The kidneys help to regulate the blood pH by excreting H+ in the urine and by reabsorbing bicarbonate. The H+ enters the filtrate in two ways: by filtration through the glomeruli and by secretion into the tubules. Most of the H+ secretion occurs across the wall of the proximal tubule in exchange for the reabsorption of Na+. This exchange is performed by a transport carrier described as "antiport," because it moves the Na+ and H+ in opposite directions (see chapter 6).
Since the kidneys normally reabsorb almost all of the filtered bicarbonate and excrete H+, normal urine contains little bicarbonate and is slightly acidic (with a pH range between 5 and 7). The mechanisms involved in acidification of the urine and reabsorption of bicarbonate are summarized in figure 17.28.
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