As the urine flows along the tubule, from Bowman's capsule on through the collecting ducts, three processes occur: filtered HCO3— is reabsorbed, titratable acid is formed, and ammonia is added to the tubular urine. All three processes involve H+ secretion (urinary acidification) by the tubular epithelium. The nature and magnitude of these processes vary in different nephron segments. Figure 25.5 summarizes measurements of tubular fluid pH along the nephron and shows ammonia movements in various nephron segments.
Acidification in the Proximal Convoluted Tubule. The pH of the glomerular ultrafiltrate, at the beginning of the proximal tubule, is identical to that of the plasma from which it is derived (7.4). H+ ions are secreted by the proximal tubule epithelium into the tubule lumen, about two thirds of this is accomplished by a Na+/H+ exchanger and about one third by H + -ATPase in the brush border membrane. Tubular fluid pH falls to a value of about 6.7 by the end of the proximal convoluted tubule (see Fig. 25.5).
The drop in pH is modest for two reasons: buffering of secreted H+ and the high permeability of the proximal tubule epithelium to H +. The glomerular filtrate and tubule fluid contain abundant buffer bases, especially HCO3 —, which soak up secreted H+, minimizing a fall in pH. The proximal tubule epithelium is rather leaky to H+, so that any gradient from urine to blood, established by H+ secretion, is soon limited by the diffusion of H+ out of the tubule lumen into the blood surrounding the tubules
Most of the H+ ions secreted by the nephron are secreted in the proximal convoluted tubule and are used to bring about the reabsorption of filtered HCO3 —. Secreted H+ ions are also buffered by filtered phosphate to form titratable acid. Ammonia is produced by proximal tubule cells, mainly from glutamine. It is secreted into the tubular urine by the diffusion of NH3, which then combines with a secreted H+ to form NH4+, or via the brush border membrane Na+/H+ exchanger, which can operate in a Na+/NH4+ exchange mode.
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