Overview
The Hormone That Decides Whether Water Leaves or Stays
Antidiuretic hormone (ADH), also called vasopressin, conserves water when the body detects that water is scarce or circulating volume is threatened.
Antidiuretic hormone (ADH), also called vasopressin, conserves water when the body detects that water is scarce or circulating volume is threatened. It is synthesized in the hypothalamus and released from the posterior pituitary. ADH acts mainly at V2 receptors in the renal collecting ducts. The signal inserts aquaporin-2 water channels into the tubular membrane, allowing water to move back into the bloodstream. Urine becomes more concentrated and urine volume falls. Serum sodium is best understood as a ratio: sodium relative to body water. ADH retains water, not sodium. Excessive ADH activity can therefore dilute serum sodium even when total body sodium is normal or increased. Conversely, absent or ineffective ADH causes loss of dilute water and can raise serum sodium. A useful clinical image is a collecting duct with adjustable water gates. ADH opens the gates when the body needs to retain water. In syndrome of inappropriate ADH secretion, the gates remain open when they should be closed. In diabetes insipidus, they do not open adequately.