MEDizzy - An 85-year-old woman presents with a fever and hypovolemic

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Renal and Urinary Physiology

An 85-year-old woman presents with a fever and hypovolemic hypotension. To assess her renal function, the filtration fraction is determined using a freely filterable substance that is neither reabsorbed nor secreted. The infusate yields a renal artery concentration of 12 mg/mL and a renal vein concentration of 9 mg/mL. Which of the following is the filtration fraction?

ExplanationBecause the amount of fluid excreted by the kidney is only a small fraction of the renal plasma flow, the volume of fluid in the vein is essentially equal to that in the artery. Thus, the difference between the arterial and venous concentrations is due to the loss of solute. Because the material is neither reabsorbed nor secreted, its removal from the plasma must have been by glomerular filtration. Therefore the filtered solute equals (12 mg/mL − 9 mg/mL), and the percent of the arterial concentration that is filtered (and therefore, the fraction of plasma filtered) is 3 mg/mL/12 mg/mL = 0.25