- The amount of H+ excreted as titratable acid depends on the amount of urinary buffer present (usually HPOf-) and the pK of the buffer.
(1) H+ and HC03" are produced in the cell from C02 and H20. The H+ is secreted into the lumen by an H+-ATPase, and the HC03~ is reabsorbed into the blood ("new" HC03~). In the urine, the secreted H+ combines with filtered HP04 2 to form H2P04~, which is excreted as titratable acid.
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