PAH (para-aminohippuric acid) is infused to measure effective renal plasma flow. At low plasma concentrations, PAH extraction ratio is approximately 0.90. Why does PAH clearance approximate effective renal plasma flow rather than total renal plasma flow?

• A PAH is only filtered, not secreted
• B PAH is metabolized by tubular cells
• C PAH is completely reabsorbed in the proximal tubule
• D PAH is both filtered and secreted in a single pass through the kidney ✓

Explanation

PAH is both glomerularly filtered and actively secreted by organic anion transporters in the proximal tubule. At low plasma levels, about 90% is removed in one pass, so PAH clearance approximates effective renal plasma flow. The remaining 10% perfuses non-secreting parenchyma (renal capsule, medulla).

Reference: BRS Physiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.