The fact that the volumetric flow rate must be the same proximal, at, and distal to a stenosis is termed the:

• A. Continuity rule
• B. Reynolds number
• C. Poiseuille's law
• D. Bernoulli effect

Answer: (A)

Conservation of mass in incompressible flow underlies this idea: the volumetric flow rate Q equals cross-sectional area A times velocity v, and this product stays constant along the vessel. When a stenosis narrows the vessel, the area decreases, so the velocity must rise to keep the same Q. That’s why the same amount of fluid moves proximal, at the narrowed point, and distal to it.

Reynolds number is about whether the flow is laminar or turbulent and depends on velocity, diameter, density, and viscosity, but it doesn’t state that the flow rate remains the same across a constriction. Poiseuille’s law describes how flow through a long, straight, narrow tube depends on radius, length, viscosity, and the pressure drop, not on maintaining equal flow across a stenosis. Bernoulli’s principle addresses energy balance along a streamline—how velocity and pressure trade off—but it doesn’t by itself impose the requirement that the volumetric flow rate remains constant through a narrowing. The continuity idea is the one that specifically captures the constancy of Q across proximal, at, and distal segments.