The adrenal glands contain elements of three distinct functional units: (1) the medulla, which secretes catecholamines; (2) the zona glomerulosa, which is responsible for mineralocorticoids (aldosterone); and (3) the zonae fasiculata and reticularis, which are the sources of glucocorticoids (cortisol). The medulla is a part of the sympathetic nervous system and releases epinephrine and norepinephrine in response to stressful stimuli. Aldosterone secretion is regulated mainly by changes in extracellular fluid-water volume and pressure, and sodium and potassium concentrations in blood. Through its actions in the kidney, aldosterone acts to retain sodium and water and enhance excretion of potassium. Glucocorticoid secretion is regulated by adrenocorticotropin from the pituitary. Cortisol has diverse actions on virtually all body tissues. One important effect of this steroid is to provide a negative feedback signal, inhibiting adrenocorticotropic hormone release.