Chapter 9: The endocrine system

Question 1

The endocrine system

Answer 1

Consists of glands widely separated from each other with no physical connections. Endocrine glands are groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones (chemical messengers) from the secretory cells into the bloodstream.

Question 2

Pituitary gland

Answer 2

The pituitary gland is a small, bean-shaped gland situated at the base of your brain, somewhat behind your nose and between your ears. Despite its small size, the gland influences nearly every part of your body. The hormones it produces help regulate important functions, such as growth, blood pressure and reproduction.

Question 3

Hypothalamus

Answer 3

A region of the forebrain below the thalamus which coordinates both the autonomic nervous system and the activity of the pituitary, controlling body temperature, thirst, hunger, and other homeostatic systems, and involved in sleep and emotional activity.
The hypothalamus controls release of hormones from both the anterior and posterior pituitary but in different ways.

Question 4

Blood supply

Answer 4

This is from branches of the internal carotid artery. The anterior lobe is supplied indirectly by blood that has already passed through a capillary bed in the hypothalamus, but the posterior lobe is supplied directly.

Question 5

venous drainage

Answer 5

Containing hormones from both lobes, venous blood leaves the gland in short veins that enter the venous sinuses between the layers of dura mater.

Question 6

anterior pituitary

Answer 6

A major organ of the endocrine system, the anterior pituitary, also called the adenohypophysis, is the glandular, anterior lobe of the pituitary gland. The anterior pituitary regulates several physiological processes, including stress, growth, reproduction, and lactation.

Question 7

Growth hormone (GH)

Answer 7

Growth hormone (GH) is a small protein that is made by the pituitary gland and secreted into the bloodstream. GH production is controlled by a complex set of hormones produced in the hypothalamus of the brain and in the intestinal tract and pancreas.
GH acts on many tissues throughout the body. In children and adolescents, it stimulates the growth of bone and cartilage. In people of all ages, GH boosts protein production, promotes the utilization of fat, interferes with the action of insulin, and raises blood sugar levels. GH also raises levels of insulin-like growth factor-1 (IGF-1).

Question 8

Thyroid-stimulating hormone (TSH)

Answer 8

Thyroid-stimulating hormone is produced and released into the bloodstream by the pituitary gland. It controls the production of the thyroid hormones, thyroxine and triiodothyronine, by the thyroid gland by binding to receptors located on cells in the thyroid gland. Thyroxine and triiodothyronine are essential to maintaining the body’s metabolic rate, heart, and digestive functions, muscle control, brain development, and maintenance of bones.

Question 9

Adrenocorticotrophic hormone (ACTH)

Answer 9

Adrenocorticotropic hormone is made in the corticotrope cells of the anterior pituitary gland. It is secreted in several intermittent pulses during the day into the bloodstream and transported around the body. Like cortisol, levels of adrenocorticotropic hormone are generally high in the morning when we wake up and fall throughout the day and lowest during sleep.
This is called a diurnal (circadian) rhythm. Once adrenocorticotropic hormone reaches the adrenal glands, it binds on to receptors causing the adrenal glands to secrete more cortisol, resulting in higher levels of cortisol in the blood. It also increases production of the chemical compounds that trigger an increase in other hormones such as adrenaline and noradrenaline.

Question 10

Prolactin

Answer 10

Prolactin is a hormone produced by your pituitary gland which sits at the bottom of the brain. Prolactin causes breasts to grow and develop and causes milk to be made after a baby is born. Normally, both men and women have small amounts of prolactin in their blood.

Question 11

Gonadotrophins

Answer 11

Gonadotropins are hormones synthesized and released by the anterior pituitary, which act on the gonads (testes and ovaries) to increase the production of sex hormones and stimulate production of either sperm or ova. Follicle stimulating hormone (FSH) and luteinizing hormones (LH) are the main gonadotropins.

Question 12

In both sexes

Answer 12

• FSH stimulates production of gametes (ova or spermatozoa) by the gonads.
• In females: LH and FSH are involved in secretion of the hormone’s estrogen and progesterone during the menstrual cycle. As the levels of estrogen and progesterone rise, secretion of LH and FSH is suppressed.
• In males: LH, also called interstitial cell stimulating hormone (ICSH) stimulates the interstitial cells of the testes to secrete the hormone testosterone

Question 13

Posterior pituitary

Answer 13

The back portion of the pituitary, a small gland in the head called the master gland. The posterior pituitary secretes the hormone oxytocin which increases uterine contractions and antidiuretic hormone (ADH) which increases reabsorption of water by the tubules of the kidney.

Question 14

Oxytocin

Answer 14

• Oxytocin is a hormone and a neurotransmitter that is involved in childbirth and breastfeeding. It is also associated with empathy, trust, sexual activity, and relationship-building.
• It is sometimes referred to as the “love hormone,” because levels of oxytocin increase during hugging and orgasm. It may also have benefits as a treatment for several conditions, including depression, anxiety, and intestinal problems.
• Oxytocin is produced in the hypothalamus, a part of the brain. Females usually have higher levels than males.

Question 15

Antidiuretic hormone

Answer 15

The anti-diuretic hormone helps to control blood pressure by acting on the kidneys and the blood vessels. Its most important role is to conserve the fluid volume of your body by reducing the amount of water passed out in the urine. It does this by allowing water in the urine to be taken back into the body in a specific area of the kidney.
Thus, more water returns to the bloodstream, urine concentration rises, and water loss is reduced. Higher concentrations of the anti-diuretic hormone cause blood vessels to constrict (become narrower) and this increases blood pressure.

Question 16

Thyroid gland

Answer 16

The thyroid gland is a butterfly-shaped organ located at the base of your neck. It releases hormones that control metabolism—the way your body uses energy.

Question 17

Thyroxine and triodothyronine

Answer 17

Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. It plays vital roles in digestion, heart and muscle function, brain development, and maintenance of bones.
Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate.

Question 18

Calcitonin

Answer 18

•This hormone is secreted by the parafollicular or C-cells in the thyroid gland. Calcitonin lowers raised blood calcium (Ca2+) levels. It does this by acting on:
-bone cells promoting their storage of calcium
-kidney tubules inhibiting the reabsorption of calcium
•Its effect is opposite to that of parathyroid hormone, the hormone secreted by the parathyroid glands. The release of calcitonin is stimulated by increased blood calcium levels.
•This hormone is important during childhood when bones undergo considerable changes in size and shape.

Question 19

Parathyroid glands

Answer 19

There are four small parathyroid glands, each weighing around 50 g, two embedded in the posterior surface of each lobe of the thyroid gland. They are surrounded by fine connective tissue capsules that contain spherical cells arranged in columns with sinusoids containing blood in between them.

Question 20

Functions of parathyroid glands

Answer 20

•These glands secrete parathyroid hormone (PTH, parathormone). Secretion is regulated by blood calcium levels. When they fall, secretion of PTH is increased and vice versa.
•Parathormone and calcitonin from the thyroid gland act in a complementary manner to maintain blood calcium levels within the normal range. This is needed for:
-muscle contraction
-transmission of nerve impulses
-blood clotting
-normal action of many enzymes.

Question 21

Adrenal glands

Answer 21

• The arterial blood supply is by branches from the abdominal aorta and renal arteries.
• The venous return is by suprarenal veins. The right gland drains into the inferior vena cava and the left into the left renal vein.
• The glands are composed of two parts which have different structures and functions. The outer part is the cortex and the inner part the medulla. The adrenal cortex is essential to life, but the medulla is not.

Question 22

Adrenal cortex

Answer 22

•The adrenal cortex produces three groups of steroid hormones from cholesterol. They are collectively called adrenocorticoids (corticosteroids). The groups are:
-glucocorticoids
-mineralocorticoids
-sex hormones (androgens).
•The hormones in each group have different characteristic actions but as they are structurally similar their actions may overlap.

Question 23

Glucocorticoids

Answer 23

Any group of corticosteroids (e.g., hydrocortisone) are involved in the metabolism of carbohydrates, proteins, and fats and have anti-inflammatory activity.

Question 24

Mineralocorticoids

Answer 24

A corticosteroid, such as an aldosterone, is involved in maintaining the salt balance in the body.

Question 25

Renin-angiotensin-aldosterone system

Answer 25

When renal blood flow is reduced or blood sodium levels fall, the enzyme renin is secreted by kidney cells. Renin converts the plasma protein angiotensinogen, produced by the liver, to angiotensin 1. Angiotensin-converting enzyme (ACE), formed in small quantities in the lungs, proximal kidney tubules, and other tissues, converts angiotensin 1 to angiotensin 2, which stimulates the secretion of aldosterone. Angiotensin 2 causes vasoconstriction and increases blood pressure closing the negative feedback loop

Question 26

Sex hormones

Answer 26

Sex hormones secreted by the adrenal cortex are mainly androgens (male sex hormones) although the amounts produced are insignificant compared with those secreted by the testes and ovaries in late puberty and adulthood.

Question 27

Adrenal medulla

Answer 27

The medulla is surrounded by the adrenal cortex. It develops from nervous tissue in the embryo and is part of the sympathetic nervous system. When stimulated by extensive sympathetic nerve supply, the glands release the hormones adrenaline (epinephrine, 80%) and noradrenaline (norepinephrine, 20%).

Question 28

Response to stress

Answer 28

• When the body is under stress homeostasis is disturbed. To restore it and, in some cases, to maintain life there are immediate and, if necessary, longer-term responses. Stressors include exercise, fasting, fright, temperature changes, infection, disease, and emotional situations.
• The immediate response is sometimes described as preparing for ‘fight or flight. This is mediated by the sympathetic nervous system.
• In the longer term, ACTH from the anterior pituitary stimulates the release of glucocorticoids and mineralocor­ticoids from the adrenal cortex providing a more prolonged response to stress

Question 29

Adrenaline and noradrenaline

Answer 29

A hormone secreted by the adrenal glands, especially in conditions of stress, increasing rates of blood circulation, breathing, and carbohydrate metabolism and preparing muscles for exertion. Also called epinephrine.
Norepinephrine (also called noradrenaline) is a neurotransmitter in both the peripheral and central nervous systems. Norepinephrine produces many effects in the body, the most notable being those associated with the ‘fight or flight response to perceived danger.

Question 30

Pancreatic islets

Answer 30

The pancreatic islets are small islands of cells that produce hormones that regulate blood glucose levels. Hormones produced in the pancreatic islets are secreted directly into the blood flow by five different types of cells. The endocrine cell subsets are: Alpha cells that produce glucagon and make up 15–20% of total islet cells.

Question 31

Insulin

Answer 31

Insulin is a hormone that is produced naturally in our bodies. Its main role is to allow cells throughout the body to uptake glucose (sugar) and convert it into a form that can be used by these cells for energy.

Question 32

Glucagon

Answer 32

•Glucagon increases blood glucose levels by stimulating:
-conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis)
-gluconeogenesis.
•Secretion of glucagon is stimulated by low blood glucose levels and exercise and decreased by somatostatin and insulin.

Question 33

Somatostatin

Answer 33

This hormone, also produced by the hypothalamus, inhibits the secretion of both insulin and glucagon in addition to inhibiting the secretion of GH from the anterior pituitary.

Question 34

Pineal gland

Answer 34

The pineal gland is a small body attached to the roof of the third ventricle and is connected to it by a short stalk containing nerves, many of which terminate in the hypothalamus. The pineal gland is about 10 mm long, reddish-brown in color, and surrounded by a capsule. The gland tends to atrophy after puberty and may become calcified in later life.

Question 35

Melatonin

Answer 35

This is the main hormone secreted by the pineal gland. Secretion is controlled by daylight and darkness; levels fluctuate during each 24-hour period, the being highest at night and the lowest around midday. Secretion is also influenced by the number of daylight hours, i.e., there may be seasonal variations. Although its functions are not fully understood, melatonin is believed to be associated with:

• coordination of the circadian and diurnal rhythms of many tissues, possibly by influencing the hypothalamus
• inhibition of growth and development of the sex organs before puberty, possibly by preventing synthesis or release.

Question 36

Histamine

Answer 36

Histamine is a chemical created in the body that is released by white blood cells into the bloodstream when the immune system is defending against a potential allergen. This release can result in an allergic reaction from allergy triggers such as pollen, mold, and certain foods.

Question 37

Serotonin

Answer 37

Serotonin is the key hormone that stabilizes our mood, feelings of well-being, and happiness. This hormone impacts your entire body. It enables brain cells and other nervous system cells to communicate with each other. Serotonin also helps with sleeping, eating, and digestion.

Question 38

Prostaglandins

Answer 38

The prostaglandins are a group of lipids made at sites of tissue damage or infection that are involved in dealing with injury and illness. They control processes such as inflammation, blood flow, the formation of blood clots, and the induction of labor.