ADH is required for the kidneys to produce concentrated urine.
Water cannot pass freely through cell membranes and requires the presence of channels, known as aquaporins. A number of these aquaporins have been identified and it is aquaporin-2 (AQP-2) which responds to ADH. When ADH binds to receptors on distal tubular cells, it stimulates the synthesis and insertion of AQP-2 water channels.
Water moves out of the collecting duct lumen under the drive of a high osmotic gradient produced by the hyperosmolar renal medullary interstitium.