Which of the following best describes the action of exhalation?

Exhalation primarily involves the relaxation of the diaphragm and the subsequent action of pushing air out of the lungs. During normal exhalation, the diaphragm, which is a dome-shaped muscle located at the base of the thoracic cavity, relaxes and moves upward. This upward movement decreases the volume of the thoracic cavity, resulting in increased pressure in the lungs compared to atmospheric pressure. This pressure difference causes air to flow out of the lungs.

The action of exhalation does not require the contraction of the diaphragm (as indicated in the first choice), nor does it involve the enlargement of the thoracic cavity (as suggested in the second choice). Additionally, inhalation is characterized by air being drawn into the lungs, which is contrary to the action of exhalation. Thus, the best description of exhalation captures the relaxation of the diaphragm and the mechanical process that leads to the expulsion of air from the lungs.