Turbulence is more likely to develop in which type of vessel diameter?

• A. Smaller
• B. Larger
• C. Both equally
• D. Neither

Answer: (A)

Turbulence in blood flow is governed by Reynolds number, which increases with the inertia of the moving blood relative to its viscosity. When flow must pass through a smaller lumen, the velocity must rise to carry the same volume of blood, and velocity in turn increases the inertial forces. Since Reynolds number scales with velocity and diameter (Re ≈ ρ v D / μ), the net effect of a smaller diameter is to raise Re, making turbulent flow more likely, especially where the flow accelerates through a constriction or changes abruptly in diameter. In small vessels, the higher velocities associated with the same flow rate promote instability and turbulent jets downstream of narrowings, so turbulence is more likely in smaller diameters.