A patient develops bilateral ureteric calculi causing obstruction. Which Starling force change across the glomerular capillary explains the fall in his GFR?

• A Decrease in glomerular capillary hydrostatic pressure
• B Decrease in Bowman capsule oncotic pressure
• C Increase in glomerular capillary colloid osmotic pressure
• D Increase in Bowman capsule hydrostatic pressure ✓

Explanation

Obstruction raises pressure within the tubules and transmits retrograde into Bowman space, increasing the hydrostatic pressure opposing filtration. Since net filtration pressure equals glomerular capillary hydrostatic pressure minus the sum of Bowman capsule hydrostatic pressure and capillary oncotic pressure, GFR falls even though renal blood flow may be preserved early on. Glomerular capillary hydrostatic pressure is actually maintained or elevated by afferent dilation in response to the falling GFR.

Reference: Ganong Review of Medical Physiology, 26th ed.

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