antidiuretic hormone

Antidiuretic hormone is just another name for vasopressin, viewed from the angle of what it does at the kidney: it stops you from losing too much water. “Diuresis” means making lots of urine; “anti-diuretic” means doing the opposite — making the kidney keep water in the body.

Mechanically, the hormone binds V2 receptors on the cells lining the kidney’s collecting ducts. This raises cyclic AMP, activates protein kinase A, and moves aquaporin-2 water channels into the membrane facing the urine. Water then flows out of the tubule and back into the blood, following the salty gradient the kidney has built in its inner regions. With the channels in place the final urine becomes concentrated; without them it stays dilute.

The “antidiuretic” name is useful clinically because it pairs naturally with the two opposite disorders. When the hormone is missing or the kidney ignores it, water escapes and urine is copious and dilute — diabetes insipidus. When there is too much of it for the body’s needs, water is retained and blood sodium falls — the syndrome of inappropriate antidiuretic hormone secretion.