Topic overview

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Inspiration is active: the diaphragm and external intercostal muscles increase thoracic volume, lowering intrapleural and alveolar pressures so air flows into the lungs. Accessory muscles can assist when breathing demand rises. At rest, expiration is usually passive, driven by lung elastic recoil and surface tension; forced expiration recruits abdominal and internal intercostal muscles. These movements depend on pressure gradients, while surfactant helps reduce surface tension and support lung compliance. Airway resistance, functional residual capacity, and the work required to breathe provide further context for how airflow is maintained. Neural signals coordinate rhythmic diaphragm contraction, and sensory feedback contributes to regulation of breathing.