Once deposited in the interstitium, some urea drifts from the inner medulla and is secreted back into the proximal tubule and loop of Henle. By reentering the tubular fluid in this manner, urea is returned to the inner medullary collecting duct to once again be reabsorbed. This process, known as urea recycling, tends to minimize urea depletion from the inner medulla.

The final elements that need to be added to this model are the capillaries of the vasa recta, which are permeable to water. If these vessels passed straight through the interstitium (Panel 7), osmotic pressure would draw out plasma and dilute the concentration gradient. Instead, the capillaries turn back upon themselves (Panel 8), and thus water that effluxes from the descending capillaries is reabsorbed in the ascending capillaries. This process is known as countercurrent exchange.

The blood leaving the medulla, however, does not completely reabsorb all of the effluxed plasma. Thus outgoing blood is slightly hyperosmotic compared with incoming blood. As a result, the anatomic confguration of the vasa recta minimizes, but does not completely prevent, solute loss from the medulla. These losses are also small, because the blood flow to the medulla is very low. Release of ADH further constricts the vasa recta capillaries, ensuring maintenance of the high interstitial concentrations required for maximal urine concentration.