Theoretical gas phase mass transfer coefficients for endogenous gases in the lungs

Gas phase mass transfer coefficients for nitric oxide (NO), ethanol (EtOH), and water vapor (H2O) were determined for typical conducting airway geomet ry and tracheal hows (5 X 10(-5) and 5 X 10(-4) m(3) s(-1)), by solving the steady-state two-dimensional diffusion equation. A constant absolute produ ction rate with first order consumption reactions in pulmonary tissue was a ssumed for NO. Far EtOH and H2O, constant concentrations were assumed in th e blood and tissue, respectively. Results, expressed in terms of the averag e Sherwood number (<(Sh)over bar>), were correlated with the Peclet (Pe(r)) number, and the length-to-diameter (L/D) ratio for each airway branch in t erms of a lumped variable, Pe(r)(L/D)(n). (<(Sh)over bar>) increases as the solubility of the gas in tissue and blood increases. In addition, <(Sh)ove r bar> passes through a minimum value at Pe(r)(D/L)(n) equal to approximate ly one when axial convection and diffusion have equal but opposite magnitud es. We conclude that <(Sh)over bar> is not a monotonic function of Pe(r)(L/ D)(n) within the entire airway tree and that it depends on the physical pro perties of the gas in the tissue. This conclusion contrasts with previous e xperimental and theoretical correlations. (C) 1999 Biomedical Engineering S ociety. [S0090-6964(99)01403-4].