Local airway heat and water vapour losses.

A previously developed time-dependent mathematical model of the heat and water vapour transport in the human respiratory tract for mouth breathing (Daviskas et al., J. Appl. Physiol. 69:362-372, 1990) was applied to calculate the local quantities of heat and water transfer. The results of the heat and water losses agreed with experimental data. The contribution of each airway to the conditioning of inspired air was found to depend on the inspired air conditions and the pattern of breathing as expected. The greater proportion of the total heat and water loss was calculated to occur within the upper airways. However, below the pharynx, the rate of water loss during hyperpnea was calculated at a much faster rate than in the resting state. The rate at which water is returned to the airways may not be adequate to keep the periciliary fluid isotonic. These findings support the proposal that the intrathoracic airways could become significantly dehydrated during hyperpnea. The use of calculated local heat and water transfer rates may help to predict the site of stimuli to exercise-induced asthma.