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Difficulty: HardExcretion and Excretory Organs

In the mammalian kidney, ultrafiltration is driven by net filtration pressure across the glomerular capillaries into the Bowman's capsule. Which combination of vascular changes would produce the highest increase in the rate of glomerular filtration?

  1. Vasodilation of the afferent arteriole combined with vasoconstriction of the efferent arterioleAnswer
  2. B
    Vasoconstriction of the afferent arteriole combined with vasodilation of the efferent arteriole
  3. C
    Vasoconstriction of both afferent and efferent arterioles to equal degrees
  4. D
    Vasodilation of the efferent arteriole combined with an increase in plasma protein concentration

Answer

Vasodilation of the afferent arteriole combined with vasoconstriction of the efferent arteriole
Dilation of the wide afferent arteriole increases blood flow into the glomerulus, while constriction of the narrow efferent arteriole impedes fluid exit. This raises the hydrostatic pressure within the glomerular capillaries, which is the main driving force for ultrafiltration across the basement membrane into the capsular space.

Step-by-Step Solution

1
Analyze the forces governing glomerular ultrafiltration in the nephron.
Net filtration pressure depends directly on glomerular capillary hydrostatic pressure pushing fluid out into Bowman's capsule, opposed by capsular hydrostatic pressure and plasma colloid osmotic pressure.
Glomerular hydrostatic pressure is the primary variable driving ultrafiltration.
2
Evaluate the hemodynamic impact of altering arteriolar resistance.
Opening (dilating) the inlet vessel (afferent arteriole) brings more blood under pressure into the glomerulus, while narrowing (constricting) the outlet vessel (efferent arteriole) creates backpressure.
This dual action traps blood in the glomerular capillaries, driving capillary hydrostatic pressure to its maximum level.
3
Select the option that maximizes net filtration pressure.
Afferent vasodilation paired with efferent vasoconstriction produces the greatest elevation in glomerular hydrostatic pressure, maximizing filtration rate.
It optimizes the pressure gradient required for ultrafiltration.

Key Concept

Renal Hemodynamics and Glomerular Ultrafiltration Mechanics
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Topics

Excretion and Excretory Organs

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