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Each segment of the nephron—proximal convoluted tubule (PCT), thick ascending limb of the loop of Henle (TAL), distal convoluted tubule (DCT), and cortical collecting tubule (CCT)—has a different mechanism for reabsorbing sodium and other ions. The subgroups of the sodium-excreting diuretics are based on these sites and processes in the nephron. Several other drugs alter water excretion predominantly. The effects of the diuretic agents are predictable from knowledge of the function of the segment of the nephron in which they act.
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The kidney filters plasma water and solutes at the glomerulus at a very high rate (180 L/day) and must recover a significant percentage of most of these substances before excretion in the urine. The major transport mechanisms for the recovery of ions and water in the various segments of the nephron are shown in Figure 15–1. Because the mechanisms for reabsorption of salt and water differ in each of the 4 major tubular segments, the diuretics acting in these segments have differing mechanisms of action. Most diuretics act from the luminal side of the membrane. An exception is the aldosterone receptor antagonist group (eg, spironolactone and eplerenone); these drugs enter the collecting tubule cell from the basolateral side and bind to the cytoplasmic aldosterone receptor.
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