Question

Substance X is found in the plasma well below the Tmax. This indicates:

1) Whatever amount of substance X was filtered, was reabsorbed.
2) Substance X is like all other physiological ions and compounds and in a healthy person, is always below Tmax
3) None of these
4) Transporters to get X across the apical membrane of proximal tubule epithelial cells must be primary active transporters
5) Substance X would readily be found in the urine

Answer 1

Substance X being below its Tmax in the plasma means that it is reabsorbed efficiently by the kidneys and not excreted in the urine, as the renal transporters are not saturated.

Step-by-step explanation:

If Substance X is found in the plasma well below the Tmax (transport maximum), this indicates that the renal transporters responsible for reabsorbing this substance from the filtrate back into the blood are not saturated. Consequently, the substance continues to be reabsorbed effectively and will not readily appear in the urine because the reabsorption capacity has not been exceeded. Therefore, the expectation is that the urine will contain little to none of Substance X, assuming that its presence in the plasma remains below the Tmax.

For a substance with renal clearance that has a set Tmax, being below that threshold means that the kidneys can reabsorb the substance more effectively from the renal tubules. When plasma concentration of a substance exceeds the Tmax, the excess is excreted in urine because reabsorption becomes limited. Keeping the concentration below Tmax is a protective mechanism to prevent waste of valuable substances such as glucose.