A 60-year-old man with congestive heart failure has elevated jugular venous pressure, hepatomegaly, and peripheral edema. His measured renal plasma flow is reduced from a normal 600 mL/min to 350 mL/min, but his GFR is relatively preserved at 100 mL/min (normal 120 mL/min). What is his filtration fraction and what does this indicate about afferent versus efferent arteriolar tone?
- A FF = 0.29 (increased); predominant afferent constriction
- B FF = 0.29 (increased); predominant efferent constriction ✓
- C FF = 0.17 (decreased); predominant efferent dilation
- D FF = 0.17 (decreased); predominant afferent dilation
Correct answer: B. FF = 0.29 (increased); predominant efferent constriction
Explanation
FF = GFR/RPF = 100/350 = 0.29 (elevated from normal 0.20). In congestive heart failure, sympathetic and angiotensin II-mediated efferent arteriolar constriction preserves GFR despite reduced RPF, raising FF. This is a classic hemodynamic pattern. Distractor A incorrectly attributes the elevated FF to afferent constriction, which would reduce both GFR and RPF without raising FF.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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