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Don, flying is tougher than walking or running, right?
飞行比走路和跑步都要困难些，对吧？
So how come birds don't get winded and run out of breath?
那为什么鸟类飞行的时候不会喘不过气来？
Actually, in terms of distance per unit energy, flying is pretty efficient.
按照单位距离来看的话，飞行是非常顺利的。
Plus birds have a super efficient respiratory system.
并且鸟类有一个特别高效的呼吸系统。
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。
Instead, birds have a complex network of air sacs that work likebellows to pump a constant stream of air through the lungs.
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。
What's more, air only goes through the lungs in one direction, which means it has a higher oxygen content than the air in human lungs.
更重要的是，空气在肺里只能流向一个方向，那就意味着它里面的氧气含量比人类肺里氧含量高。
When a bird inhales, the air goes into the rear air sacs, and when it exhales, that air movesforward to the lungs, where oxygen is exchanged for carbon dioxide waste.
鸟在吸气时，空气进入鸟肺泡的底部，而当呼气时，空气就向肺的前面移动，在那里氧气被交换成二氧化碳废气。
Then, when the bird inhales again, the air is pushed out of the lungs and into the front air sacs.
然后，鸟再次吸气，气体就从肺中进入肺泡。
When it exhales asecond time, the air leaves the body.
当第二次呼气时，气体就从身体出来了。
So it takes two breaths to process a packet of air.
所以鸟类需要呼吸两次才能处理气体了。
Yep, Plus the air sacs mean that birds weigh less than you'd expect, which makes flying lesscostly in terms of energy.
是的。加上肺泡的话就说明鸟比我们想象的要轻，那样飞行就不会那么耗能量。
But scientists still don't fully understand how birds breathe.
但是科学家们现在也不完全明白鸟类是如何呼吸的。
Birds fly over Mount Everest at altitudes where the air is so thin humans can't function, and no one knows howthey do it.
鸟可以在珠穆朗玛峰上空飞行，但山上空气太稀薄人类无法呼吸，没有人知道它们是怎么做到的。
