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