肺由称为肺泡的小气囊组成，当健康的人呼吸时会充满空气。
The lungs are made up of small sacs called alveoli, which fill with air when a healthy person breathes.
相反地，鸟类有一个复杂的肺泡结构，就像一个风箱把源源不断的空气打进肺里。
Instead, birds have a complex network of air sacs that work like bellows to pump a constant stream of air through the lungs.
肺泡是空气相互交换的地方。
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