Respiratory heat and water exchange: physiological and clinical implications.

Recent evidence demonstrates that the conditioning of inspired air is not confined to the upper airways as formerly thought but rather involves as much of the tracheobronchial tree as necessary to complete the process. As the need to condition more air is increased by raising ventilation and/or lowering inspired temperature (and so water content), the point at which the inspirate reaches body conditions moves progressively deeper into the lungs, and under extreme conditions thermal transfers can be measured in airways less than 2 mm in diameter. The decrease in airway temperature that develops from the movement of heat and water from the mucosa during inspiration not only facilitates recovery during expiration by reversing the thermal gradients, but it may also produce airway obstruction in susceptible individuals by an as yet undefined mechanism. Respiratory heat exchange may also interact with airway secretory processes and mucociliary transport mechanisms and may help regulate bronchial blood supply.