MODELLING OF HEAT AND WATER VAPOR TRANSPORT IN THE HUMAN RESPIRATORY TRACT (AIR-CONDITIONING)

A theoretical model is developed which for the first time describes local heat and water vapor transport between the flowing airstream and the mucosal surface along the human respiratory tract. The model provides new insight into the design of the respiratory system as a heat and mass exchanger and suggests that respiratory air-conditioning plays a role in many respiratory disease processes. Because of the geometrical complexity of the respiratory tract, the local mass transfer coefficients were experimentally determined within an acrylic model of the human upper respiratory tract cast from a cadaver split sagitally. The heat transport coefficients were evaluated from the mass transfer coefficients assuming a heat and mass transport analogy. The theoretical model together with the experimentally determined local heat and mass transport coefficients was solved numerically using the predictor-corrector method to yield the five unknown variables of the process as functions of distance into the airways. The five variables of the model are: the airstream temperature and water vapor concentration, the temperature and water vapor concentration at the air-mucosal interface, and the water flux at the air-mucosal interface. The numerical model shows that the airway wall blood temperature and the nasal cross-sectional area are the most important physiological parameters controlling heat and water loss or gain from the various regions of the respiratory tract. In particular, over a breathing cycle the net heat and water flux from the pharyngeal mucous membrane is shown to be most sensitive to these parameters. Generally, the pharyngeal mucosa shows a net gain of heat and water but can experience a net loss dependent on the above parameters. This finding suggests that the pharynx is vulnerable to drying which in turn can lead to respiratory tract infection. The role of the treacheo-bronchial tree in respiratory air-conditioning is also shown to have been previously underestimated providing more insight into the association of respiratory heat loss and bronchoconstriction of exercise-induced asthma. The model also predicts that increasing bronchial mucus layer depth substantially reduces heat and water loss during bronchoconstriction supporting the hypothesis that the function of mucus is to restrict heat and water loss