The adrenal gland and the stress response · Biology… | StemPy Lab
Course 18.2·Lesson 1 of 3
The adrenal gland and the stress response
The adrenal glands sit on the kidneys. The medulla makes adrenaline and noradrenaline for an emergency. The cortex makes glucocorticoids such as cortisol and mineralocorticoids such as aldosterone. Too little of these hormones gives Addison's disease, and too much cortisol gives Cushing's syndrome.
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A fright in the road
A cyclist hears a horn behind her. In a second her heart pounds, her breathing is fast, her pupils widen and her palms sweat. She has not decided anything. A pair of glands on top of the kidneys has released a hormone, and the hormone has prepared the whole body for action.
The adrenal gland
The body has one pair of adrenal glands, one at the top (the anterior part) of each kidney. Each gland has two different parts that make different hormones.
A drawing of an adrenal gland cut in half, sitting on top of a kidney. The outer part is the cortex, in three layers. From the outside: the zona glomerulosa, the zona fasciculata and the zona reticularis, which is the innermost layer. In the centre is the medulla. Labels on the right give the hormones: the cortex makes corticoids, which are glucocorticoids such as cortisol and mineralocorticoids such as aldosterone, and also small amounts of androgens. The medulla makes adrenaline and noradrenaline, the catecholamines.
The adrenal medulla is the central part.
The adrenal cortex is the outer part. It has three layers. From the outside to the inside they are the zona glomerulosa, the zona fasciculata and the zona reticularis.
The adrenal medulla
The medulla makes adrenaline (also called epinephrine) and noradrenaline (norepinephrine). Together they are the catecholamines. They are released very quickly in response to stress of any kind and in emergencies, so they are called the emergency hormones or the hormones of fight or flight.
Two routes of the stress response drawn side by side. Route 1 is fast and nervous: a stress signal goes to the hypothalamus, nerves go to the adrenal medulla, and the medulla releases adrenaline and noradrenaline within seconds. The effects listed are a faster heartbeat, a stronger contraction, faster breathing, wider pupils, sweating, raised hairs and the breakdown of glycogen to glucose. Route 2 is slower and hormonal: the hypothalamus signals the pituitary, the pituitary releases ACTH, and the adrenal cortex releases cortisol. A red arrow with a minus sign runs from cortisol back to the hypothalamus and the pituitary.
What they do.
They increase alertness, widen the pupils, raise the hairs (piloerection) and make the body sweat.
They increase the heartbeat, the strength of the contraction of the heart and the rate of breathing.
They stimulate the breakdown of glycogen, so the glucose of the blood rises. They also stimulate the breakdown of lipids and proteins.
The medulla is switched on by nerves, not by a pituitary hormone. This is why the answer is so quick.
The adrenal cortex
The cortex makes many hormones, called corticoids. They are steroids. There are three groups.
Glucocorticoids regulate the metabolism of carbohydrates. The main one in the body is cortisol.
They stimulate gluconeogenesis (the making of glucose from other substances), lipolysis (the breakdown of fats) and proteolysis (the breakdown of proteins).
They stop cells from taking up and using amino acids.
They help to maintain the cardiovascular system and the work of the kidneys.
They have an anti-inflammatory action and suppress the immune response. This is why doctors use steroid medicines against swelling and allergy.
Cortisol stimulates the making of red blood cells.
Mineralocorticoids regulate the balance of water and electrolytes. The main one is aldosterone. It acts on the renal tubules. It stimulates the reabsorption of sodium ions and water, and the excretion of potassium and phosphate ions. So it helps to maintain the electrolytes, the volume of the body fluids, the osmotic pressure and the blood pressure.
Androgenic steroids are made in small amounts. They play a part in the growth of the axial, pubic and facial hair at puberty.
A way to remember the layers: from the outside, the layers make salt, sugar and sex hormones. That is aldosterone, cortisol and androgens. NCERT names the layers and the groups of hormones but does not join them. Standard physiology books do.
How the cortex is controlled
Cortisol is controlled by a loop. The hypothalamus signals the pituitary, the pituitary releases ACTH, and ACTH stimulates the cortex to make glucocorticoids. A high level of cortisol acts back on the hypothalamus and the pituitary and switches ACTH down. It is a negative feedback loop, like the loop of the thyroid.
When the cortex goes wrong
Addison's disease is the result of a lack of the hormones of the adrenal cortex. The person is weak and tired and loses weight. Because aldosterone is short, the body loses sodium and water, and the blood pressure falls.
Cushing's syndrome is the result of too much cortisol. The face becomes round, fat collects on the trunk, the blood sugar and the blood pressure rise, and the skin is thin. This part is not in the NEET syllabus text, but A-Level and IB ask it.
You should find that a weak cortex gives low cortisol and high ACTH, which is the pattern of Addison's disease. A steroid medicine does something different. It adds cortisol from outside, so the ACTH falls and the gland makes less of its own. This is why a person on a long course of steroid tablets must not stop them suddenly. The model is a teaching device, not a measurement.
Try it in Python
The program finds the levels of cortisol and ACTH for a given strength of the cortex and a given dose of medicine.
Practise with new numbers
Common mistakes
Saying that adrenaline comes from the cortex. Adrenaline and noradrenaline come from the medulla.
Mixing up the two groups of corticoids. Cortisol is a glucocorticoid for sugar. Aldosterone is a mineralocorticoid for salt and water.
Saying that the medulla is controlled by ACTH. The medulla is controlled by nerves. ACTH controls the cortex.
Saying that aldosterone makes the body lose sodium. It makes the kidney keep sodium and lose potassium.
Mixing up Addison's disease and Cushing's syndrome. The first is too little, the second is too much cortisol.
Remember these
Adrenal medulla: adrenaline and noradrenaline, the catecholamines, the hormones of fight or flight. They raise the heartbeat, the strength of contraction and the rate of breathing, and they raise the glucose.
Adrenal cortex: three layers (from outside) glomerulosa, fasciculata and reticularis. Glucocorticoids (cortisol) and mineralocorticoids (aldosterone), and small amounts of androgens.
Cortisol: gluconeogenesis, lipolysis and proteolysis, anti-inflammatory, suppresses immunity. Aldosterone: reabsorbs Na+ and water, excretes K+.
Addison's disease: lack of cortical hormones. ACTH controls the cortex.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Which part of the adrenal gland makes adrenaline? Why is it called a hormone of fight or flight?
Name three effects of adrenaline on the body.
Name the main glucocorticoid and the main mineralocorticoid. What does each regulate?
How does aldosterone act on the kidney?
A man has weakness, weight loss and low blood pressure because his adrenal cortex hormones are too low. What is the disorder?
References
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapter on adrenocortical hormones.
NCERT (2023). Biology, Textbook for Class XI, chapter 19.
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