Renal Acid Excretion
The process of renal acid excretion is complex. In order to conceptualize this process, lets consider the follow equation:
HCl + NaHCO3 ↔ NaCl + H2CO3 ↔ CO2 + H2O + NACl.
The above equation represents the process of buffering of the daily nonvolatile acid load. Buffering minimizes the effect that strong acids such as HCl would have on the pH. Nonetheless the pH will be affected if the bicarbonate lost in this process is not regenerated, because as we will learn; loss of bicarbonate from the ECF lowers the extracellular pH, leading to acidosis. One way to regenerate the lost bicarbonate would be for the kidney to reverse the above equation, and excrete HCl in the urine as free H+ ions. Unfortunately this would require a urine pH of 1.0, an impossible task since the minimum attainable urine pH is 4.0 to 4.5.
In order to maintain acid base balance, the kidney must accomplish two tasks:
1) Reabsorption of all filtered bicarbonate
2) Excrete the daily acid load
The kidney achieves these three tasks effectively via the processes of hydrogen secretion, bicarbonate reabsorption and excretion of hydrogen ions with urinary buffers( titratable acids and ammonium) .
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