How the Adrenal Axis Works


What is the adrenal axis?

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

Adrenal Cortex

The adrenal cortex is responsible for producing cortisol and aldosterone.

Cortisol

  • 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

  • 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

Adrenal Medulla

  • 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

Cortisol Regulation

Glucocorticoid production is regulated by the hypothalamic-pituitary-adrenal-axis.

Physiology

  1. Corticotropin-releasing hormone (CRH) is secreted by the hypothalamus (release is influenced by stress levels, time of day and serum cortisol levels).
  2. CRH travels in the blood and binds to specific receptors on the pituitary gland. This binding causes increased production of adrenocorticotropic hormone (ACTH).
  3. ACTH is released into the bloodstream and travels to the adrenal glands, where it binds to specific receptors on the adrenal cortex.
  4. 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.
  5. Increased serum cortisol levels inhibit the production of CRH and ACTH via negative feedback, preventing the overproduction of cortisol.
Hypothalamic-Pituitary-Adrenal Axis Diagram
Hypothalamic-Pituitary-Adrenal axis diagram
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Aldosterone Regulation

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:

  • Sodium reabsorption
  • Water reabsorption
  • Potassium excretion

4. Aldosterone, therefore, causes a net increase in blood volume.
5. Increased blood volume inhibits the production of angiotensin II via negative feedback.

 

Renin-angiotensin-aldosterone system
Renin-angiotensin-aldosterone system 1

References

  1. By A. Rad – Own work, CC BY-SA 3.0. Available from: [LINK]

 

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