Are the lungs active in expelling air during expiration?

The lungs are considered passive during expiration, meaning they do not actively squeeze out air. Instead, expiration primarily relies on the elastic recoil of the lung tissue and the thoracic cavity. When the diaphragm and intercostal muscles relax after inhalation, the lung volume decreases, which increases the pressure within the alveoli compared to the outside atmosphere. This pressure difference allows air to flow out of the lungs naturally, without requiring the lungs themselves to exert effort.

The passive nature of expiration is crucial, especially when considering the mechanics of breathing in resting conditions. During exercise, there may be changes in respiratory techniques that can involve muscular activity for greater air expulsion, but this does not apply to normal, quiet breathing where the lungs return to their original shape due to elastic recoil. Therefore, the assertion that the lungs are passive during expiration accurately reflects the physiological process occurring in typical breathing patterns.