UWorld question explanation

[SeventhSon]

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In a renal phys question, an explanation had a graph with TF/P for many substances at the end of the proximal tubule. It had inulin having a higher TF/P at the end of the proximal tubule than creatinine. Is this correct? Creatinine is slightly secreted by peritubular capillaries, so wouldn't it have a higher TF/P than inulin at the end of the PT?

thanks..

 

[TAbrown]

The only explanation that I can come up with is this:

TF/P = substance in the proximal tubular fluid/concentration in the plasma

Theoretically, inulin's plasma concentration should decrease by 20% with each passing through the kidneys, since it's not being made in the body.

Creatinine, which is continually being made, should not have this plasma decrease.

Thus, with inulin, the ratio of inulin in the proximal tubular fluid to concentration of inulin in the blood should continually increase along the length of the proximal tubule.
Creatinine's TF/P still increases along the P.T. too but is not as dramatic as that of inulin, since creatinine is continually being made by muscles (plasma concentration of creatinine is not decreased nor increased (constant)).