The Reynolds number provides insight into which aspect of flow in vessels?

• A. Pressure gradient
• B. Flow regime
• C. Blood viscosity
• D. Vessel diameter

Answer: (B)

The Reynolds number directly indicates the flow regime in a vessel—whether flow is smooth and laminar or chaotic and turbulent. It’s calculated from factors like velocity, vessel diameter, density, and viscosity, and it reflects the balance between inertial forces and viscous forces. A low Reynolds number means viscous forces dominate and the flow stays orderly; a high Reynolds number means inertial forces dominate and turbulence is likely. In medical vessels, flow is usually laminar, but large vessels or high velocities can raise Re toward turbulent ranges, which can show up as spectral broadening on Doppler. So this measure tells you about the nature of the flow pattern, not directly about a pressure gradient or viscosity itself, even though viscosity is a component of the calculation.