The tiny working units of the kidneys are the nephrons (Fig. 1). Each of these microscopic structures is basically a single tubule coiled and folded into various shapes. At the beginning of the tubule is the cup-shaped Bowman capsule, which is part of the blood-filtering device of the nephron. The tubule then folds into the proximal convoluted tubule, straightens out to form the loop of Henle, coils again into the distal convoluted tubule, and then finally straightens out to form a collecting tubule.

FIGURE 1. A nephron and its blood supply. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's  The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

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