Mechanism of Breathing II: Expiration

During expiration, the diaphragm relaxes and moves upward, reducing the volume in the thoracic cavity. This relaxation is the primary driver of the initial expiration phase. As the diaphragm moves upward, the pleural cavity contracts and exerts pressure on the lungs, causing lung volume to decrease and air to flow outward.

As lung volume decreases during expiration, the alveoli are compressed, which increases alveolar pressure. When alveolar pressure exceeds atmospheric pressure outside the body, air flows out of the lungs. This pressure gradient is essential for moving air from the conducting zone to the external environment.

The pleural cavity provides a frictionless space between the lungs and chest wall, allowing smooth expansion and contraction during breathing. Expiration results from elastic recoil of the lungs, which does not require muscular contractions during quiet breathing. This passive mechanism makes normal expiration an effortless process.

During labored exhalation, the abdominal muscles contract to push the diaphragm further upward, intensifying thoracic volume reduction. The internal intercostal muscles lower the ribcage and decrease thoracic volume. These accessory muscles activate during vigorous breathing, singing, or heavy physical exertion like weightlifting.

Voluntary expiration involves conscious control over breathing to regulate airflow, such as during speech or singing. Involuntary expiration is an automatic process that occurs continuously without conscious effort, even during sleep, and is vital for sustaining metabolic functions and life.

The intercostal muscles, situated between the ribs, contract during expiration to pull the ribs downward and inward. This action decreases the size of the chest cavity and reduces thoracic volume. During quiet breathing, this muscular effort combines with elastic recoil to facilitate smooth airflow out of the lungs.

Expiration is passive during quiet breathing because it relies on elastic recoil of lung tissue and relaxation of inspiratory muscles rather than active muscular contractions. The natural elasticity of the lungs, combined with decreased pleural pressure, drives air out without requiring conscious effort or significant muscle work.