Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Kidney

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The glomerular basement membrane consists of a homogeneous acellular meshwork of collagen fibers, glycoproteins, and mucopolysaccharides (Fig. 24-5C). Because the endothelial and epithelial layers of the glo- merular capillary have porous structures, the basement membrane determines the permeability of the capillary membrane. The spaces between the fibers that make up the basement membrane represent the pores of a filter and determine the size-dependent permeability barrier of the glomerulus. The size of the pores in the basement membrane normally prevents red blood cells and plasma proteins from passing into the filtrate. Alterations in the structure and function of the glomerular basement mem- brane are responsible for the leakage of proteins and blood cells into the filtrate that occurs in many forms of glomerular disease. The visceral layer of the Bowman capsule is com- posed of epithelial cells that are highly modified to perform a filtering function. These large cells, called podocytes, have numerous finger-like processes that completely encircle the outer surface of the capillaries (see Fig. 24-5C). The elongated spaces between the inter- digitating foot processes, called filtration slits, function as a size-selective filter that prevents proteins and mac- romolecules that have crossed the basement membrane from entering Bowman space. Another important component of the glomerulus is the mesangium. In some areas, the capillary endothe- lium and the basement membrane do not completely surround each capillary tuft. Instead, the mesangial cells, which lie between the tufts, provide support for the glomerulus in these areas (Fig. 24-5B). The mesan- gial cells produce an intercellular substance similar to that of the basement membrane. This substance cov- ers the endothelial cells where they are not covered by basement membrane. The mesangial cells also exhibit contractile properties and are thought to contribute to the regulation of blood flow through the glomerulus; possess phagocytic properties and remove macromo- lecular materials that enter the intercapillary spaces; and are capable of proliferation. Although the mesan- gial area is normally narrow and contains only a small number of cells, mesangial hyperplasia and increased mesangial matrix develop in many forms of glomerular disease. Tubular Components of the Nephron The nephron tubule is divided into several segments: the proximal tubule, which drains Bowman capsule; a thin looped structure, the loop of Henle; a distal coiled por- tion, the distal convoluted tubule; and the final segment, the collecting tubule (see Fig. 24-4A). The filtrate passes through each of these segments before reaching the pel- vis of the kidney. The proximal tubule is a highly coiled structure that lies in the cortex of the kidney and dips toward the renal pelvis to become the loop of Henle. The loop consists of a descending and ascending limb. The ascending loop of Henle returns to the region of the renal corpuscle, where it becomes much thicker, and is referred to as the

thick ascending limb . Beyond the thick ascending limb of Henle is the distal convoluted tubule, which like the proximal tubule lies in the renal cortex. The distal con- voluted tubule is divided into two segments: the diluting segment and the late distal tubule. The late distal tubule fuses with the collecting tubule The collecting tubule is divided into two segments: the cortical tubule and the medullary collecting tubule. The initial parts of 8 or 10 cortical collecting tubules join to form a single large tubule that moves down into the medulla to become the medullary collecting tubule. Throughout its course, the tubule is composed of a single layer of epithelial cells resting on a basement membrane. The structure of the epithelial cells varies with tubular function. The cells of the proximal tubule have a fine, villous structure that increases the surface area for reabsorption; they also are rich in mitochon- dria, which support active transport processes. The epi- thelial layer thins in segments of the loop of Henle and has few mitochondria, indicating minimal metabolic activity and reabsorptive function. ■■ The kidneys are paired, bean-shaped organs that lie outside the peritoneal cavity in the posterior abdomen, one on either side of the vertebral column. On longitudinal section, a kidney can be divided into an outer cortex and an inner medulla.The cortex and medulla are composed of nephrons, blood vessels, and nerves. ■■ The nephrons, which are the functional units of the kidney, consist of a renal capsule (glomerulus and Bowman capsule) where blood is filtered, and tubular structures (proximal tubule, loop of Henle, distal tubule, and collecting tubule) where water, electrolytes, and soluble nutrients are reabsorbed into the blood and waste products are secreted from the blood into the tubular fluid. ■■ The nephrons are supplied by two capillary systems, a glomerulus and peritubular capillary network.The glomerulus is a unique, high- pressure capillary filtration system located between two arterioles—the afferent and the efferent arterioles.The peritubular capillaries, which originate from the efferent arteriole and surround the tubules, are low-pressure vessels adapted for reabsorption rather than filtration. SUMMARY CONCEPTS

Urine Formation Urine formation involves the filtration of blood by the glomerulus to form an ultrafiltrate of urine, the selective reabsorption by the renal tubules of substances needed