Venturi effect

The Venturi effect describes what happens when a fluid flows through a narrowed section of a tube: the fluid speeds up through the constriction, and its pressure drops. The counterintuitive part is the pressure — most people expect a squeezed fluid to push harder, but the pressure in the narrow throat is actually lower than in the wide sections on either side.

Two principles combine to produce the effect. The continuity equation (A₁v₁ = A₂v₂) says that for an incompressible fluid, the volume flow rate must be the same everywhere in the tube — so when the cross-sectional area shrinks, velocity must rise proportionally. Halve the area and the speed doubles. Bernoulli's principle then connects speed to pressure: along a streamline, higher velocity means lower pressure, because the energy that accelerates the fluid comes at the expense of pressure energy. Faster flow in the throat therefore means lower pressure there.

The effect has wide applications. Venturi meters measure flow rate from the pressure difference between the wide and narrow sections; atomizers, carburetors, and aspirators use the low-pressure throat to draw in a second fluid. In physiology, the same physics applies to blood flowing through a narrowed (stenotic) vessel: velocity rises and lateral pressure falls across the narrowed segment — a useful idealization, even though real blood vessels and pulsatile flow add complications.

The MCAT tests the Venturi effect in its chemistry and physics section on fluids, often in the context of the circulatory system. Be comfortable pairing the continuity equation with Bernoulli's equation and predicting how velocity and pressure change as vessel or pipe diameter changes.