# MCAT Physics Practice Question – Fluid Dynamics

• Reviewed By: Liz Flagge
• Which of the following statements are true?

I. The blood velocity and pressure are higher in arteries than in capillaries.
II. Blood vessels do NOT follow Bernoulli’s principle if viscosity of blood is taken into account.
III. For an incompressible fluid undergoing laminar flow through a pipe with both narrow and wide sections, the fluid velocity is higher in the narrow areas than in wider areas.
IV. For an incompressible fluid undergoing laminar flow through a pipe with both narrow and wide sections, the fluid pressure is higher in narrow areas than in wide areas.

A. I only

B. I and III only

C. III and IV only

D. I, II, and III only

Explanation

The resistance of capillaries to blood flow is much higher than that of arteries. This causes the capillaries to contain a  lower blood pressure than arteries. In addition, due to the larger total cross-sectional area of capillaries compared to arteries, the blood velocity in capillaries is lower than in arteries (I).

Bernoulli’s principle has two underlying assumptions: incompressibility and non-viscosity of the fluid. These assumptions are not true of blood (or of any real fluid), so blood vessels don’t follow Bernoulli’s principle. As blood moves from arteries to capillaries, both pressure and velocity drops. This change cannot be explained by Bernoulli’s equation, which is given as P + ½ρv2 + ρgh = constant (II).

The continuity equation holds that A1V1 = A2V2, where A is cross-sectional area and V is velocity. In narrowed sections, the area is smaller; therefore the velocity will be larger (III). Therefore, D. is the answer.

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