To understand the changes that affect the glomerular filtration rate (GFR) and the filtration fraction, we must first examine the basic renal physiology involved in determining these values.
Concepts and Definitions
- Glomerular Filtration Rate (GFR): The volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time. It is influenced by the net filtration pressure, surface area, and permeability of the filtration membrane.
- Filtration Fraction: The ratio of the GFR to the renal plasma flow (RPF).
- Afferent Arteriole: The blood vessel that supplies blood to the glomerulus.
- Efferent Arteriole: The blood vessel that carries blood away from the glomerulus.
Effect of Arteriole Changes
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Constriction of the Afferent Arteriole: Constriction here reduces blood flow into the glomerulus, reducing the glomerular pressure and thus decreasing GFR.
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Constriction of the Efferent Arteriole: Constriction here increases glomerular pressure as it impedes blood outflow, raising the GFR. Because the filtration fraction is the ratio of GFR to RPF, and RPF would reduce due to increased resistance, the filtration fraction increases.
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Dilation of the Afferent Arteriole: This increases blood flow into the glomerulus, increasing GFR. However, without affecting the efferent output significantly, it doesn’t necessarily change the filtration fraction as substantially.
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Dilation of the Efferent Arteriole: This would relieve the back-pressure and likely decrease GFR due to decreased glomerular pressure.
Conclusion
Considering the effects above, constriction of the efferent arteriole is the only mechanism that increases both the GFR and the filtration fraction. Thus, the correct answer is "Constriction of the efferent arteriole."