Adrenal cortex: hormones, physiological effects and control of their secretion.
The adrenal glands are paired, pyramid-shaped structures in the retroperitoneal space above each kidney. They are composed of two distinct glands: the adrenal medulla and the adrenal cortex. The adrenal cortex makes up approximately 80% of the gland and has three layers:
- Zona glomerulosa: the outermost layer; produces mineralocorticoids such as aldosterone.
- Zona fasciculata: the middle and largest layer; produces glucocorticoids such as cortisol, as well as androgens.
- Zona reticularis: the innermost layer; also produces glucocorticoids and androgens.
Physiological Effects of Glucocorticoids
- Metabolism:
- Stimulate gluconeogenesis (glucose formation from non-carbohydrate sources) in the liver.
- Increase protein breakdown (proteolysis) in muscle to provide amino acids for gluconeogenesis.
- Increase lipolysis (fat breakdown) in adipose tissue to provide free fatty acids and glycerol for gluconeogenesis.
- Decrease glucose utilization by tissues.
- Decrease insulin sensitivity of adipose tissue.
- These metabolic actions help maintain blood glucose levels during fasting and stress.
- Anti-inflammatory & Immunosuppressive Effects
- Inhibit the production of inflammatory mediators such as prostaglandins, leukotrienes, and cytokines.
- Suppress the immune response by inhibiting lymphocyte proliferation and antibody production.
- These actions are useful in treating inflammatory and autoimmune diseases but can also make individuals more susceptible to infections.
- Vascular Responsiveness: enhance vascular responsiveness to catecholamines (epinephrine and norepinephrine), which helps maintain blood pressure.
- CNS Effects:
- Decrease REM sleep.
- Increase slow-wave sleep. o Increase awake time.
- Affect mood and behavior.
- Other Effects:
- Inhibit bone formation by decreasing collagen synthesis, osteoblast activity, and intestinal Ca2+ absorption.
- Increase glomerular filtration rate (GFR) by causing vasodilation of afferent arterioles in the kidney.
- Stimulate erythropoietin production, which increases red blood cell count.
- Facilitate fetal development, particularly maturation of the respiratory system (including surfactant production) and gastrointestinal enzymes.
Glucocorticoid secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis:
- 1. Corticotropin-releasing hormone (CRH) is released from the hypothalamus and travels to the anterior pituitary via the hypothalamic-hypophysial portal blood vessels.
- 2. CRH stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary.
- 3. ACTH acts on the adrenal cortex to stimulate the synthesis and secretion of cortisol.
- 4. Cortisol exerts negative feedback on both the hypothalamus and the anterior pituitary, inhibiting the release of CRH and ACTH, respectively.
ACTH and cortisol levels follow a diurnal (daily) pattern, with peak levels in the morning and lowest levels at night.
Mineralocorticoids (Aldosterone)
The primary mineralocorticoid is aldosterone, which acts on the kidneys to regulate electrolyte and fluid balance:-