Filtration Fraction Normal — FRCA Primary MCQ

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HardPhysiology - RenalFiltration Fraction NormalFRCA Primary

If GFR is 125 mL/min and renal plasma flow is 625 mL/min, what is the filtration fraction?

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Correct answer: C — 20%, calculated as 125 divided by 625

The best answer is “20%, calculated as 125 divided by 625”. Filtration fraction is GFR divided by renal plasma flow. Therefore 125/625 = 0.20, or 20%. It is not the proportion of renal blood flow because plasma flow excludes erythrocyte volume. Changes in afferent or efferent arteriolar tone alter GFR and renal plasma flow differently, changing filtration fraction.

Reference: Royal College of Anaesthetists, Primary FRCA Syllabus 2021: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf