A freely filterable substance that is neither reabsorbed nor secreted has a renal aery concentration of 12 mg/mL and a renal vein concentration of 9 mg/mL. Which of the following is the filtration fraction?

Because the amount of fluid excreted by the kidney is only a small fraction ot the renal plasma flow, the volume of fluid in the vein is essentially equal to that in the aery Thus, the difference between the aerial 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 aerial concentration that is filtered (and therefore, the fraction of plasma filtered) is