exhalation

Exhalation, or expiration, is the phase of respiration in which air leaves the lungs. It follows inspiration and is essential for removing carbon dioxide produced by cellular metabolism. In a resting state, expiration is typically a passive process driven by the elastic recoil of the lungs and chest wall after the inspiratory muscles relax.

In quiet breathing the diaphragm relaxes and the rib cage falls inward; the lungs and chest wall recoil, raising intrapulmonary pressure above atmospheric pressure and driving airflow outward until the alveolar pressure again equals ambient pressure.

During exertion or when a faster rate of ventilation is needed, expiration can become active. The abdominal muscles and internal intercostal muscles contract to raise intrathoracic pressure, pushing air out more forcefully. Lung compliance and airway resistance also influence expiratory flow, and the system remains under central nervous control.

Breathing is regulated by the brainstem. Central chemoreceptors monitor CO2-related pH in cerebrospinal fluid, and peripheral chemoreceptors in the carotid and aortic bodies monitor O2, CO2, and pH. The resulting adjustments in rate and depth help maintain arterial CO2, pH, and overall gas exchange, influencing functional residual capacity and alveolar ventilation. Expiration thus integrates mechanical, chemical, and neural factors to sustain respiration.