What occurs when the diaphragm relaxes, leading to the expulsion of air from the lungs?

The process that occurs when the diaphragm relaxes, resulting in the expulsion of air from the lungs, is called expiration. During expiration, the diaphragm, a thin muscle that separates the chest cavity from the abdominal cavity, moves upward and relaxes. This relaxation decreases the volume of the thoracic cavity, which in turn increases the pressure in the lungs. As a result, air is expelled from the lungs and out of the body through the respiratory tract.

In contrast, inspiration is the process of inhaling air into the lungs, which involves the contraction of the diaphragm and other respiratory muscles to expand the chest cavity. Peristalsis refers to the wave-like muscle contractions that move food through the digestive tract, which is unrelated to respiratory mechanics. Breathing encompasses both inspiration and expiration, but in this question, the specific action described is expiration, which is why it is the correct answer.