describe the function

ADH is synthetised in the hypothalamus & is transported to the posterior pituitary.

ADH acts primarily to promote water conservation by the kidney by increasing the permeability of the distal convoluted tubules and collecting tubules to water. It thus allows water reabsorption and excretion of a smaller volume of concentrated urine - antidiuresis. 

ADH also increases the permeability of the kidney's papillary portion of the collecting duct to urea, allowing increased reabsorption of urea into the medullary interstitium, down the concentration gradient created from the removal of water in the cortical collecting duct.

It also stimulates sodium reabsorption in the thick-ascending loop of Henle.