The adrenal glands vary in size, shape, and nerve supply in other animal species. In some vertebrates the cells of the two parts are to varying degrees interspersed.

The adrenal medulla is composed of cells of neural origin that produce epinephrine and norepinephrine. These hormones belong to a class of chemicals called catecholamines, which are darkened when oxidized by potassium dichromate. The adrenal medulla is therefore referred to as chromaffin tissue.

In adult humans the outer cortex comprises about 90 percent of each adrenal gland. It is composed of three structurally different concentric zones. From the outside in, they are the zona glomerulosa, zona fasciculata, and zona reticularis. The zona glomerulosa produces aldosterone, which acts on the kidneys to conserve salt and water. The production of aldosterone is regulated primarily by the renin-angiotensin system. Renin is produced by the kidney when salt and water are lost—for example, during diarrhea or excessive perspiration. Renin stimulates the production of angiotensin from its inactive precursor, angiotensinogen, in the blood. Angiotensin then stimulates the secretion of aldosterone, which acts on the kidneys to stimulate salt and water retention and blood vessel constriction, thereby raising blood pressure.

The inner two zones of the adrenal cortex—the zona fasciculata and the zona reticularis—function as a physiological unit and are controlled by corticotropin (adrenocorticotropic hormone), a hormone secreted by the anterior pituitary gland. These two zones produce cortisol and adrenal androgens (male hormones), with dehydroepiandrosterone, a weak androgen, being the major product. Cortisol has two primary actions: (1) stimulation of gluconeogenesis—i.e., the breakdown of protein and fat in muscle and their conversion to glucose in the liver; and (2) anti-inflammatory actions. Cortisol and synthetic derivatives of it, such as prednisone and dexamethasone, are known as glucocorticoids, so named because of their ability to stimulate gluconeogenesis. In severely stressed patients these compounds not only facilitate glucose production but also raise blood pressure and reduce inflammation. Because of their anti-inflammatory properties, they are often given to patients with inflammatory diseases such as rheumatoid arthritis and asthma. Glucocorticoids also reduce the function and action of the immune system, making them useful for protecting against transplant rejection and ameliorating autoimmune (autoimmunity) and allergic (allergy) diseases.

Diseases of the adrenal glands may be divided into those of the medulla and those of the cortex. The only known disease of the adrenal medulla is a tumour known as a pheochromocytoma. Pheochromocytomas secrete excessive quantities of epinephrine and norepinephrine. Many patients with these tumours have periodic episodes of hypertension (high blood pressure), palpitations of the heart, sweating, headaches, and anxiety, whereas other patients have persistently high blood pressure. High blood pressure and the other symptoms can be treated with drugs that block the action of epinephrine and norepinephrine; however, the most effective treatment is surgical removal of the tumour.

Diseases of the adrenal cortex may be manifested as hyperfunction (excessive secretion of adrenocortical hormones) or hypofunction (insufficient secretion of these hormones), also known as Addison disease.

Adrenocortical hyperfunction may be congenital or acquired. Congenital hyperfunction is always due to hyperplasia (enlargement) of both adrenal glands, whereas acquired hyperfunction may be due to either an adrenal tumour or hyperplasia. Congenital adrenal hyperplasia, also known as the adrenogenital syndrome (congenital adrenal hyperplasia), is a disorder in which there is an inherited defect in one of the enzymes needed for the production of cortisol. Excessive amounts of adrenal androgens must be produced to overcome the block in cortisol production. In female infants this results in masculinization with pseudohermaphroditism (anomalous development of genital organs), whereas in male infants it results in premature sexual development (sexual precocity).

Acquired adrenocortical hyperfunction is manifested by either cortisol excess ( Cushing syndrome), androgen excess, or aldosterone excess (primary aldosteronism).

Cushing syndrome is characterized by obesity, rounding and reddening of the face, high blood pressure, diabetes mellitus, osteoporosis, thinning and easy bruising of the skin, muscle weakness, depression, and, in women, cessation of menstrual cycles ( amenorrhea). The major causes of Cushing syndrome are a corticotropin-producing tumour of the pituitary gland (known as Cushing disease), production of corticotropin by a nonendocrine tumour, or a benign or malignant adrenal tumour. All of these disorders are treated most effectively by surgical removal of the tumour.

Androgen excess in women is characterized by excessive hair growth on the face and other regions and amenorrhea; in contrast, androgen excess has few effects in men. The major causes of adrenal androgen excess in adults are late-onset congenital adrenal hyperplasia and adrenal tumours.

Primary aldosteronism (hyperaldosteronism) is characterized by high blood pressure, caused by increased retention of salt and water by the kidneys, and low serum potassium concentrations (hypokalemia), caused by excess excretion of potassium in the urine. The symptoms and signs of aldosterone excess include not only hypertension but also muscle weakness and cramps and increased thirst and urination. Primary aldosteronism is usually caused by a benign adrenal tumour (adenoma), but some patients have hyperplasia of both adrenal glands. Successful removal of the adrenal tumour usually results in reduction in blood pressure and cessation of potassium loss; patients with bilateral adrenal hyperplasia are treated with antihypertensive drugs. See also endocrine system (endocrine system, human).