A 58-year-old man with renovascular hypertension undergoes evaluation of the stenotic kidney. Within that kidney, angiotensin II constricts the efferent arteriole while the afferent arteriole pressure falls. Which change is expected in the affected kidney?

• A Both GFR and renal plasma flow fall proportionally, so filtration fraction is unchanged
• B Both GFR and renal plasma flow rise, keeping the filtration fraction constant
• C GFR falls more than renal plasma flow, lowering the filtration fraction
• D GFR is relatively preserved while renal plasma flow falls, raising the filtration fraction ✓

Explanation

Efferent arteriolar constriction raises glomerular capillary hydrostatic pressure, partially defending GFR against the reduced perfusion pressure. Renal plasma flow, however, drops substantially because overall vascular resistance rises. Since GFR falls less than plasma flow, the ratio of GFR to renal plasma flow, the filtration fraction, increases. This is why the kidney distal to a renal artery stenosis characteristically shows a high filtration fraction.

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

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