The effect of flow generated by a rhythmically expanding pulmonary acinus on aerosol dynamics

The effects of a rhythmically expanding alveolus on aerosol kinematics is investigated. The alveolus and alveolar duct are modeled as cyclically self-similar expanding, spherical cap attached at its rim to a circular opening in an expanding plane. The flow induced by the alveolar wall motion plays a significant role in determining aerosol trajectories and deposition location. Under the combined effects of the flow and gravity, particles of 0.5–2 μm in diameter tend to form a two-peaks deposition distribution over the alveolar walls; one peak is formed near the alveolar opening and the other around the alveolar center. This is markedly different from the single-peaked deposition pattern, where higher deposition concentration is observed at the alveolar center, in a simulation performed in the absence of alveolar wall motion. In addition, wall motion has a pronounced effect on particle residence time inside the alveolus. For a given set of physiological conditions, particles tend to stay more than ten times longer before colliding with the alveolus wall, due to their cyclic excursions. In microgravity circumstances (e.g. in space), the simulation shows that 10 μm particles initially placed outside the alveolus tend to slowly migrate away from it, while particles placed initially inside the alveolus tend to cross streamlines due to inertial effects and subsequently deposit on the alveolar walls after a large number of breathing periods.