Adrenal Cortex

The adrenal cortex makes up about 90% of the adrenal gland and has cells divided into three layers (Fig. 64-5). Mineralocorticoids are the hormones produced in the zona glomerulosa and help control the body’s sodium and potassium content. Glucocorticoids, androgens, and estrogens are produced in the zona fasciculata and zona reticularis. The hormones secreted by the cortex are often called adrenal steroids or corticosteroids.

FIG. 64-5 The structural detail of the adrenal gland.

Mineralocorticoids are produced and secreted by the adrenal cortex to help control body fluids and electrolytes. Aldosterone is the major mineralocorticoid and maintains extracellular fluid volume. It promotes sodium and water reabsorption and potassium excretion in the kidney tubules. Aldosterone secretion is regulated by the renin-angiotensin system, serum potassium ion concentration, and adrenocorticotropic hormone (ACTH).

Renin is produced by specialized cells of the renal afferent arterioles. Its release is triggered by a decrease in extracellular fluid volume from blood loss, sodium loss, or posture changes. Renin converts renin substrate (formerly called angiotensinogen), a plasma protein, to angiotensin I. Angiotensin I is converted by a converting enzyme to form angiotensin II, the active form of angiotensin. In turn, angiotensin II stimulates the secretion of aldosterone. Chapter 13 (see Fig. 13-6) further explains the renin-angiotensin system. Aldosterone causes the kidney to reabsorb sodium and water to bring the plasma volume and osmolarity back to normal.

Serum potassium level also controls aldosterone secretion. The adrenal cortex secretes aldosterone when the serum potassium level increases above normal by as little as 0.1 mEq/L. Aldosterone then enhances kidney excretion of potassium to reduce the blood level back to normal.

Glucocorticoids are produced by the adrenal cortex and are essential for life. The main glucocorticoid produced by the adrenal cortex is cortisol. Cortisol affects:

Cortisol also influences other important body processes. For example, it must be present for catecholamine action and maintaining the normal excitability of the heart muscle cells. Glucocorticoid functions are listed in Table 64-3.

TABLE 64-3 FUNCTIONS OF GLUCOCORTICOID HORMONES

Glucocorticoid release is regulated directly by the anterior pituitary hormone ACTH and indirectly by the hypothalamic corticotropin-releasing hormone (CRH). The release of CRH and ACTH is affected by the serum level of free cortisol, the normal sleep-wake cycle, and stress.

As described earlier and shown in Fig. 64-3, when blood cortisol levels are low, the hypothalamus secretes CRH, which triggers the pituitary to release ACTH. Then ACTH triggers the adrenal cortex to secrete cortisol. Adequate or elevated blood levels of cortisol inhibit the release of CRH and ACTH. This inhibitory effect is an example of a negative feedback system.

Glucocorticoid release peaks in the morning and reaches its lowest level 12 hours after each peak. Emotional, chemical, or physical stress increases the release of glucocorticoids.

Sex hormones (androgens and estrogens) are secreted in low levels by the adrenal cortex in both genders. Adrenal secretion of these hormones is usually not significant because the gonads (testes and ovaries) secrete much larger amounts of androgens and estrogens. In women, however, the adrenal gland is the major source of androgens. Women who have adrenal insufficiency or who have had surgical removal of the adrenals may need a small amount of testosterone replacement.

Adrenal Medulla

The adrenal medulla is a sympathetic nerve ganglion that has secretory cells. Stimulation of the sympathetic nervous system causes the release of adrenal medullary hormones, the catecholamines (which include epinephrine and norepinephrine). These hormones travel to all areas of the body through the blood and exert their effects on target cells. The adrenal medullary hormones are not essential for life because they also are secreted by other body tissues, but they do play a role in the physiologic stress response.

The adrenal medulla secretes about 15% norepinephrine (NE) and 85% epinephrine. Hormone effects vary with the specific receptor in the cell membranes of the target tissue.

These receptors are of two types: alpha adrenergic and beta adrenergic. Both types of receptors are further classified as alpha₁ and alpha₂ receptors and beta₁, beta₂, and beta₃ receptors. NE acts mainly on alpha-adrenergic receptors, and epinephrine acts mainly on beta-adrenergic receptors.

Catecholamines exert their actions on many target organs (Table 64-4). Activation of the sympathetic nervous system, which then releases adrenal medullary catecholamines, is an important part of the body’s response to stress. Catecholamines are secreted in small amounts at all times to maintain homeostasis. Stress triggers increased secretion of these hormones. This sympathetic activation results in the “fight-or-flight” response, a state of heightened physical and emotional awareness.

TABLE 64-4 CATECHOLAMINE RECEPTORS AND EFFECTS OF ADRENAL MEDULLARY HORMONE STIMULATION ON SELECTED ORGANS AND TISSUES