In: Anatomy and Physiology
If you wanted to determine the tubular maximum for glucose reabsorption, and you could measure amount of glucose in serum and urine, how would you proceed?
We know that glomerular filtration rate = 125 mL/min and urine production rate = 1 mL/min
Normally all the glucose filtered through glomerulus is reabsorbed such that glucose is absent in urine. So, we have to artificially increase the plasma glucose concentration such that the filtered glucose concentration exceeds the maximum glucose tubular reabsorption rate so that glucose appears in urine.
Assume plasma glucose concentration is increased to 3 mg/mL. Then, rate of filtration of plasma glucose = 3 X 125 = 375 mg/min
Due to osmotic diuretic effect of glucose, urine production increases to 3 mL/min. Assume urine glucose concentration to be 25 mg/mL. Then, rate of excretion of urine glucose = 3 X 25 = 75 mg/min
Therefore, from 375 mg/min glucose filtered at glomerulus, glucose is excreted out at a rate of 75 mg/min.
Hence, tubular maximum for glucose reabsorption is given by,
Tmax. = rate of plasma glucose filtration - rate of urine glucose excretion = 375 - 75 = 300 mg/min