BLOOD SUPPLY TO THE NEPHRONS

The kidneys receive an enormous amount of blood relative to their mass. Blood enters each kidney via a renal artery, which then divides into progressively smaller branches: interlobar, arcuate, and finally interlobular arteries (usually called cortical radial arteries because they radiate outward toward the kidney surface). As each of the interlobular arteries projects toward the outer kidney surface, a series of parallel arterioles branch off at right angles, each of which leads to a glomerulus. These are called afferent arterioles. Note that these arteries and glomeruli are found only in the cortex, never in the medulla.
Normally about 20% of the plasma (and none of the erythrocytes) entering the glomerulus is filtered from the glomerulus into Bowman’s capsule, leaving the remaining 80% to flow on to the next vascular segment. In most organs, capillaries recombine to form the beginnings of the venous system, but the glomerular capillaries instead recombine to form another set of arterioles called the efferent
arterioles. Thus, blood enters each glomerulus through a single afferent arteriole and leaves via a single efferent arteriole at the vascular pole of Bowman’s capsule. The afferent and efferent arterioles both penetrate Bowman’s capsule on the same side, with the thick ascending limb of the nephron that originated from that capsule passing between and in contact with each arteriole. The efferent arterioles soon subdivide into a second set of capillaries. These are usually the peritubular capillaries, which are profusely distributed throughout the cortex. The peritubular capillaries then rejoin to form the veins by which blood ultimately leaves the kidney.
The medulla receives much less blood than does the cortex, and in a quite different manner. There are no glomeruli in the medulla. In contrast to most efferent arterioles in the cortex, those from juxtamedullary glomeruli do not branch into peritubular capillaries, but rather descend downward into the outer medulla, where they divide many times to form bundles of parallel vessels that penetrate deep into the medulla. These are called descending vasa recta (Latin recta for “straight” and vasa for “vessels”). Although it is still uncertain, a small fraction of the descending vasa recta may branch off from the cortical radial arteries before the glomeruli, not after. The vasa recta on the outside of the vascular bundles “peel off” and give rise to interbundle plexi of capillaries that surround Henle’s loops and the collecting ducts in the outer medulla. Only the center-most vasa recta supply capillaries in the inner medulla; thus, limited blood flows into the papilla. The capillaries from the inner medulla re-form into ascending vasa recta that run in close association with the descending vasa recta within the vascular bundles. The structural and functional properties of the vasa recta are rather complex. The beginnings of the descending vasa recta are like arterioles, with pericytes containing smooth muscle in their walls, but become more capillary like as they descend. The ascending vasa recta have a fenestrated endothelium like that found in the glomerular capillaries. Therefore, the vasa recta, in addition to being conduits for blood, also participate in exchanging water and solutes between plasma and interstitium. The whole arrangement of descending and ascending blood flowing in parallel has major significance for the formation of both concentrated and diluted urine because plasma and medullary interstitial constituents exchange between descending and ascending vessels.