Urea is important to maintaining the concentration gradient in the interstitial fluid by a process called

Urea is important to maintain the concentration gradient in the interstitial fluid by a process called urea recycling.  

Urea recycling is a process which occurs in collecting ducts of the inner medulla. Antidiuretic hormone (ADH) increases both water and urea permeability in those ducts and that adds to the osmotic gradient and helps drive water reabsorption.