In the outer part of the medulla, the osmotic concentration of interstitial fluid is 300mosmol/litre. It gradually increases and reaches a value of 1200mosmol/litre in the inner part of the medulla. This gradient is maintained for the concentration of urine as it passes through the collecting tubule. The vasa recta contribute to the maintenance of the hyperosmotic condition of ECF in the medulla by a mechanism known as counter-current exchange. The counter-current mechanism works by exchange of water and electrolytes between Loop of Henle and vasa recta.