Aldosterone, the principal mineralocorticoid, maintains extracellular fluid volume as part of the renin-angiotensin-aldosterone system. Secreted in response to decreased extracellular volume, aldosterone stimulates the kidneys to reabsorb sodium and water and excrete potassium in an effort to increase plasma fluid volume and restore normal osmolality. Other factors that stimulate aldosterone secretion include elevated serum potassium levels and adrenocorticotropic hormone (ACTH) stimulation.