The kidney is shaped like a kidney bean. When sliced along its long axis on the midline, its key anatomical features are revealed (Figure 51.9B). The ureter and the renal artery and renal vein enter the kidney on its concave (punched-in) side. Inside the kidney, the ureter is continuous with structures that envelop kidney tissues called renal pyramids. The renal pyramids make up the internal core, or medulla, of the kidney. The medulla is covered by an outer layer, or cortex, that has a granular appearance. Between the cortex and the medulla, the renal artery divides into the many arterioles that serve the nephrons. In this same region, the renal vein collects blood from the many venules that drain the peritubular capillaries.

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The organization of nephrons within the kidney is very regular. All of the glomeruli with their Bowman’s capsules are located in the cortex. The initial segments of the renal tubules that come from the Bowman’s capsules are called the proximal convoluted tubules—“proximal” because they are closest to the glomerulus, and “convoluted” because they are twisted (Figure 51.9C). All of the proximal convoluted tubules are located in the cortex.

At the point at which the proximal convoluted tubule enters the medulla, it becomes thin, straight, and descends directly down toward the tip of a renal pyramid. In the medulla the tubule makes a hairpin turn and ascends back to the cortex, forming what is called a loop of Henle. Some nephrons have longer loops of Henle than others. Some 20–30 percent of human nephrons that have glomeruli deep in the cortex (i.e., near the border with the medulla) have long loops of Henle that go deep into the medulla. Nephrons that have glomeruli farther up in the cortex generally have short loops of Henle that descend only a short distance into the medulla. As you will see, the long loops are the critical adaptation of the mammalian nephron that enables the kidney to concentrate the urine.

The ascending limb of the loop of Henle becomes the distal convoluted tubule when it reaches the cortex—“distal” because it is farther from the glomerulus. The distal convoluted tubules of many nephrons join a common collecting duct in the cortex. The collecting ducts descend back down through the renal pyramid, parallel to and past the tips of the loops of Henle, and empty into a funnel-shaped structure called the pelvis. Divisions of the pelvis that surround each renal pyramid join together to leave the kidney as the ureter (see Figure 51.9B).

The organization of the blood vessels of the kidney closely parallels the organization of the nephrons (see Figure 51.9C). Smaller arteries branch from the renal artery and radiate into the cortex, forming the afferent arterioles that carry blood to each glomerulus. Each glomerulus is drained by an efferent arteriole that gives rise to the peritubular capillaries, most of which surround the proximal and distal convoluted tubules. The intimate associations of the glomerular and peritubular capillaries with the renal tubules enable exchanges between the blood and the specialized regions of the tubules.

Some of the peritubular capillaries run into the medulla in parallel with the loops of Henle and the collecting ducts, forming a vascular network called the vasa recta. All of the peritubular capillaries from a nephron join back together into a venule that joins with venules from other nephrons and eventually leads to the renal vein. As you will see, the concentrating ability of the mammalian kidney depends on water reabsorption in the renal medulla, and the vasa recta are the avenue by which that water gets out of the renal medulla and back into the circulation.

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