In a steady-state subject, the clearance of para-aminohippuric acid (PAH) is 600 mL/min and the hematocrit is 40 percent. What is the renal blood flow?

• A 360 mL/min
• B 840 mL/min
• C 1000 mL/min ✓
• D 1500 mL/min

Explanation

PAH is almost completely cleared from renal plasma in a single pass, so its clearance approximates effective renal plasma flow, here 600 mL/min. Renal blood flow = RPF divided by (1 minus hematocrit) = 600 divided by 0.6 = 1000 mL/min. Note that PAH extraction is roughly 90 percent rather than complete, so its clearance slightly underestimates true RPF, but the standard exam convention treats PAH clearance as equal to effective RPF.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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