lipids by adipose tissue.

These effects are known as the “fight or flight response” and are elicited by sympathetic nervous stimulation in response to stress.

Epinephrine will inhibit certain vegetative functions, such as digestion, which are not immediately important for survival.

Both of these hormones are also neurotransmitters, proteins used by neurons to transmit signals.

The release of these hormones is stimulated during sympathetic activation by sympathetic preganglionic fibers.

Question 14

Pituitary Gland

Answer 14

The pituitary (hypophysis) is a small, trilobed gland at the base of the brain.

The two main lobes, anterior and posterior, are functionally distinct.

(In humans, the third lobe, the intermediate lobe, is rudimentary.)

Specifically, the pituitary gland hangs below the hypothalamus and is connected by a slender cord known as the infundibulum.

Question 15

Anterior Pituitary

Answer 15

Synthesizes both direct hormones, which directly stimulate their target organs, and tropic hormones, which stimulate other endocrine glands to release hormones.

The hormonal secretions of the anterior pituitary are regulated by hypothalamic secretions called releasing/inhibiting hormones or factors.

Question 16

Direct Hormones

Answer 16

1) Growth Hormone (GH, somatotropin)

2) Prolactin

Question 17

Growth Hormone

Answer 17

Growth Hormone (GH, somatotropin) - GH promotes bone and muscle growth. GH also promotes protein synthesis and lipid mobilization and catabolism.

• In children, a GH deficiency can lead to stunted growth (dwarfism), while overproduction of GH results in gigantism.
• Overproduction of GH in adults cause acromegaly, a disorder characterized by a disproportionate overgrowth of bone, localized especially in the skull, jaw, feet, and hands.

Question 18

Prolactin

Answer 18

Prolactin stimulates milk production and secretion in female mammary glands.

Question 19

Tropic Hormones

Answer 19

1) Adrenocorticotropic Hormone (ACTH) - stimulates the adrenal cortex to synthesize and secrete glucocorticoids and is regulated by the releasing hormone corticotrophin-releasing factor (CRF).
2) Thyroid-stimulating Hormone (TSH)
3) Luteinizing Hormone (LH)
4) Follicle-stimulating Hormone (FSH)
5) Melanocyte-stimulating Hormone (MSH)
6) Endorphins

Question 20

Thyroid-stimulating Hormone (TSH)

Answer 20

stimulates the thyroid gland to synthesize and release thyroid hormones, including thyroxin.

Question 21

Luteinizing Hormone (LH)

Answer 21

In women, LH stimulates ovulation and formation of the corpus luteum. LH is also responsible for regulating progesterone secretion in women. In men, LH stimulates the interstitial cells of the testes to synthesis testosterone.

Question 22

Follicle-stimulating Hormone (FSH)

Answer 22

In women, FSH causes maturation of ovarian follicles that begin secreting estrogen; in men, FSH stimulates maturation of the seminiferous tubules and sperm production.

Question 23

Melanocyte-stimulating Hormone (MSH)

Answer 23

secreted by the intermediate love of the pituitary. In mammals, the function of MSH is unclear, but in frogs, MSH causes darkening of the skin via induced dispersion of molecules of pignment in melanophore cells.

Question 24

Endorphins

Answer 24

neurostransmitters that have pain-relieving properties.

Question 25

Posterior Pituitary

Answer 25

The posterior pituitary (neyrohypophysis) does not synthesize hormones; it stores and rleases peptide hormones oxyocin and antiduretic hormone, which are produced by the neurosecretory cells of the hypothalamus.

Hormone secretion is stimulated by action potentials descending from the hypothalamus.

Question 26

Oxytocin

Answer 26

Secreted during childbirth.

Increases the strength and frequency of uterine muscle contractions.

Oxytocin secretion is also induced by suckling; oxytocin stimulates milk secretion in the mammary glands.

Question 27

Antidiuretic Hormone (ADH; vasopressin)

Answer 27

ADH increases the permeability of the nephron’s collecting duct to water, thereby promoting water reabsorption and increasing blood volume, which subsequently increases blood pressure.

ADH is secreted when plasma osmolarity increases, as sensed by osmoreceptors in the hypotalamus, or when blood volume decreases, as sensed by baroreceptors in the circulatory system.

Question 28

Hypothalamus

Answer 28

Part of the forebrain and is located directly above the pituitary gland.

The hypotalamus receives neural transmissions from other parts of the brain and from peripheral nerves that trigger specific responses from its neurosecretory cells.

The neurosecretory cells regulate pituitary gland secretions via negative feedback mechanisms and through the actions inhibiting and releasing hormones.

Question 29

Hypothalamus and the Anterior Pituitary

Answer 29

Hypothalamic-releasing hormones stimulate or inhibit the secretions of the anterior pituitary.

For example, GnRH stimulates the anterior pituitary to secrete FSH and LH. Releasing hormones are secreted into the hypothalamic-hypophyseal portal system. In this circulatory pathway, blood from the capillary bed in the hypothalamus flows through a portal vein into the anterior pituitary, where it diverges into a second capillary network. In this way, releasing hormones can immediately reach the anterior pituitary.

A complicated feedback system regulates the secretions of the endocrine system.

For example, when the plasma levels of adrenal cortical hormones drop, hypothalamic cells (via a negative feedback mechanism) release ACTH-releasing factor (ACTH-RF) into the portal system. When the plasma concentration of corticosteroids exceeds the normal plasma level, the steroids themselves exert an inhibitory effect on the hypothalamus.

Question 30

Hypothalamus and the Posterior Pituitary

Answer 30

Neurosecretory cells in the hypothalamus synthesize both oxytocin and ADH and transport them via their axons into the posterior pituitary for storage and secretion.

Question 31

Thyroid Hormones

Answer 31

Thyroid hormones affect the function of nearly every organ system in the body.

In children, thyroid hormones are essential for growth and development; in adults, thyroid hormones are essential for maintenance of metabolic stability.

Thyroid hormones are necessary for growth and neurological development in children. They increase the rate of metabolism throughout the body.

Question 32

Thyroxine (T4) & Triiodothyronine (T3)

Answer 32

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are formed on the glycoprotein thyroglobulin, which is synthesized in the thyroid cell.

Question 33

Hypothyroidism

Answer 33

Thyroid hormones are undersecreted or not at all.

Common symptons of hypothyroidism include a slowed heart rate and respiratory rate, fatigue, cold intolerance, and weight gain.

Hypothyroidism in newborn infants, called cretinism, is characterized by mental retardation and short stature.

The thyroid often enlarges, forming a bulge in the beck called a goiter.

Question 34

Where T3 & T4 Are Derived From

Answer 34

Thyroxine (T4) and triiodothyronine (T3) are derived from the iodination of the amino acid tyrosine.

Question 35

T3 vs. T4

Answer 35

Because of the specific tertiary structure of this glycoprotein, iodinated tyrosine residues present in thyroglobulin are able to bind together to form active thyroid hormones. With respect to thyroid hormone characteristics, the following principles apple:

• T3 is five times more potent than T4.
• T4 and T3 are transported in the blood by the proteins TBG, TBPA, and albumin. Approximately 99.5% of these hormones are bound to these proteins. Note: Only an unbound hormone is able to enter a cell and elicit a cellular response.
• All of the T4 in the body is formed and secreted by the thyroid gland; however, only 20% of T3 is produced by the thyroid gland.
• The majority of T3 is produced by the conversion of T4 to T3 by the enzyme 5’-monodeiodase, found primarily int he peripheral tissues.

Question 36

Hyperthyroidism

Answer 36

The thyroid is overstimulated, resulting in the oversecretion of thyroid hormones.

Symptoms often include increased metabolic rate, feelings of excessive warmth, profuse sweating, palpitations, weight loss, and protruding eyes.

The thyroid often enlarges, forming a bulge in the beck called a goiter.

Question 37

Calcitonin

Answer 37

Decreases plasma calcium concentration by inhibiting the release of Ca2+ from bone.

Calcitonin secretion is regulated by plasma Ca2+ levels.

Calcitonin is antagonistic to parathyroid hormone.

Question 38

Pancreas

Answer 38

Both an exocrine and an endocrine organ.

The exocrine function is performed by the cells that secrete digestive enzymes into the small intestine via a series of ducts.

The endocrine function is performed by small glandular structures called the islets of Langerhans, which are composed of alpha and beta cells.

Alpha cells produce and secrete glucagon; beta cells produce and secrete insulin.

Question 39

Glucagon

Answer 39

Stimulates protein and fat degradation, the conversion of glycogen to glucose, and gluconeogensis, all of which serve to increase the blood glucose levels.

Glucagon’s actions are largely antagonistic to those of insulin.

Question 40

Insulin

Answer 40

A protein hormone secreted in response to a high blood glucose concentration.

It stimulates the uptake of glucose by muscle and adipose cells and the storage of glucose as glycogen in muscle and liver cells, thus lowering blood glucose levels.

It also stimulates the synthesis of fats from glucose and the uptake of amino acids.

Insulin’s actions are antagonistic to those of glucagon and glucocorticoids.

Question 41

Diabeties

Answer 41

Underproduction of insulin, or an insensitivity to insulin, leads to diabetes mellitus, which is characterized by hyperglycemia (high blood glucose levels).

Diabetes is the most common endocrine disorder and is characterized by long-term complications involving the eyes, nerves, kidneys, and blood vessels.

Question 42

Parathyroid Glands

Answer 42

Four small, pea-shaped structures embedded in the posterior surface of the thyroid.

These glands synthesize and secrete parathyroid hormone (PTH), which regulates plasma calcium concentration.

PTH raises the calcium concentration in the blood by stimulating calcium release from the bone and decreasing calcium excretion in the kidneys.

Calcium in bone is bonded to phosphate, and breakdown of the bone releases phosphate as well as calcium.

Parathyroid hormone compensates for this by stimulating excretion of phosphate by the kidneys.

Question 43

Renin

Answer 43

When blood volume falls, the kidneys produce renin - an enzyme that converts the plasma protein angiotensinogen to angiotensin I.

Angiotensin I is converted to angiotensin II, which stimulates the adrenal cortex to secrete aldosterone.

Aldosterone helps to restore blood volume by increasing sodium reabsorption at the kidney, leading to an increase in water. This removes the initial stimulus for renin production.

Question 44

Erythropoietin (ERP)

Answer 44

The kidneys also produce erythropoietin (ERP). EPO is a glycoprotein that stimulates red blood cell production; it is normally produced in the kidneys. This hormone causes the following:

• Stimulation of the stem cells to differentiate into rubriblasts (least mature erythrocyte)
• Increased rate of mitosis
• Increased release of reticulocytes from the bone marrow.
• Increased hemoglobin (HgB) formation, which allows the critical HgB concentration necessary for maturity to be reached at a more rapid rate.

Question 45

Gastrin

Answer 45

Ingested food stimulates to release the hormone gastrin.

Gastrin is carried to the gastric glands and stimulates the glands to secrete GCl in response to food in the stomach.

Question 46

Secretin

Answer 46

Secretion of pancreatic juice, or the exocrine secretion of the pancreas, is also under hormonal control; the hormone secretin is released by the small intestine when acidic food material enters from the stomach.

Secretin stimulates the secretion of an alkaline bicarbonate solution from the pancreas that neutralizes the acidity of the chyme (partially digested food coming from the stomach).

Question 47

Cholecystokinin

Answer 47

The hormone cholecystokinin is released from the small intestine in response to the presence of fats and causes the contraction of the gallbladder and release of bile into the small intestine.

Question 48

Bile

Answer 48

Bile is involved in emulsification and digestion of fats.

Question 49

Pineal Gland

Answer 49

A tiny structure at the base of the brain that secretes the hormone melatonin.

Question 50

Melatonin

Answer 50

The role of melatonin in humans is unclear, but it is believed to play a role in the regulation of circadian rhythms-physiological cycles lasting 24 hours.

Melatonin secretion is regulated by light and dark cycles in the environment.

In primitive vertebrates, melatonin lightens the skin by concentrating pigment granules in melanophores. (Melatonin is an antagonist to MSH.)

Question 51

Peptide Hormones

Answer 51

Range from simple short peptides (amino acid chains), such as ADH, to complex polypeptides, such as insulin.

Peptide hormones act as first messengers. Their binding to specific receptors on the surface of their target cells triggers a series of enzymatic reactions within each cell, the first of which may be the conversion of ATP to cyclic adenosine monophosphate (cAMP);

this reaction is catalyzed by the membrane-bound enzyme adenylate cyclase. Cyclic AMP acts as a second messenger, relating messages from the extracellular peptide hormone to cytoplasmic enzymes and initiating a series of successive reactions in the cell.

This is an example of a cascade effect; which each step, the hormone’s effects are amplified. Cyclic AMP activity is inactivated by the cytoplasmic enzyme phosphodiesterase.

Question 52

Steroid Hormones

Answer 52

Examples are estrogen and aldosterone. Belong to a class of lipid-derived molecules with a characteristic ring structure.

They are produced by the testes, ovaries, placenta, and adrenal cortex.

Because they are lipid soluble, steroid hormones cross the phospholipid bilayer and enter their target cells directly in order to bind to specific receptor proteins in the cytoplasm.

This receptor-hormone complex enters the nucleus and directly activates the expression of specific genes by binding to receptors on the chromatin. This induces a change in mRNA transcription and protein synthesis.