英語聽力文摘 鳥是如何呼吸的？
How Birds Breathe
鳥是如何呼吸的
Y: Don, flying is tougher than walking or running, right? So how come birds don’t get winded and run out of breath?
D: Actually, in terms of distance per unit energy, flying is pretty efficient. Plus birds have a super efficientrespiratory system. 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. Instead, birds have a complex network of air sacs that work like bellows to pump a constant stream of air through the lungs. 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.
When a bird inhales, the air goes into the rear air sacs, and when it exhales, that air moves forward to the lungs, where oxygen is exchanged for carbon dioxide waste. Then, when the bird inhales again, the air is pushed out of the lungs and into the front air sacs. When it exhales a second time, the air leaves the body.
Y: So it takes two breaths to process a packet of air.
D: Yep, Plus the air sacs mean that birds weigh less than you’d expect, which makes flying less costly in terms of energy. But scientists still don’t fully understand how birds breathe. Birds fly over Mount Everest at altitudes where the air is so thin humans can’t function, and no one knows how they do it.
飛行比走路和跑步都要困難些，對吧?那為什么鳥類飛行的時候不會喘不過氣來?
按照單位距離來看的話，飛行是非常順利的。并且鳥類有一個特別高效的呼吸系統(tǒng)。當鳥呼吸系統(tǒng)占據(jù)它身體的五分之一時，它的肺比較僵硬，也比較小，不會完全緊縮。相反地，鳥類有一個復雜的肺泡結構，就像一個風箱把源源不斷的空氣打進肺里。更重要的是，空氣在肺里只能流向一個方向，那就意味著它里面的氧氣含量比人類肺里氧含量高。
鳥在吸氣時，空氣進入鳥肺泡的底部，而當呼氣時，空氣就向肺的前面移動，在那里氧氣被交換成二氧化碳廢氣。然后，鳥再次吸氣，氣體就從肺中進入肺泡。當?shù)诙魏魵鈺r，氣體就從身體出來了。
所以鳥類需要呼吸兩次才能處理氣體了。
是的。加上肺泡的話就說明鳥比我們想象的要輕，那樣飛行就不會那么耗能量。但是科學家們現(xiàn)在也不完全明白鳥類是如何呼吸的。鳥可以在珠穆朗瑪峰上空飛行，但山上空氣太稀薄人類無法呼吸，沒有人知道它們是怎么做到的。
Notes:
air sacs: 肺泡
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