Adrenal Gland (Physiology)

Question 1

List the main hormones produced by the adrenal glands and their functions

Answer 1

Adrenal medulla

• Upon stimulation by the nerve fibres - usually in a situation of fear, pain or other stress - the chromaffin cells release a mixture of catecholamines called epinephrine, norepinephrine, and dopamine.
• These catecholamines act as hormones to increase alertness and prepare the body in several ways for physical activity.
• They mobilise high-energy fuels such as lactate, fatty acids, and glucose.
• Epinephrine has a glucose-sparing effect as it inhibits the secretion of insulin so the muscles and other insulin-dependent organs absorb less glucose. They use alternative fuels such as fatty acids, while the blood glucose is left for use by the brain, which is more glucose-dependent but not insulin-dependent.

Adrenal cortex

• Produces steroid hormones collectively known as corticosteroids or corticoids. The five most important fall into three categories: mineralocorticoids, glucocorticoids, and sex steroids.
• Aldosterone is the most significant mineralocorticoid and is only produced by the zona glomerulosa. It stimulates the kidneys to retain sodium. Water is retained with it by osmosis, so aldosterone helps to maintain blood volume and pressure.
• Cortisol is the most potent glucocorticoid. Glucocorticoids are secreted by the zona fasciculata and zona reticularis in respinse to ACTH from the pituitary. They stimulate fat and protein catabolism, gluconeogenesis, and the release of fatty acids and glucose into the blood. This helps the body adapt to stress and repair damaged tissues. They also have an anti-inflammatory effect.
• Androgens are the primary adrenal sex steroids. The major androgen is dehydroepiandrosterone (DHEA). Many tissues convert it to the more potent forms, testosterone and dihydrotestosterone. DHEA plays an important role in the prenatal development of the male reproductive tract. At puberty in both sexes, androgens induce growth of pubic and axillary hair and their associated apocrine scent glands.
• Estradiol is the main adrenal oestrogen. After menopause, the ovaries no longer function and only the adrenals secrete oestrogen. Both androgens and oestrogen promote adolescent skeletal growth and help to sustain adult bone mass.

Question 2

Describe the hypothalamic-pituitary-adrenal axis and its role in control of glucocorticoid secretion

Answer 2

• Physical, emotional, or chemical stress stimulates the hypothalamus to release corticotropin-releasing hormone.
• The corticotropin-releasing hormone travels down the hypophyseal portal system into the anterior pituitary.
• The corticotropin-releasing hormone binds to receptors in the anterior pituitary which responds by releasing adrenocorticotrophic hormone.
• The adrenocorticotrophic hormone travels to the adrenal gland and binds to receptors on the adrenal cortex. This stimulates the adrenal cortex to release glucocorticoids (especially cortisol).
• Once a certain blood concentration of cortisol is reached, the cortisol exerts negative feedback to the hypothalamic release of corticotrophin-releasing hormone and the pituitary release of adrenocorticotrophic hormone.
• At this point, systemic homeostasis returns.

Question 3

List the main actions of glucocorticoids like cortisol

Answer 3

• Stimulate fat and protein catabolism
• Stimulates gluconeogenesis in the liver
• Stimulates the release of fatty acids and glucose into the blood
• Anti-inflammatory effect e.g. hydrocortisone used in ointments to relieve swelling
• Excessive glucocorticoid secretion or medical use suppresses the immune system

Question 4

Explain the significance of cortisol during major physical stress

Answer 4

• Cortisol (along with epinephrine) is best known for its involvement in the “fight or flight” response.
• Cortisol stimulates fat and protein catabolism, gluconeogenesis, and the release of fatty acids and glucose into the blood. Cortisol also inhibits insulin production in an attempt to prevent glucose from being stored, favouring its immediate use.
• These combined factors help supply an immediate source of energy to large muscles.
• Cortisol narrows the arteries while the epinephrine increases heart rate, both of which contribute to an increase in blood pressure.
• Together these responses enable the body to adapt to major physical stress.

Question 5

Describe the mineralocorticoid effects of aldosterone in terms of its renal actions

Answer 5

• Baroreceptors in major arteries detect a fall in blood pressure.
• This causes more renin to be produced by juxtaglomerular cells in the kidney, and angiotensinogen to be produced by the liver.
• Renin and angiotensinogen combine to form angiotensin I, which is then converted to angiotensin II in the lungs by angiotensin converting enzyme.
• An increase in angiotensin II stimulates the adrenal cortex to secrete more aldosterone. The aldosterone increases sodium and water reabsorption in the kidneys.
• This response stimulated by aldosterone is known as the
  renin-angiotensin-aldosterone (RAA) system.

Question 6

Briefly describe the effects of excess or deficient cortisol secretion

Answer 6

Cortisol excess

• Cushing’s syndrome
• Hyperglycaemia - increase in blood glucose levels x2 normal “adrenal diabetes”
• Effects on protein metabolism
• Protein shortage causes muscle weakness
• Stretch lines (striae)
• Easy bruising
• Thinning of skin
• Suppression of the immune system
• Osteoporosis

Cortisol deficiency

• Hypoglycaemia
• Reduction in fat and protein metabolism
• Weight loss
• Poor exercise tolerance
• Poor stress tolerance - death

Question 7

Briefly describe the effects of excess or deficient aldosterone secretion

Answer 7

Aldosterone excess

• Increased sodium and water absorption
• Increased potassium and hydrogen secretion
• Increased blood volume and blood pressure

Aldosterone deficiency

• Increased loss of sodium and water in the urine - dehydration, plasma depletion, and hypotension
• Renal retention of potassium and hyperkalemia which increases cardiac excitability and can cause ventricular fibrillation (heart beats quickly and abnormally).
• Renal retention of hydrogen producing a metabolic acidosis.

Question 8

Briefly describe the effects of excess or deficient adrenal androgen secretion

Answer 8

Androgen excess

• Adrenogenital syndrome in children often causes enlargement of the penis or clitoris, and the premature onset of puberty.
• In women, adrenogenital syndrome produces masculinising effects such as increased body hair, deepening of the voice, and beard growth.

Androgen deficiency (in women)

• Lethargy
• Loss of muscle mass and strength
• Loss of libido

Androgen deficiency (in men)

• Breast development
• Increased body fat, particularly around the abdomen
• Loss of body hair

Question 9

Describe the main effects of normal and excess catecholamine secretion

Answer 9

Normal secretion

• Increase alertness and prepare the body for physical activity.
• They mobilise high-energy fuels such as lactate, fatty acids, and glucose.
• The liver boosts glucose levels by glycogenolysis and gluconeogenesis.
• Epinephrine has a glucose-sparing effect by inhibiting insulin secretion, so the muscles consume less glucose. Instead they fall back on fatty acids while the blood glucose is left for use by the brain.
• Raise the heart rate and blood pressure, stimulate circulation to the muscles, increase pulmonary airflow, and raise the metabolic rate.
• Inhibit temporarily inessential functions such as digestion and urine production so that they do not compete for blood flow and energy.

Excess secretion (symptoms of pheochromocytoma - a catecholamine-secreting tumour)

• Hypertension
• Headache
• Sweating
• Palpitations
• Chest pain
• Anxiety
• Glucose intolerance
• Increased metabolic rate

Question 10

What are the differences between adrenaline and noradrenaline

Answer 10

• Alpha receptors have a greater affinity for noradrenaline whereas beta receptors have a greater affinity for adrenaline.
• Adrenaline causes a greater increase in cardiac stimulation.
• Noradrenaline causes greater vasoconstriction.
• Adrenaline increases metabolic rate more greatly.