Quantitative renal excretion of acid base
To balance acid-base, net acid excretion must be equal to the amount of non-volatile acid produced in the body. In acidosis, net acid excretion increases markedly, especially due to increased secretion of NH4 +, thereby removing acid from the blood.
Based on these principles, we can quantify renal acid excretion or the amount of absorption or elimination of HCO3 А from the blood as follows:
Bicarbonate excretion is calculated as the volume of urine volume with urinary HCO3 concentration. This number indicates the rapid removal of HCO3 А from the blood (similar to adding H + to the blood). In acidosis, the loss of HCO3 А helps to return the pH to normal.
The amount of new HCO3 А deposited into the blood at any time is equal to the amount of H + secreted by the end of the renal tubules with the non-bicarbonate urinary system. As discussed previously, the non-bicarbonate urinary system is composed of NH4 + and phosphate. Therefore, the amount of HCO3 А added to the blood (and H + excreted by NH4 +) is calculated by measuring the amount of NH4 + excretion (urine volume multiplied by the NH4 + concentration).
The remainder of the non-bicarbonate, non-NH4 + buffer in the urine is calculated by the determination of a value called a titratable acid. The amount of titratable acid in urine is calculated in urine titrated with a strong base, such as NaOH, with pH 7.4, normal blood pH, and glomerular dialysis solution pH. Therefore, the mEq number of NaOH is needed to bring the urinary pH to 7.4 equal to the amount of mEq H + added to the tubular fluid, combined with phosphate and other buffers. Quantitative acid titration does not include H + in NH4 + since the pK of NH3-NH4 + reaches 9.2, and titration with NaOH to pH 7.4 does not lose H + in NH4 +.
Therefore, the net renal excretion of acid can be calculated by:
Acid excreted net = NH4 + excreted + acid titratable urinary - HCO3- excreted.
The reason for the excretion of HCO3 А is the loss of HCO3 А like the addition of H + to the blood. To balance acid-base, net acid excretion must be equal to the amount of non-volatile acid produced in the body. In acidosis, net acid excretion increases markedly, especially due to increased secretion of NH4 +, thereby removing acid from the blood. The amount of net acid excreted is also equal to the number of net HCO3 А added to the blood. Therefore, in acidosis, more HCO3 А is added to the blood than NH4 + and titratable uric acid is excreted.
In alkali, titratable acid and NH4 + excretion decreased to 0, while increased excretion of HCO3 А. Therefore, the amount of net acid is secreted negative, meaning that there is a net loss of HCO3 А from the blood (the same as adding H + to the blood) and no new HCO3 А by the kidneys.