听力文摘：鸟是如何呼吸的

How Birds Breathe

鸟是如何呼吸的

Don, flying is tougher than walking or running, right? So how come birds don’t get winded and run out of breath?

飞行比走路和跑步都要困难些，对吧？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. Plus birds have a super efficientrespiratory system. While a bird’s respiratory system takes up about one fifth of its body, its lungs are relatively¹ rigid² and small, and don’t fully³ deflate. Instead, birds have a complex network of air sacs that work like bellows⁴ 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.

按照单位距离来看的话，飞行是非常顺利的。并且鸟类有一个特别高效的呼吸系统。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。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。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 inhales⁵, the air goes into the rear air sacs, and when it exhales⁶, 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.

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。然后，鸟再次吸气，气体就从肺中进入肺泡。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 less costly⁷ 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.

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。但是科学家们现在也不完全明白鸟类是如何呼吸的。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。

1 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

2 rigid   

adj.严格的，死板的；刚硬的，僵硬的

参考例句：

• She became as rigid as adamant.她变得如顽石般的固执。
• The examination was so rigid that nearly all aspirants were ruled out.考试很严，几乎所有的考生都被淘汰了。

3 fully   

adv.完全地，全部地，彻底地；充分地

参考例句：

• The doctor asked me to breathe in,then to breathe out fully.医生让我先吸气，然后全部呼出。
• They soon became fully integrated into the local community.他们很快就完全融入了当地人的圈子。

4 bellows   

n.风箱；发出吼叫声，咆哮（尤指因痛苦）( bellow的名词复数 )；（愤怒地）说出（某事），大叫v.发出吼叫声，咆哮（尤指因痛苦）( bellow的第三人称单数 )；（愤怒地）说出（某事），大叫

参考例句：

• His job is to blow the bellows for the blacksmith. 他的工作是给铁匠拉风箱。 来自辞典例句
• You could, I suppose, compare me to a blacksmith's bellows. 我想，你可能把我比作铁匠的风箱。 来自辞典例句

5 inhales   

v.吸入( inhale的第三人称单数 )

参考例句：

• Long, slow, full breaths with exhales at least as long as the inhales. 深长、缓慢、充分的呼吸，呼气至少要同吸气一样长。 来自互联网
• An impressive pile forms. Heywood bends down and inhales deeply, smelling the aroma. Rapture. 一小排香烟。海沃德低下头使劲地闻着香烟的气味，高兴不已。 来自互联网

6 exhales   

v.呼出，发散出( exhale的第三人称单数 )；吐出（肺中的空气、烟等），呼气

参考例句：

• He shivers, exhales, gets the ball and races back to his friends. 他浑身一颤，舒了口气，捡起球，跑回到他的朋友们那里。 来自互联网
• A smoker exhales in a pub in Richmond, London. 一名吸菸者在伦敦瑞旗蒙一家酒吧吞云吐雾。 来自互联网

7 costly   

adj.昂贵的，价值高的，豪华的

参考例句：

• It must be very costly to keep up a house like this.维修这么一幢房子一定很昂贵。
• This dictionary is very useful,only it is a bit costly.这本词典很有用，左不过贵了些。