vasopressin (ADH)

Vasopressin is the body’s “water-saving switch.” When you are short on water — after a salty meal, a long run, or a night without drinking — this hormone tells your kidneys to hold onto water instead of pouring it into the urine. The result is a smaller volume of more concentrated urine, which protects the amount of water in your blood.

It is a small peptide of nine amino acids made by neurons in the hypothalamus and stored in the posterior pituitary, from which it is released into the bloodstream. Its main trigger is a rise in blood osmolality (the concentration of dissolved particles), detected by hypothalamic osmoreceptors; a large fall in blood volume or pressure also stimulates it. At the kidney it acts on collecting-duct cells through V2 receptors, raising cyclic AMP and driving water channels into the membrane so water can be reabsorbed.

The second name, antidiuretic hormone, simply describes this water-conserving action (“anti-diuresis” means reducing urine output). The same molecule also constricts blood vessels through V1 receptors, which is the origin of the name “vasopressin,” though this pressor effect matters most during severe blood loss. Too little of it, or kidneys that cannot respond, causes diabetes insipidus; too much causes water retention and dilutional low sodium.