Heat and water exchange in human airways.

The intrathoracic airways of humans play a prominent part in conditioning inspired air. During inspiration the air is warmed and humidified by the movement of heat and water from the mucosa as a direct function of the temperature and vapor pressure gradients that exist. In this process, the mucosa is cooled. During expiration, the gradients are reversed, and heat and water are given back into the airways. At low levels of ventilation, most of the conditioning process takes place in the upper air passages; however, as ventilation rises, more and more of the tracheobronchial tree becomes involved, and incompletely conditioned air penetrates deeply into the distal airways before it is brought to equilibrium with body conditions. It is likely that the heat required to condition the inspired air is derived from the bronchial circulation, but this has not yet been definitely proved. In normal persons, the thermal events associated with the conditioning of inspired air do not produce any changes in pulmonary mechanics. In contrast, in asthmatics, the airway cooling of hyperpnea and the rapid rewarming that develops when hyperpnea is terminated evoke bronchoconstriction.