The quasi-steady creeping flow in a spherical model alveolus has been studied by solving the Stokes equations numerically in order to understand the airflow in the alveolar region of the human lung. The alveolar shape is allowed to change during the respiratory cycle such that the alevolar mouth opening as a fraction of the surface is a function of the lung volume. The complete velocity field in the alveolus is determined from which a hypothetical interface between the tidal and residential airs before and after a respiratory cycle is calculated. The result shows that there is significant mechanical mixing between the two airs and the calculated percentages of mixing in the lung agree favorably with the existing data from single breath measurement using half-micron aerosols.

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