Which factor increases vascular resistance according to Poiseuille-like principles?

• A. Increased viscosity increases resistance
• B. Shorter vessel length increases resistance
• C. Increased radius increases resistance
• D. Decreased viscosity increases resistance

Answer: (A)

Poiseuille-like principles show that vascular resistance depends on viscosity (η), vessel length (l), and radius (r) as R ∝ η l / r^4. This means any factor that increases the numerator or reduces the denominator raises resistance. Increasing viscosity makes the blood flow more sluggish, so more pressure is needed to maintain the same flow, directly raising resistance. In contrast, shortening the vessel decreases resistance, and widening the radius lowers resistance far more because the radius is to the fourth power in the denominator. Decreasing viscosity would also lower resistance. Therefore, the factor that increases vascular resistance is increased viscosity.