Multicomponent diffusion in the lung.

Since breathing involves more than two gases, the physics of multicomponent diffusion is relevant to respiratory physiology. Fundamental principles and important studies in this area are reviewed, some pertain to equimolar diffusion, others to nonequimolar diffusion. As the lung is an open-system nonequimolar exchanger, emphasis is placed on the latter. A mathematical model that has been verified by both physical and biological experiments is presented. This model is used to study ternary diffusion in the alveolar spaces and to obtain effective diffusion coefficients. Based on the cumulative studies up to now, it may be concluded that Fick's law still holds when normal air is breathed under normal pressure and when tracer amounts of inert gases are added to normal air. Fick's law no longer holds when helium is used as a carrier gas or when breathing is under extreme hyperbaric conditions.