aldosterone

Aldosterone is the body's chief salt-keeping hormone. When you are short on fluid — after sweating heavily, losing blood, or simply eating too little salt — aldosterone tells your kidneys to hold onto sodium, and water follows, helping to defend your blood volume and blood pressure. Think of it as the body's drought-management hormone, dialing up water conservation when supplies run low.

It is the dominant mineralocorticoid, made in the zona glomerulosa of the adrenal cortex. In the kidney's distal nephron, aldosterone increases the number and activity of sodium channels and pumps, so more sodium is reabsorbed from urine back into blood. In exchange, potassium (and hydrogen ions) are secreted into the urine. The net result is sodium and water retention with potassium loss.

Aldosterone secretion is driven mainly by the renin-angiotensin system and by rising blood potassium, and only weakly by ACTH. This separate control is clinically important: in primary hyperaldosteronism (Conn's syndrome) the gland makes too much aldosterone on its own, causing high blood pressure and low potassium, whereas in Addison's disease aldosterone is deficient, causing salt wasting and high potassium.