The vasa recta are the long, straight blood vessels that extend into the medulla of the kidney and parallel to the course of the loop of Henle. There is a distinct difference in solute concentration between the fluid in the loop of Henle and the plasma concentration in the vessels of the vasa recta. This mechanism is called counter current exchange; it results in the reabsorption of water and solutes between the fluid in the loop of Henle and blood.