How does the renal HCO3- reabsorption increase blood pH?

Submitted by HugoBossPT t3_105dbr3 in askscience

Hello, this thing is completely destroying my brain.

HCO3- in renal tubules is "reabsorved" into the circulatory system through a bunch of reactions involving the carbonic anhydrase.

Basically:

- It combines with H+ forming CO2 + H2O, that can enter the tubular cells;

- The reverse reaction happens inside these cells: forming HCO3- + H+. The HCO3- is reabsorved into the blood, and the H+ goes back to the renal tubules to restart the cycle with another HCO3-.

I dont get how this is supposed to eliminate H+. Isn't the H+ net equal to 0 ?

Also, how does this make the pH higher? If it reabsorbs the HCO3-, then the H+ used wont be eliminated and vice versa, so I don't see how any of these actions (one or another) could make the pH higher.

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Sorry if it is confusing and thank you.

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Comments

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Aggressive-Visual-44 t1_j3aj5jg wrote

Your body secretes H+ and HCO3- through the kidneys. They combine to form H2CO3 (carbonic acid). An enzyme, carbonic anhydrase, splits the carbonic acid into H2O and CO2 in the renal tubular lumen and this water and carbon dioxide enter the renal tubular cell passively. This is because carbon dioxide is lipid soluble and can easily pass through the cell membrane. Water can also move easily across the cell membrane.

Once the H2O and CO2 are inside the renal tubular cell, carbonic anyhydrase acts on them again to form carbonic acid. This carbonic acid splits into H+ and HCO3- again. Now the H+ ion is excreted from the renal tubular cell through a ion exchanger known as the Na-H exchanger where the H+ is exchanged for Na+ in the renal tubular lumen.

Thus, the same H+ ion is used for reabsorption of HCO3- and Na+.

I hope this helps.

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uh-okay-I-guess t1_j3ehodi wrote

It doesn't directly eliminate H^+ from the body. As you said, there is net zero transport of H^+. Instead, a base (bicarbonate) is reabsorbed into the body.

You should not believe that every bicarbonate ion actually reacts with an H^+ ion -- there is a complex buffered equilibrium. But each reabsorbed bicarbonate shifts that equilibrium slightly toward the alkaline side, because it has the capacity to accept an H^+.

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HugoBossPT OP t1_j3h0bm6 wrote

That makes sense. So some ventilation problem that would cause CO2 levels to rise would shift the equilibrium to the H+ (and HCO3-) side, lowering pH. To compensate, the body would reabsorb as much HCO3- as possible to "reshift" the equilibrium to the original state. Am I thinking right? Thank you!!

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uh-okay-I-guess t1_j3k2c1m wrote

Yes, the kidney will retain bicarbonate, but unfortunately, under normal conditions the kidney already reabsorbs close to 100% of bicarbonate. So unfortunately this mechanism cannot be increased enough to fully compensate for respiratory acidosis.

The kidney has other methods to indirectly raise the pH, like excreting ammonium (which would otherwise be reacted with bicarbonate in the liver). Other acids can also be excreted. However, renal compensation for respiratory acidosis takes days. The lungs are much faster at removing acid from the body than the kidney is.

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