The countercurrent exchanger

The vasa recta function as counter current exchangers, maintaining the osmotic gradient established by the cycling of salt while delivering blood cells in the area and removing reabsorbed water and solutes. These vessels receive only about 10% of the renal blood supply, making blood flow through the vasa recta sluggish. Moreover, they are freely permeable to water and NaCl, allowing blood to make passive exchanges with the surrounding interstitial fluid. Consequently, as the blood flows into the medullary depths, it loses water and gains salt (becomes hypertonic). Then, as it emerges from the medulla into the cortex, the process is reversed. It picks up water and loses salt. The water picked up by the ascending vasa recta includes not only water lost from the descending vasa recta, but also water  reabsorbed from the loop of Henle and collecting duct. As a result, the volume of the blood at the end of the vasa recta is greater than at beginning.

Because blood leaving and reentering the cortex via the vasa recta has nearly the same solute concentration, the vessels of the vasa recta act as countercurrent exchangers. This system does not create the medullary gradient, but it protects it by preventing rapid removal of salt from the medullary interstitial space, and by removing reabsorbed water,