Adrenal hormones are secreted by two functionally and embryologically distinct portions of the adrenal gland. The adrenal cortex, which is of mesodermal origin, secretes three types of steroids: (1) glucocorticoids, which affect carbohydrate metabolism; (2) mineralocorticoids, which promote potassium excretion and sodium retention by the kidneys; and (3) adrenal androgens, which the liver converts to testosterone. Cortisol is the predominant glucocorticoid, whereas aldosterone is the predominant mineralocorticoid. Production and secretion of cortisol and adrenal androgens are stimulated by ACTH. Although ACTH also may enhance aldosterone production, the usual stimulants are either increased serum potassium or decreased serum sodium.
The adrenal medulla, which constitutes only about one-tenth of the volume of the adrenal glands, derives from the ectoderm and physiologically belongs to the sympathetic nervous system. The hormones secreted by the adrenal medulla are epinephrine and norepinephrine, which are collectively known as the catecholamines. Epinephrine is secreted in response to sympathetic stimulation, hypoglycemia, or hypotension. Most norepinephrine is manufactured by and secreted from sympathetic nerve endings; only a small amount is normally secreted by the adrenal medulla.52