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Avian Flow-Through Lungs and Air Sacs

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Bird lungs connect to air sacs that keep air flowing through in one direction on both inhalation and exhalation, unlike a mammal's in-and-out flow.

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One-directional airflow lets a bird's lung contact fresh air continuously, supporting flight at low-oxygen altitude.

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One-directional flow needs two full breathing cycles to move one volume of air completely through.

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