MCAT Question A Day - 3/16/14 - Answer!

During respiratory ventilation, impulses along the phrenic nerves stimulate the diaphragm, causing it to contract and move downward. When this occurs, which of the following takes place in the thoracic cavity and lungs?

A. Volume increases, pressure increases, and air moves out.

B. Volume decreases, pressure decreases, and air moves out.

C. Volume increases, pressure decreases, and air moves in.

D. Volume decreases, pressure increases, and air moves in.

The correct answer is (C). Basic knowledge of respiratory anatomy and the gas laws are required. When the diaphragm contracts (moves downward), the volume in the thoracic cavity (and lungs) increases, causing the pressure to decrease. As a result, the air outside the body is under higher pressure than the air in the lungs, and air rushes in (inspiration).

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