Air flow in an avian lung was studied numerically to determine the effects of air sac compliance on flow in the parabronchi. In this preliminary study, the geometry of the avian respiratory system was simplified to capture the characteristics of respiratory flow. The pressure fluctuation within air sacs caused by inflation and deflation was expressed by a lumped parameter model. The results demonstrate that the flow direction in the parabronchi varied, depending upon the compliance of the air sacs. A unidirectional flow in the parabronchi was achieved for compliances where pressure fluctuations in all air sacs were in phase. Air sac compliance significantly affected the pressures in the anterior and posterior air sacs and thus the pressure difference over the parabronchi that drove the flow in the parabronchi. These results address the importance of air sac compliance in the avian respiratory system and suggest that the compliance of air sacs would be optimized to accomplish unidirectional flow in the parabronchi.
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