urine pH 4,5 - 8

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May 2, 2022
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Hi ! Why does urine have maximum (and minimum) pH? :help:
I get that H-ATPases establish a H+ gradient of 1000:1 so the minimum pH indeed is 7,4 - log 1000 = approx. 4,5, but what I can't comprehend is how to actually calculate the maximum pH?
Thanks in advance:biglove:

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I don't know if OP is still active but in case anyone else needs this information:

4.5~8 is the general range for urine pH. Generally it stays within this range because of limitations of the kidney, but there are some exceptions. Namely, a urea-splitting organism infection will generate ammonia from urea and cause Urine pH to exceed 8.

The acid-base equilibrium of urine is established by H+ ATPase (secretion into urine) and HCO3- ATPAse (reabsorption from urine). The minimum and maximum mentioned are based on the actions of these two functioning features.

The lower limit (4.5) is usually caused by some form of acidosis and full function of the H+ ATPase function; and the upper limit is a combination of H+ ATPase failure and HCO3- ATPase failure (which can happen for many reasons).

Although I couldn't find the specific numbers for the HCO3- ATPase, I presume that calculating the maximum Urine pH would be some equilibrium calculation assuming both the H+ ATPase and HCO3- ATPase fail.
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Thank you ! This makes a lot of sense.

I found this scheme so I'll post it here + I'll add some of my thoughts, please correct me if I'm overexaggerating it and if it's wrong:
acid-base balance.jpg

If the HCO3- ATPase fails, does that mean that those 4,32 mole HCO3-/day shown in picture can't be reabsorbed and they're instead secreted in urine? If so, when you put those numbers in a formula 7,4 + log(4,32), you'll get 8,04 and that's (mathematically) the amount we're looking for, but is it physiologically correct?

And thanks again for helping, it means a lot !
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Nephrology definitely isn't my specialty though so anyone please feel free to correct me but I believe that would be more or so the correct reasoning, especially given that the numbers seem to match up