<prompt> Site of action of ADH is:March 2008PCTVasa rectaLoop of henleCollecting ducts  <response>: Ans. D: Collecting ductsBecause one of its principal physiologic effects is the retention of water by the kidney, vasopressin is often called the antidiuretic hormone ADH.It increases the permeability of the collecting ducts of the kidney, so that water enters the hypeonic interstitium of the renal pyramids.The urine becomes concentrated, and its volume decreases. The overall effect is therefore retention of water in excess of solute; consequently, the effective osmotic pressure of the body fluids is decreased. In the absence of vasopressin, the urine is hypotonic to plasma, urine volume is increased, and there is a net water loss. Consequently, the osmolality of the body fluid rises. 