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OBJECTIVES
After studying this chapter, you should be able to:
Describe the morphology of a typical nephron and its blood supply.
Explain autoregulation and how it regulates renal function.
Define glomerular filtration rate (GFR), describe how it can be measured, and list the major factors that alter GFR.
Outline how each tubular segment of the nephron handles Na+ and water.
Explain tubular reabsorption and secretion of glucose and K+.
Describe how the kidney operates to produce hypertonic or hypotonic urine.
Define the major classes of diuretics and explain where each operates to increase urine flow.
Describe the voiding reflex and draw a cystometrogram.
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Each individual renal tubule and its glomerulus is a unit (nephron). The size of the kidneys between species varies, as does the number of nephrons they contain. Each human kidney has approximately 1 million nephrons. The specific structures of the nephron are shown in diagrammatic manner in Figure 37–1.
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The glomerulus, which is about 200 µm in diameter, is formed by the invagination of a tuft of capillaries into the dilated, blind end of the nephron (Bowman’s capsule). The capillaries are supplied by an afferent arteriole and drained by the efferent arteriole (Figure 37–2), and it is from the glomerulus that the filtrate is formed. The diameter of the afferent arteriole is larger than the efferent arteriole. Two cellular layers separate the blood from the glomerular filtrate in Bowman’s capsule: the capillary endothelium and the specialized epithelium of the capsule. The endothelium of the glomerular capillaries is fenestrated, with pores that are 70–90 nm in diameter. The endothelium of the glomerular capillaries is completely surrounded by the glomerular basement membrane along with specialized cells called podocytes. Podocytes have numerous pseudopodia that interdigitate (Figure 37–2) to form filtration slits along the capillary wall. The slits are approximately 25 nm wide, and each is closed by a thin membrane. The glomerular basement membrane, the basal lamina, does not contain visible gaps or pores. Stellate cells called mesangial cells are located between the basal lamina and the endothelium. They are similar to cells called pericytes, which are found in the walls of capillaries elsewhere in the body. Mesangial cells are especially common between two neighboring capillaries, and in these locations the basal membrane forms a sheath shared by both capillaries (Figure 37–2). The mesangial cells are contractile and play a role in the regulation of glomerular filtration. Mesangial cells secrete the extracellular matrix, take up immune complexes, and are involved in the progression of glomerular disease.
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