Topic overview

Start with the big picture

The respiratory tract has a conducting zone, which carries air without gas exchange, and a respiratory zone, where exchange occurs in structures including the alveoli. As airways branch, their combined cross-sectional area increases toward the smaller passages. Ventilation depends on pressure and volume changes in the thorax: the diaphragm is the primary inspiratory muscle, inspiration is active, and expiration is passive at rest but can become active during exertion. Intrapleural pressure remains negative, helping prevent lung collapse. Compliance describes how readily the lungs distend; surfactant from Type II pneumocytes lowers alveolar surface tension and helps prevent collapse. These relationships provide a foundation for understanding respiratory mechanics.