Renal urea handling is the part of renal physiology that deals with the reabsorption and secretion of urea. Movement of large amounts of urea across cell membranes is made possible by urea transporter proteins. About 40% of the urea filtered is normally found in the final urine, since there is more reabsorption than secretion along the nephron. It is regulated by antidiuretic hormone, which controls the amount reabsorbed in the collecting duct system and secreted into the loop of Henle.