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 bodies response to stress, immune function, energy expenditure, mood, emotions & libido. It is therefore an incredibly important body system which can cause huge problems if it malfunctions
What are the adrenal glands?
There is an Adrenal gland located above each kidney
They both produce a number of hormones
The Adrenal glands are composed of a 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)which is released under stress & low steroid levels
It’s main function is to increase blood glucose levels by promoting gluconeogenesis
Cortisol also suppresses the immune system & increases fat, protein & carb metabolism
Random Fact!
You may recognise cortisol’s pharmaceutical name which is Hydrocortisone
Hydrocortisone is used to reduce inflammation leveraging Cortisol’s immunosuppresive effect
Aldosterone
Aldosterone is also a steroid hormone (mineralocorticoid)
It’s main function is to increase blood volume
It causes reabsorption of sodium and water as well as causing excretion of potassium
Random Fact!
Drugs that interfere with the secretion or action of Aldosterone are in used as antihypertensives
An example is Spironolactone which blocks Aldosterone receptorsAdrenal Medulla
The adrenal medulla produces adrenaline & noradreniline (both have similar actions)
They are released as an acute response to stress & imitate many of the effects of cortisol
They essentially prepare the body for “fight or flight” – ↑HR, ↑BP, ↑Blood flow to muscles
Their release is under direct control from the hypothalamus – allowing fast response to stress
Adrenaline & Noradreniline’s effects are not as long lasting as cortisol
How Cortisol is Regulated
- Corticotrophic releasing hormone (CRH) is secreted from the Hypothalamus
- This release is influenced by stress levels, time of day & serum cortisol levels
- CRH travels in the blood & binds to specific receptors on the Pituitary Gland
- This binding causes increased production of ACTH (adrenocorticotrophic hormone)
- ACTH is released into the blood stream where it travels to the Adrenal glands
- ACTH binds to specific receptors on the Adrenal Cortex
- This stimulates the Adrenal Cortex to release Cortisol into the blood
- Cortisol enables the body to cope with stress in a more effective manner
- Increased levels of Cortisol also have an immunosuppresive effect
- Blood glucose is also increased via breakdown of glycogen, protein & fat
- Increased serum Cortisol inhibits production of CRH & ACTH via negative feedback
How Aldosterone is Regulated
Low blood volume triggers release of Angiotensin II
Angiotensin II stimulates the Adrenal Cortex to produce Aldosterone
Aldosterone has a number of different actions;
- Sodium Reabsorption
- Water Reabsorption
- Potassium Excretion
Aldosterone’s actions result in a net increase in blood volume
Increased blood volume inhibits production of Angiotensin II via negative feedback
