Modeling soluble gas exchange in the airways and alveoli.

A mathematical model of heat, water and soluble gas exchange in the airways and alveoli was used to predict the location of soluble gas exchange in the lung. A previously published model of heat, water and soluble gas exchange in the airways was improved by incorporating anatomical data on the airway wall to better describe the bronchial circulation and expanding the model to include a time varying description of soluble gas concentration in the alveoli. Next, the model was validated using two experimental data sets from the literature: (1) ethanol expirograms and (2) the uptake of seven soluble gases. Then, the model simulated the excretion of ten soluble gases whose blood:air partition coefficient (lambda(b:a)), a measure of blood solubility, ranged over 5 orders of magnitude. We found that gases with lambda(b:a) < 10 exchange almost solely in the alveoli and gases with lambda(b:a) > 100 exchange almost exclusively in the airways. Gases with lambda(b:a) between 10 and 100 have significant interaction with the airways and alveoli. These results suggest that the airways play a larger role in pulmonary gas exchange than previously assumed and may require a reevaluation of pulmonary tests that involve exhaled samples of gases with lambda(b:a) > 10.