While a bird's respiratory system takes up about one fifth of its body, its lungs are relatively rigid and small, and don't fully deflate.

当鸟呼吸系统占据它身体的五分之一时，它的肺比较僵硬，也比较小，不会完全紧缩。

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This results in an equivalent rigid body system that can be analyzed by relatively simple standard procedures.

在等效刚体系统这个结果可以相对简单的分析标准程序。

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