Which mechanism is primarily involved in passive expiration?

Passive expiration primarily relies on the elastic recoil of the lungs. During normal breathing, when an individual exhales passively, the diaphragm and other chest muscles relax after a contraction. This relaxation allows the lungs to return to their resting state due to the inherent elasticity of lung tissue and the thoracic cavity. As the lungs and chest wall recoil, air is expelled from the lungs without the need for active muscle contractions.

This process differs from active expiration, which would involve controlled contraction of various respiratory muscles, requiring more energy and conscious effort. In passive expiration, because the lungs naturally rebound due to elastic properties, it occurs effortlessly and spontaneously. This mechanism allows for effective and efficient breathing patterns, especially during rest or light physical activity where energy conservation is essential.