The adrenal cortex (zona glomerulosa, zona fasciculata, zona reticularis) and adrenal medulla. Functions and regulation of the hormones produced: mineralocorticoids (aldosterone), glucocorticoids (cortisol), androgens (DHEA), and sympathetic epinephrine (adrenaline) and norepinephrine (noradrenaline)
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The adrenal glands are a pair of glands sitting on top of the kidneys. Each gland consists of an outer cortex of glandular tissue, and an inner medulla of nervous tissue.
The adrenal cortex produces a variety of steroid hormones, all of which are based on cholesterol. The cortex has three layers: zona glomerulosa, zona fasciculata, and zona reticularis. Each layer secretes its own set of hormones.
The outermost layer, zona glomerulosa, produces mineralocorticoids, so named for their role in regulating minerals, in particular sodium and potassium. These hormones primarily act on the kidneys to control fluid and electrolyte balance.
The major mineralocorticoid is aldosterone. Aldosterone increases potassium excretion, as well as sodium and water retention by the kidneys, thereby increasing blood volume and blood pressure. The main regulators of aldosterone are the renin-angiotensin-aldosterone system, which activates in response to low blood pressure; and elevated potassium levels.
The middle layer, zona fasciculata, produces glucocorticoids, named for their role in glucose metabolism. The most important of these is cortisol, best known as the stress hormone. Cortisol helps the body stay on high alert to deal with long-term stress. Cortisol overall effect is to increase energy supply:
- In the liver, it promotes catabolism of glycogen to glucose.
- In the muscles, it acts to reduce glucose uptake and consumption, and increase degradation of muscle proteins into amino acids, which can then be used to synthesize glucose, fats, or generate energy.
- In adipose tissue, cortisol promotes lipid breakdown for additional energy.
- In the pancreas, it acts to decrease insulin and increase glucagon secretion, which together leads to increased blood glucose.
In addition, cortisol maintains higher blood pressure by increasing the sensitivity of vascular smooth muscle to vasoconstrictors and inhibiting the release of vasodilators.
Cortisol also reduces inflammation and suppresses the immune system, which is the mechanism of action of glucocorticoid drugs.
Secretion of cortisol is under the control of the hypothalamus-pituitary-adrenal (HPA) axis. In response to stressors, the hypothalamus produces corticotropin-releasing hormone (CRH). CRH promotes the secretion of adrenocorticotropic hormone, ACTH, from the anterior pituitary. ACTH then stimulates the production of cortisol by the adrenal gland. A negative feedback loop exists to prevent overproduction of glucocorticoids. The HPA axis also follows a circadian rhythm, producing higher cortisol levels in the morning and lower at night.
The inner layer of the adrenal cortex, zona reticularis, produces male hormones known as androgens. Its major product is a weak androgen named DHEA. DHEA is produced in response to ACTH from the anterior pituitary, and is converted, in other tissues, to either testosterone or estrogen. Adrenal androgens only make up a small proportion of androgens in males, but they are a major source of androgens in females, especially after menopause.
The core of adrenal gland - adrenal medulla, is actually part of the sympathetic nervous system. It is composed of modified postganglionic neurons called chromaffin cells. During a “fight or flight” response to acute, short-term stress, sympathetic preganglionic fibers stimulate these cells to produce epinephrine and norepinephrine. These hormones increase cardiac output, accelerate respiratory rate, and release stored energy. By binding to different receptors, they cause vasoconstriction in most organs but vasodilation in the heart and skeletal muscles, and thus directing blood flow to where it is most needed.
