A 28-year-old man passes a 5 mm ureteral calculus. The stone obstructs the ureter and raises hydrostatic pressure within the renal tubules and Bowman's capsule. What happens to his GFR and why?
- A GFR falls because raised Bowman's capsule hydrostatic pressure reduces net filtration pressure across the glomerulus ✓
- B GFR is unchanged because afferent arteriolar tone compensates fully for any back pressure
- C GFR rises because elevated capsular pressure stretches the filtration barrier open
- D GFR falls because obstruction eliminates the oncotic pressure difference across the glomerular capillary
Correct answer: A. GFR falls because raised Bowman's capsule hydrostatic pressure reduces net filtration pressure across the glomerulus
Explanation
Net filtration pressure equals glomerular capillary hydrostatic pressure minus the sum of Bowman's capsule hydrostatic pressure and capillary oncotic pressure. Obstruction raises the hydrostatic pressure inside the tubules backward into Bowman's space, opposing filtration directly and reducing GFR. Plasma oncotic pressure at the capillary is unaffected by a downstream blockage, and afferent dilation cannot fully offset severe back pressure in complete obstruction.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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