Airway heat and gas exchange

The usual approach to analysis of pulmonary gas exchange deals with the airways as a dead space that does not participate in gas exchange and acts simply as a conduit for the passage of air between the outside environment and the alveoli. In reality, however, inspired air undergoes some change during its trans-airway passage. The relatively cool and dry air is heated and humidified protecting the delicate lower airway epithelium from thermal and desiccation injuries. During inspiration relatively dry and cool air passes through the nasal cavity (or mouth) and pharynx and into the trachea. Then, the air encounters a branching network of airways. The airway surface is lined with a moist mucous layer with which air exchanges water, heat, and soluble gases. This mucous layer exchanges with the underlying epithelial tissue which, in turn, exchanges with the subepithelial tissue and the bronchial circulation.