a. Na+-glucose cotransport in the proximal tubule reabsorbs glucose from tubular fluid into the blood. There are a limited number of Na+-glucose carriers.
b. At plasma glucose concentrations less than 250 mg/dl, all of the filtered glucose can be reabsorbed because plenty of carriers are available; in this range, the line for reabsorption is the same as that for filtration.
c. At plasma glucose concentrations greater than 350 mg/dl, the carriers are saturated. Therefore, increases in plasma concentration above 350 mg/dl do not result in increased rates of reabsorption. The point at which the carriers are saturated is the Tm.
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