Davies Publishing SPI Practice Test 2026 - Free SPI Practice Questions and Study Guide

Reynolds number measures the balance between inertial forces and viscous forces in a flow. For a simple view, it scales with density times velocity times a length scale (such as vessel diameter) and is inversely proportional to viscosity: Re ≈ ρ v L / μ. So increasing flow speed raises inertial effects, increasing vessel diameter enlarges the scale of the flow, and increasing density boosts inertia as well. All three factors push Re higher, while higher viscosity would push it down. In that context, Reynolds number increases with flow speed, vessel diameter, and blood density, making the best choice all of the above.