What process describes the flow of air out of the lungs?

The flow of air out of the lungs is accurately described by passive expiration under normal resting conditions. During passive expiration, the diaphragm and external intercostal muscles relax after inhalation, leading to an increase in intrathoracic pressure and causing air to flow out of the lungs due to elastic recoil of the lung tissue.

In this process, energy is not actively expended by the body; rather, the natural elasticity of the lungs and the thoracic cage facilitate the movement of air. This is why passive expiration is a key feature of normal breathing, allowing for a smooth transition between inhalation and exhalation without requiring additional muscular effort.

Activities like heavy exercise or anything requiring forceful exhalation might utilize other mechanisms, such as forced expiration, which involves the contraction of additional muscles to push air out more vigorously. However, these apply to specific conditions and do not represent the typical resting state of pulmonary function.