Question

Which of the following changes will increase both glomerular filtration rate (GFR) and filtration fraction?

• A. Constriction of the afferent arteriole
• B. Constriction of the efferent arteriole
• C. Dilation of the afferent arteriole
• D. Dilation of the efferent arteriole

Hint:

Efferent arteriole constriction increases glomerular pressure → increases GFR → decreases RPF → increases filtration fraction.

Answer 1

The Correct Option is B

Step 1: Understanding GFR and Filtration Fraction.
Glomerular Filtration Rate (GFR) depends on the hydrostatic pressure within the glomerular capillariesFiltration fraction (FF) is defined as the ratio of GFR to renal plasma flow (RPF)
\[ FF = \frac{GFR}{RPF} \]
Step 2: Effect of Afferent Arteriole Changes.
(A) Constriction of the afferent arteriole reduces blood flow into the glomerulusThis decreases glomerular capillary pressure, thereby decreasing GFR and FF
(C) Dilation of the afferent arteriole increases GFR but also increases renal plasma flow proportionallyThus, filtration fraction does not increase significantly
Step 3: Effect of Efferent Arteriole Changes.
(B) Constriction of the efferent arteriole causes blood to back up into the glomerulusThis increases glomerular capillary hydrostatic pressure, thereby increasing GFRHowever, renal plasma flow decreases due to increased resistanceSince GFR increases while RPF decreases, filtration fraction increases
(D) Dilation of the efferent arteriole reduces glomerular pressure, decreasing GFR and filtration fraction
Step 4: Conclusion.
Only constriction of the efferent arteriole increases both GFR and filtration fraction