13.5 : 呼吸机制 II：呼气

Expiration begins when the inspiratory muscles relax, causing air to move out of the conducting zone to the external environment.

Relaxation of the diaphragm causes the pleural cavity to contract and exert pressure on the lungs.

As lung volume decreases, it compresses the alveoli and increases alveolar pressure.

When alveolar pressure exceeds atmospheric pressure, air flows out.

The pleural cavity provides a frictionless space for the lung to expand and contract during breathing.

In addition, expiration results from the elastic recoil of the lungs and does not involve any muscular contractions.

During labored exhalation, abdominal muscles push forward the diaphragm, causing the thoracic cavity to contract; simultaneously, the internal intercostal muscles lower the ribcage and decrease the thoracic volume.

Expiration can be voluntary or involuntary.

In voluntary expiration, the air in the lungs is held and released at a fixed rate—for example, controlled exhalation during speech.

Conversely, involuntary expiration is vital for metabolic functions and is not in conscious control, like breathing during sleep.