Physiologists are studying how the plasma concentration of various substances affects the degree to which they are excreted in the urine. During one experiment, a substance freely filtered at the glomerulus is infused intravenously at various rates. The plasma concentration (mg/mL) and urinary excretion rate (mg/min) of the substance are monitored. Assuming a constant glomerular filtration rate (GFR) of 100 mL/min, the following observations are made: Which of the following substances is most likely to demonstrate a similar pattern of filtration and excretion in a healthy adult?

Question

Physiologists are studying how the plasma concentration of various substances affects the degree to which they are excreted in the urine.  During one experiment, a substance freely filtered at the glomerulus is infused intravenously at various rates.  The plasma concentration (mg/mL) and urinary excretion rate (mg/min) of the substance are monitored.  Assuming a constant glomerular filtration rate (GFR) of 100 mL/min, the following observations are made:  Which of the following substances is most likely to demonstrate a similar pattern of filtration and excretion in a healthy adult?

Quizplus · Accepted Answer

11ec160b_3cf5_4173_9249_cd3987cb6f05_MD0001_00 At physiologic levels (preinfusion), the experimental substance is filtered at the glomerulus but completely reabsorbed, and no amount is excreted in the urine. However, as the concentration and filtered load are increased, the substance begins to spill into the urine, suggesting a limit to the rate at which it can be reabsorbed. This rate, called the transport maximum (Tmax) of a substance, is determined by the capacity of transporters available for active reabsorption. The reabsorption pattern is similar to that of glucose, which is filtered in the glomerulus and reabsorbed completely in the proximal tubule under normal serum concentrations. However, once glucose reaches its Tmax, the excess filtered load passes unabsorbed through the tubules. The serum concentration at which glucosuria begins, called the threshold of glucose, is approximately 200 mg/dL. (Choice A) Albumin is a large, polarized plasma protein that is not filtered in a normal, healthy glomerulus, so urinary excretion would remain at 0 mg/min. (Choices B and D) Creatinine and mannitol (an osmotic diuretic) are freely filtered in the glomerulus and are not reabsorbed. As a result, the urinary excretion of these substances is dependent on the glomerular filtration rate and remains equal to the filtered load of the substance regardless of the serum concentration (note that urinary excretion of creatinine is slightly more than the filtered rate due to tubular secretion). (Choice E) Urea is filtered and then reabsorbed by passive diffusion into the peritubular capillaries. Approximately 20%-50% of the filtered load is normally reabsorbed. Urea has no Tmax because it is passively reabsorbed. Educational objective: At normal plasma concentrations of glucose, the renal tubules reabsorb the entire filtered load of glucose because it is below the maximum tubular reabsorption ability (transport maximum of glucose). At higher plasma concentrations, glucose is excreted when the filtered amount exceeds the transport maximum. The serum concentration at which glucosuria begins, called the threshold of glucose, is approximately 200 mg/dL.

Physiologists Are Studying How the Plasma Concentration of Various Substances