Acidification of the urine also occurs in the connecting tubule and collecting duct. The residual amounts of bicarbonate are reabsorbed, and the pH of the final urine is lowered so that substantial amounts of titrated buffers can be excreted. Protons (H+) are actively secreted by intercalated cells that appear among the principal cells from the midregion of the distal convoluted tubule into the medullary collecting duct, although they are absent in the inner medullary collecting duct. The regulation of proton secretion in the distal nephron is an important determinant of acid-base balance, as is discussed in Chapter 31. Figure 8 shows the luminal surfaces of intercalated cells surrounded by principal cells in the cortical collecting duct.
Intercalated cells acidify the urine by secreting protons in much the same way as the proximal tubule cell. As in most cells, the usual intracellular reaction between H+, HCO—, carbonic acid, CO2, and water is catalyzed by carbonic anhydrase (see Chapter 26, Fig. 9). Thus, CO2 from metabolism within the intercalated cell and diffusing from the plasma provides a ready supply of H+ and HCO— ions within the cell. As in the proximal tubule, protons are then secreted into the lumen by an active transport process, and HCO3— anions selectively diffuse across the basolateral membrane back
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