A healthy volunteer has a GFR of 125 mL/min and an effective renal plasma flow of 660 mL/min. If her hematocrit is 45%, what is her filtration fraction and her total renal blood flow?

• A Filtration fraction 19%, renal blood flow 1200 mL/min ✓
• B Filtration fraction 15%, renal blood flow 1100 mL/min
• C Filtration fraction 19%, renal blood flow 990 mL/min
• D Filtration fraction 25%, renal blood flow 1320 mL/min

Explanation

Filtration fraction equals GFR divided by renal plasma flow: 125/660, approximately 0.19 or 19%, close to the normal value of about 20%. Renal blood flow equals renal plasma flow divided by (1 minus hematocrit): 660/0.55 gives 1200 mL/min. Option B uses the normal fractional extraction of PAH incorrectly, and option D divides by hematocrit itself rather than by plasma fraction.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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