The Human Body

Adrenal Gland

Adrenal Gland, vital endocrine gland that secretes hormones into the bloodstream, situated, in humans, on top of the upper end of each kidney. The two parts of the gland—the inner portion, or medulla, and the outer portion, or cortex—are like separate organs: They are composed of different types of tissue and perform different functions. The adrenal medulla, composed of chromaffin cells, secretes the hormone epinephrine, also called adrenaline, in response to stimulation of the sympathetic nervous system at times of stress. The medulla also secretes the hormone norepinephrine, which plays a role in maintaining normal blood circulation. The hormones of the medulla are called catecholamines. Unlike the adrenal cortex, the medulla can be removed without endangering the life of an individual.

The adrenal outer layer, or cortex, secretes about 30 steroid hormones, but only a few are secreted in significant amounts. Aldosterone, one of the most important hormones, regulates the balance of salt and water in the body. Cortisone and hydrocortisone are necessary to regulate fat, carbohydrate, and protein metabolism. Adrenal sex steroids have a minor influence on the reproductive system. Modified steroids, now produced synthetically, are superior to naturally secreted steroids for treatment of Addison's disease and other disorders. See: Hormone; Hydrocortisone.