Glomerular Filtration: Role of Afferent and Efferent Resistance on GFR

Key Concepts:

• Glomerular capillary hydrostatic pressure (PG)
• Glomerular capillary colloid osmotic pressure (πG)
• Glomerular filtration rate (GFR)
• Afferent arteriolar resistance (RA)
• Efferent arteriolar resistance (RE)
• Renal plasma flow (RPF)
• Filtration fraction (FF)
• Paradoxical effect

1. Regulation of GFR by Afferent and Efferent Arteriolar Resistance

• Basics: Blood enters the afferent arteriole at approximately 100 mmHg. Afferent arteriolar resistance (RA) reduces this pressure to about 50 mmHg at the glomerular capillaries to prevent damage. As plasma flows through the glomerular capillaries, fluid filtration causes hydrostatic pressure to decrease. Efferent arteriolar resistance (RE) helps maintain glomerular capillary hydrostatic pressure (PG) at around 50 mmHg, resulting in a typical GFR of 125 ml/min.

2. Impact of Afferent Arteriolar Resistance on GFR

• Afferent Resistance Changes: Decreasing RA increases PG, while increasing RA decreases PG. GFR directly follows these changes in PG because the afferent arteriole is upstream of the glomerular capillary.
• Example: A typical RA results in a PG of 50 mmHg.

3. Impact of Efferent Arteriolar Resistance on GFR

• Efferent Resistance Changes: Decreasing RE decreases PG, while increasing RE increases PG. This is because the efferent arteriole is downstream of the glomerular capillary.
• Paradoxical Effect: Initially, increasing RE increases GFR, but GFR plateaus and then decreases at mid to high RE. This is the "paradoxical effect."

4. Explanation of the Paradoxical Effect

• Hydrostatic vs. Colloid Osmotic Pressure: Increased RE leads to increased PG, which should increase GFR. However, at mid to high RE, GFR decreases due to an increase in glomerular capillary colloid osmotic pressure (πG).
• Plasma Protein Concentration: Increases in πG are due to increased plasma protein concentration, resulting from increased fluid loss during filtration. This increased fluid loss is primarily due to a reduction in renal plasma flow (RPF).

5. Role of Renal Plasma Flow (RPF) and Filtration Fraction (FF)

• Normal Physiological Conditions: Normal RPF is about 600 ml/min, while GFR is about 125 ml/min.
• RPF and GFR Relationship: Increasing RPF leads to a slight increase in GFR, while decreasing RPF leads to a dramatic decrease in GFR.
• Filtration Fraction: At a typical RPF of 600 ml/min, the filtration fraction (FF) is about 20%. Decreasing RPF leads to a larger FF because fluid moves more slowly across the glomerular capillary, allowing more time for filtration. At very low RPF, FF can reach a maximum of about 40%.
• Formula: Filtration Fraction (FF) = GFR / RPF

6. Synthesis/Conclusion

• The paradoxical effect is caused by a reduction in RPF, leading to an increased FF. This results in an increase in glomerular capillary colloid osmotic pressure (πG), which counteracts the effects of increased hydrostatic pressure (PG), ultimately causing GFR to plateau and then decrease at mid to high efferent arteriolar resistance.