Airway heat and water fluxes and the tracheobronchial circulation.

Breathing dry gas at high minute ventilation leads to heat and water losses from the pulmonary airways. Although these thermal exchanges tend to promote airway cooling and drying, net changes in airway wall temperature and hydration reflect the balance of these losses and restoration of heat and water from circulatory replenishing sources. Because its blood flow rate is small, the tracheobronchial circulation replaces little of the heat loss from the bronchi to the atmosphere; instead, the pulmonary arteries represent the major circulatory heat source for intrapulmonary bronchi. Nonetheless, the tracheobronchial circulation probably acts as an important water replenishing source that helps the bronchi, resist drying, though the precise role of the airway circulation in this regard has not yet been established. Conversely, airway heat and water fluxes caused by dry gas breathing affect the tracheobronchial circulation in two ways. They increase airway systemic blood flow rate, and induce bronchovascular hyperpermeability. Each effect appears primarily as a local response to local airway heat/water losses, but the mechanisms leading to these changes remain uncertain. Furthermore, these vascular responses probably enhance the ability of the tracheobronchial circulation to modulate airway cooling or drying. Thus, airway heat and water exchanges share bidirectional interactions with the airway circulation.