The Countercurrent Exchanger; The Vasa Recta
The vasa recta, like all capillaries, are freely permeable to water and all solutes, except large proteins like albumin, and equilibrate readily with the immediately surrounding interstitial fluid. The flow pattern through the hair pin loops of the vasa recta and the associated flow and transport characteristics of the loop of Henle allows for development of a cortico-medullary osmolar concentration gradient.
As noted above part of the NaCl transported by the thick ascending limb enters the descending limb of the vasa recta and as the blood flows through the descending vasa recta its solutes equilibrate by diffusion with the surrounding tissue fluid causing a progressive increase in the osmolarity of the ISF in the inner zone of the medulla. Because some of the solute reabsorbed from the thick ascending limb of Henles loop entered the ascending vasa recta in the outer medulla and has already been removed, the blood flowing back up the ascending limb of the vasa recta from the inner medulla re-equilibrates with the less concentrated ISF in the outer zone of the medulla. The figure shows the approximate concentrations of the major solutes contributing to the osmolar gradient in the vasa recta and the renal medullary ISF.
Thus, although the osmolarity of the blood leaving the ascending limb of the vasa recta is slightly more concentrated than the blood entering the descending limb of the vasa recta it is much less concentrated than the blood and the ISF at the tips of the hair pin loops of the vasa recta and the long loops of Henle.