The capillaries that accompany the long loops of Henle on their trip to the medulla and back are called the vasa recta. These capillaries also form long loops and run in parallel to the loop of Henle. The arterial and venous ends of the capillaries run in parallel with each other. This allows a process called countercurrent exchange to occur. Countercurrent exchange allows exchange of solute between the ascending and descending limbs of the capillaries and prevents them from dissipating the medullary concentration gradient. Among the solutes exchanged in this process are oxygen and carbon dioxide.
The vasa recta also have a countercurrent multiplier mechanism that prevents washout of solutes from the medulla, thereby maintaining the medullary concentration.