The modern atmosphere supports many forms of complex life that would not have been able to exist in Earth's first atmosphere because the oxygen level was too low.

现代大气层支持着许多复杂的生命形式，而这些生命在地球最初的大气层中是不可能存在的，因为当时的氧气含量太低。

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People with trouble sleeping can use this app to record their blood-oxygen level during the night — data that can help a doctor diagnose whether they have sleep apnea.

有睡眠障碍的人可在晚间使用该应用记录他们的血氧饱和度——该数据可帮助医生诊断他们是否患有睡眠呼吸暂停症。

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Scientists are not certain why the oxygen level dropped.

科学家们现在还不能断定为什么含氧量会下降。

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Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light.

VOA: special.2010.11.10

The respiratory system plays an important role in maintaining our internal environment at the proper level of oxygen, by bringing the right amount in.

呼吸系统的重要性在于,通过向体内输入适量的氧气,维持人体内环境中的氧平衡

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Next Roth lowered the oxygen level in their cage to 5% from 21%.

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As the oxygen level fell, though, something else rose: the level of urea.

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This involves shining a light into the brain to discern how much activity is there, and examining the corresponding oxygen level.

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