The ability to concentrate urine depends upon the generation of a hyperosmolar environment in the medulla. The loops of Henle are set up so as to concentrate osmolarity in the deepest part of the medulla. This occurs because the ascending and descending limbs have different permeabilities to salt and water.

 Blood flow to the medulla is set up so as to maintain the osmotic gradient. The vasa recta are capillaries that flow in parallel to the loops of Henle. The blood flow in the vasa recta is sluggish, allowing time for the plasma in the vasa recta to equilibrate with the surrounding interstitial fluid. The osmolarity of the plasma inside the vasa recta increases as it descends into the medulla, and then decreases again on the ascending side. This allows blood to flow to the medulla, without eliminating the osmotic gradient.