Antidiuretic hormone

Antidiuretic hormone (ADH), also called vasopressin, is a small peptide of nine amino acids. It is made by nerve cells mainly in the supraoptic nucleus of the hypothalamus, carried down their axons and stored in the posterior pituitary until needed. Its name describes its main job: it opposes diuresis, the production of large volumes of urine.

Osmoreceptors in the hypothalamus track the concentration of solutes in the blood. When it rises, as in dehydration or after a salty meal, more ADH is released; a large drop in blood volume or pressure also boosts release. In the kidney, ADH acts on cells lining the collecting ducts, causing them to insert water channels called aquaporins into the membrane facing the forming urine. Water then moves by osmosis into the salty tissue of the kidney medulla and back to the blood, leaving a small volume of concentrated urine. At high levels ADH also narrows blood vessels, which is where the name vasopressin comes from.

Too little ADH, or kidneys that cannot respond to it, causes diabetes insipidus, with heavy urine output and thirst. Too much, as in the syndrome of inappropriate ADH secretion, dilutes the blood and lowers its sodium. Alcohol suppresses ADH release, which is why drinking increases urination.