Which statement best describes exhalation during normal breathing?

• A. is a passive process caused by increased intrathoracic pressure.
• B. requires muscular effort to effectively expel air from the lungs.
• C. is an active process caused by decreased intrathoracic pressure.
• D. occurs when the diaphragm lowers and expels air from the lungs.

Answer: (A)

Exhalation during normal breathing is a passive process driven by the elastic recoil of the lungs and chest wall. When the inspiratory muscles relax, the chest returns to its resting size and the lungs recoil, which increases intrathoracic pressure. That rise in pressure pushes air out of the lungs without the need for active muscle effort.

This aligns with quiet breathing, where no extra work is required. The other descriptions describe conditions outside normal quiet breathing: exhalation that requires muscle effort corresponds to forced expiration, not resting breath; expiration described as due to decreased intrathoracic pressure would actually apply to inspiration; and the action of the diaphragm lowering is what occurs during inspiration, not expiration.