The adrenal axis (hypothalamic-pituitary-adrenal axis) refers to a complex set of interactions and feedback loops between the hypothalamus, pituitary and adrenal glands. This system regulates the body’s response to stress, immune function, energy expenditure, mood, emotions and libido.
What are the adrenal glands?
An adrenal gland is located above each kidney
The glands produce a number of different hormones (discussed below)
Each gland is composed of an outer cortex and an inner medulla
The adrenal cortex is responsible for producing cortisol and aldosterone.
Cortisol is a steroid hormone (glucocorticoid)
It is released in response to stress and low blood-glucose concentration
Some of its functions include:
Increasing blood glucose levels by promoting gluconeogenesis
Suppressing the immune system
Aiding the metabolism of fat, protein and carbohydrates
Aldosterone is also a steroid hormone (mineralocorticoid)
Its main function is to increase blood volume
It promotes reabsorption of sodium and water, as well as excretion of potassium
The adrenal medulla produces adrenaline and noradrenaline (both have similar actions).
Adrenaline and noradrenaline are released during periods of stress, as part of the “fight or flight” response (↑HR, ↑BP, ↑ blood flow to muscles)
The release of adrenaline and noradrenaline is under the direct control of the hypothalamus
Glucocorticoid production is regulated by the hypothalamic-pituitary-adrenal-axis.
Corticotropin-releasing hormone (CRH) is secreted by the hypothalamus (release is influenced by stress levels, time of day and serum cortisol levels).
CRH travels in the blood and binds to specific receptors on the pituitary gland. This binding causes increased production of adrenocorticotropic hormone (ACTH).
ACTH is released into the bloodstream and travels to the adrenal glands, where it binds to specific receptors on the adrenal cortex.
The binding of ACTH to the receptors on the adrenal cortex stimulates the release of cortisol into the bloodstream, where it has a wide range of effects.
Increased serum cortisol levels inhibit the production of CRH and ACTH via negative feedback, preventing the overproduction of cortisol.
1. Low blood volume stimulates the release of angiotensin II. 2. Angiotensin II stimulates the adrenal cortex to produce aldosterone.
3. Aldosterone has a number of different actions including:
4. Aldosterone, therefore, causes a net increase in blood volume. 5. Increased blood volume inhibits the production of angiotensin II via negative feedback.
By A. Rad – Own work, CC BY-SA 3.0. Available from: [LINK]