My question concerns the differential treatment of bicarbonate versus other buffer anions in human acid-base physiology. In Guyton (2021)'s physiology textbook, we see the following argument:

Because $\ce{HCO3−}$ must react with a secreted $\ce{H+}$ to form $\ce{H2CO3}$ before it can be reabsorbed, 4320 mEq of $\ce{H+}$ must be secreted each day just to reabsorb the filtered $\ce{HCO3−}$. Then, an additional 80 mEq of $\ce{H+}$ must be secreted to rid the body of the nonvolatile acids produced each day for a total of 4400 mEq of $\ce{H+}$ secreted into the tubular fluid each day. (p. 410)

Similar passages can be found in Boron (2017)'s textbook:

(...) $\ce{HCO3−}$ reabsorption does not represent net $\ce{H+}$ excretion into the urine. It merely prevents the loss of the filtered alkali. (p. 823)

In our example, the kidneys secrete 4390 mmol/day of $\ce{H+}$ into the tubule lumen. The kidneys use most of this secreted acid—4320 mmol/day or ~98% of the total—to reclaim filtered $\ce{HCO3−}$. The balance of the total secreted $\ce{H+}$, 70 mmol/day, the kidneys use to generate new $\ce{HCO3−}$. (p. 825)

On the other hand, as tubule cells excrete $\ce{H+}$ into the lumen, some titrate $\ce{HPO4^2-}$ to form $\ce{H2PO4-}$. This is viewed as net acid excretion, instead of reclamation of filtered $\ce{HPO4^2-}$.

Mathematically, the form of the net acid excretion formula expresses this differential treatment:

$\rm NAE = TA + [NH_4^+] - [HCO_3^-]$

Whereas any amount of filtered $\ce{HCO3−}$ contributes negatively to NAE, since TA is defined to be the amount of base necessary to titrate urine back to pH 7.4, the amount of $\ce{HPO4^2-}$ and $\ce{H2PO4-}$ in the initial filtrate do not affect NAE (assuming that pH of the initial filtrate is 7.4). What matters is the additional amount of titration done.

Why the difference? Any amount of $\ce{HPO4^2-}$ filtered would also mean a loss of buffering capacity of blood. Therefore, shouldn't the formula instead be $\rm NAE = [NH_4^+] - [HCO3^-] - [HPO_4^{2-}]$?

One could argue that since urine contains $\ce{H2PO4-}$ as well as $\ce{HPO4^2-}$, and since $\ce{H2PO4-}$ is a Bronsted-Lowry acid, we must account for the effects of both ions. When the two effects precisely neutralize each other, pH should equal 7.4. Acid is excreted only if phosphate is protonated more than the degree of protonation at pH 7.4. Therefore, instead of subtracting $\rm[HPO_4^{2-}]$, we subtract the difference $\rm[HPO_4^{2-}] - [HPO_4^{2-}]_{pH=7.4}$, which equals -TA by definition.

But then why can't this argument be applied to bicarbonate? Shouldn't we subtract the difference $\rm[HCO_3^-] - [HCO_3^-]_{pH=7.4} = [HCO_3^-] - 24$?



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