Pulmonary interdependence of gas transport.

We have examined the interaction of convection and gas-phase diffusion among parallel pathways of the human lung by solving the differential equation for gas transport in a solid geometric model. Two trumpet-shaped units with a threefold differences in volume flow were joined at a branch point that could be varied in position along the airway tree. Because diffusion dominates gas transport in peripheral airways, or when time for diffusion is large, alveolar concentrations are more homogeneous than predicted from volume flows when the branch point is peripheral to respiratory bronchioles, or when total flow rate is small. When the branch point is in the larger airways, subtending large units of lung, diffusion is less important, so that alveolar concentration of each unit depends almost completely on its volume flow. These simulations provide a possible explanation for experimental findings of partial separation of inert gases of differing diffusivities and of improved O2 exchange when dense gases are breathed.