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The kidneys are located in the retroperitoneum and weigh 130–150 g each. The surface is smooth and invested in a capsule, which in turn is surrounded by perinephric fat and Gerota's fascia.
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The anatomic unit of the kidney is the nephron, which is composed of the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting tubule. Each kidney contains approximately 1 million nephrons.
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The glomerulus (Figure 47-1) is composed of (1) an afferent and efferent arteriole; (2) intervening capillaries lined by endothelial cells (glomerular tuft); (3) the outer surface of the capillaries, which is covered by epithelial cells (podocytes), continuous with the epithelium of Bowman's space and the proximal tubule; (4) the mesangium, composed of mesangial cells and matrix; and (5) the basement membrane.
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Glomerular Filtration
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Ultrafiltration of plasma occurs in the glomerular capillaries, driven by the hydrostatic pressure head in the arteriolar end of the capillary (Table 47-1). The normal glomerular filtration membrane forms a barrier to molecules in the plasma that is based largely on (1) capillary endothelial cell junctional fenestrations, which normally restrict the passage of molecules with molecular weights greater than 70,000 (Figure 47-1); (2) basement membrane polyanions such as acidic proteoglycans, which impart a negative charge that selectively prevents the filtration of anionic particles such as albumin; and (3) the epithelial cell, which acts as a physical barrier.
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The glomerular filtration rate (GFR) is normally about 120 mL/min. It may be measured accurately by the clearance of exogenous inulin (inulin clearance test) or endogenous creatinine (creatinine clearance test).
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Eighty percent of the glomerular ultrafiltrate volume is absorbed actively in the proximal tubule. Potassium, glucose, and amino acids are completely reabsorbed.
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Approximately 30 mL/min of isotonic fluid is delivered to the loop of Henle. The loop of Henle passes down into the medulla and establishes a countercurrent exchange mechanism that causes a progressive increase in tonicity from the cortico-medullary junction to the tip of the papillae. This mechanism is enhanced by the action of the vasa recta that accompany ...