NaCl transported by the ascending limb of loop of Henle is exchanged with

The countercurrent exchange mechanism between the ascending limb of the loop of Henle and the descending limb of the vasa recta is responsible for kidney's ability to establish a concentration gradient in the medulla and concentrate urine.

In the descending limb of the vasa recta (a specialised network of capillaries surrounding the loop of Henle), blood flows in the opposite direction of the filtrate in the loop of Henle. As blood travels down the descending vasa recta, it encounters the concentrated interstitial fluid in the medulla. This interstitial fluid is created by the reabsorption of sodium chloride (NaCl) in the ascending limb of the loop of Henle.

Due to the osmotic gradient between the descending vasa recta and the interstitial fluid, water flows out of the capillaries and into the blood, resulting in the concentration of solutes in the blood. This countercurrent exchange helps to prevent the dilution of the concentrated interstitial fluid in the medulla and further aids in maintaining the osmotic gradient needed for the kidney's urine-concentrating ability.