 Renal vasa recta

The renal vasa recta are straight, hairpin-arranged blood vessels supplying the renal medulla, especially alongside the long loops of a juxtamedullary nephron. They carry blood, whereas the neighboring Loop of Henle and collecting ducts carry tubular fluid. Keeping these compartments separate is essential to understanding how the kidney returns reabsorbed water to the circulation.^[ref]

Their geometry supports countercurrent exchange. As blood descends into increasingly concentrated surroundings, it can gain solute and lose water; on its return toward the renal cortex, the local differences favor exchange in the opposite directions. Much of the solute taken up on descent can therefore be returned to the medulla instead of being carried away. Exchange helps preserve a gradient; it is distinct from the active transport that helps generate it.

For an idealized accounting example, if descending blood gains 10 units of a solute and returning blood gives 9 units back, its net removal is only 1 unit. These are hypothetical amounts, illustrating retention through exchange rather than a measured percentage in every vessel.

Direct renal micropuncture supplied an important spatial test in 1959: fluid sampled from loop bends, adjacent collecting ducts and renal vasa recta at the same deep medullary level was similarly hyperosmotic. The vascular route thus shared the concentrated environment even though its contents were distinct from tubular fluid.^[ref]