Going with the flow: updating old techniques to gain insight into regional kidney hemodynamics

Renal blood flow occurs both in series and parallel. While all the blood entering the kidney first enters the renal cortex, two distinct populations of nephrons give rise to remarkably different post‐glomerular capillary circulations. Superficial and mid‐cortical nephrons give rise to the cortical peritubular capillary network, a web of capillaries that supplies the cortical parenchyma and supports reabsorption of the bulk of the glomerular filtrate. In contrast, the majority of juxta‐medullary nephrons, whose afferent arterioles arise near the corticomedullary junction, give rise to vascular bundles containing descending vasa recta capillaries that supply blood to the renal medulla. Some of these descending vasa recta traverse all the way to the tip of the papilla, before looping around to giving rise to ascending vasa recta capillaries. Descending vasa recta on the periphery of these vascular bundles give rise to the dense plexus of capillaries that supply the medullary parenchyma. The relatively low rate of perfusion (less than 10% of total renal blood flow) and the looped structure of the medullary circulation limit the washout of solutes including NaCl and urea in this region, maximizing the ability to form concentrated urine