Blood Supply

Although the kidneys account for only 0.4% of the body weight, they receive about 21% of the cardiac output (the renal fraction). This attests to their importance in controlling blood volume and composition.

The larger divisions of the renal circulation are shown in figure 23.7a. Each kidney is supplied by a renal artery (occasionally two or more) arising from the aorta. Just before or after entering the hilum, the renal artery divides and eventually gives rise to a few interlobar arteries. One interlobar artery penetrates each renal column and travels between the pyramids to the corticomedullary junction, the boundary between the cortex and medulla. Here it branches again to form the arcuate arteries, which make a sharp 90° bend and travel along the base of the pyramid. Each arcuate artery gives rise to several interlobular arteries, which pass upward into the cortex.

The finer branches of the renal circulation are shown in figure 23.5b. As an interlobular artery ascends through the cortex, a series of afferent arterioles arise from it like the limbs of a pine tree. Each afferent arteriole supplies blood to one nephron and ends in the glomerulus described earlier. The glomerulus is drained by an efferent arteriole.

The afferent and efferent arterioles penetrate one side of the glomerular capsule together. Just outside the capsule, they contact the first part of the distal convoluted tubule and with it, form a juxtaglomerular (JUX-tuh-glo-MER-you-lur) apparatus. This is a device that enables a nephron to monitor and stabilize its own performance and compensate for fluctuations in blood pressure. It will be described in detail when we consider renal autoregulation.

The efferent arteriole leads next to a plexus of per-itubular capillaries, named for the fact that they form a network around the renal tubules. Blood flows from the peritubular capillaries to, in order, the interlobular veins, arcuate veins, interlobar veins, and renal vein, which travel parallel to the arteries of the same names. The renal vein leaves the hilum and drains into the inferior vena cava.

The renal medulla receives only 1% to 2% of the total renal blood flow, supplied by a network of vessels called the vasa recta.11 In the juxtamedullary nephrons, the efferent arterioles descend immediately into the medulla and give rise to the vasa recta instead of giving rise to peritubular capillaries. The capillaries of the vasa

10juxta = next to

" vasa = vessels + recta = straight

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

886 Part Four Regulation and Maintenance

886 Part Four Regulation and Maintenance

Flow Chart Renal Circulation
Figure 23.7 Renal Circulation. (a) The larger blood vessels of the kidney. (b) Flow chart of renal circulation. The pathway through the vasa recta (instead of peritubular capillaries) applies only to the juxtamedullary nephrons.

recta lead into venules that ascend and empty into the arcuate and interlobular veins. The route of renal blood flow is summarized in figure 23.7b.

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