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Regeneration of Bicarbonate, the Role of Titratable Acid:
Definition of Titratable Acid

The regeneration of HCO3- occurs by the secretion of a H+ that reacts, not with HCO3-, but with nonbicarbonate buffers present in the glomerular filtrate. This results in the acidification of the tubular fluid. The H+ secretory mechanism is rate limited and in the collecting ducts the pH may fall as low as low as 4.4 when maximal rates of H+ secretion are achieved. At this pH the rate of H+ back diffusion equals the rate of H+ secretion. The urine is acidified in this process and the amount of H+ secreted in the regeneration of HCO3- can be estimated by measuring the amount of NaOH required to titrate the urine back to pH 7.4, hence the term titratable acid.
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