Urinary concentration is achieved by countercurrent multiplication in the inner medulla. It takes place in juxtamedullary nephron. Urea reabsorbed from collecting duct to medullary interstitum produces the hyperosmotic medullary interstitium. The active ion pump of the thick ascending limb on the loop of Henle reduces the concentration inside the tubule and raises the interstitial concentration. The tubular fluid in the descending limb and the interstitial fluid quickly reach osmotic equilibrium.