Introduction:
The two adrenal (suprarenal) glands are small endocrine glands located on the upper poles of each kidney, enclosed within the renal fascia. Each gland is about 4 cm long and 3 cm thick.
Structurally, the adrenal gland consists of two distinct parts with different functions:
- Outer cortex
- Inner medulla
The adrenal cortex is essential for life, while the adrenal medulla is not vital but supports the body’s stress responses.
Adrenal Cortex:
The adrenal cortex produces three groups of steroid hormones from cholesterol, collectively known as adreno-corticocoids (corticosteroids).
These are:
- Glucocorticoids
- Mineralocorticoids
- Sex hormones (Androgens)
Although each group has distinct actions, some of their effects may overlap due to structural similarities.
1. Glucocorticoids:
The main glucocorticoid is Cortisol (Hydrocortisone), with small amounts of Corticosterone and Cortisone also produced.
These hormones are essential for life, helping regulate metabolism and the body’s response to stress.
Control of Secretion:
- Secretion is controlled by a negative feedback system involving the hypothalamus and anterior pituitary.
- ACTH (Adrenocorticotropic Hormone) from the pituitary stimulates cortisol secretion.
- Levels are influenced by stress, infection, and surgery.
- Cortisol shows circadian variation, peaking between 4 a.m. and 8 a.m., and lowest between midnight and 3 a.m.
Functions of Glucocorticoids:
Glucocorticoids mainly promote catabolic (breakdown) processes, making nutrients available for energy production:
- Increase blood glucose (Hyperglycemia) by:
- Breaking down glycogen
- Stimulating gluconeogenesis (formation of new glucose from protein)
- Enhance lipolysis, breaking down fat (triglycerides) into fatty acids and glycerol.
- Stimulate protein breakdown, releasing amino acids for energy and protein synthesis.
- Promote sodium and water absorption from renal tubules (mild mineralocorticoid effect).
Additional Pharmacological Effects:
In higher doses, glucocorticoids also:
- Act as anti-inflammatory agents
- Suppress immune responses
- Delay wound healing
When administered therapeutically (e.g., for asthma), high corticosteroid levels suppress natural CRH and ACTH secretion through negative feedback.
2. Mineralocorticoids (Aldosterone):
Aldosterone is the main mineralocorticoid responsible for maintaining water and electrolyte balance.
Functions:
- Stimulates reabsorption of sodium (Na⁺) and excretion of potassium (K⁺) in renal tubules.
- Promotes water retention, thus helping regulate blood volume and blood pressure.
Regulation of Secretion:
- High blood potassium → stimulates aldosterone secretion.
- Low blood potassium → suppresses secretion.
- Angiotensin II, from the Renin–Angiotensin–Aldosterone System (RAAS), also promotes aldosterone release.
Renin–Angiotensin–Aldosterone System (RAAS)
When renal blood flow decreases or sodium levels fall, the enzyme renin is released by the kidneys.
Renin converts angiotensinogen (from the liver) into angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE).
Angiotensin II:
- Stimulates aldosterone secretion
- Causes vasoconstriction
- Raises blood pressure
This system ensures normal blood pressure and fluid balance through a negative feedback mechanism.
3. Sex Hormones (Androgens):
The adrenal cortex secretes small amounts of sex hormones, primarily androgens such as dehydroepiandrosterone (DHEA).
Functions and Importance:
- In males, adrenal androgens have minimal effects because the testes produce much higher amounts of testosterone after puberty.
- In females, adrenal androgens:
- Maintain libido (sex drive)
- Are converted into estrogens after menopause (when ovarian estrogen stops)
- Stimulate growth of axillary and pubic hair
- Contribute to prepubertal growth spurts
The secretion of adrenal androgens is primarily controlled by ACTH.
Adrenal Medulla:
The adrenal medulla lies at the center of the gland, completely surrounded by the cortex.
It develops from embryonic nervous tissue and acts as part of the sympathetic nervous system.
When stimulated by sympathetic nerves, the medulla releases two key hormones:
- Adrenaline (Epinephrine) – 80%
- Noradrenaline (Norepinephrine) – 20%
Adrenaline and Noradrenaline:
Overview:
- Noradrenaline acts as a postganglionic neurotransmitter in the sympathetic nervous system.
- Both hormones are released into the bloodstream during stress or emergency situations, reinforcing the fight or flight response.
Physiological Effects:
- Increase heart rate and blood pressure
- Redirect blood flow to vital organs (heart, brain, skeletal muscles)
- Constrict vessels in less essential organs (skin, GI tract)
- Increase metabolic rate
- Dilate pupils
Adrenaline has a stronger effect on the heart and metabolism, while noradrenaline mainly affects blood vessel constriction.
Detailed Notes:
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