The Gas Exchange System
A-Level Biology Topic 9 6:58 English narration · English + 中文 subtitles burned in
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Spread the inside of your lungs out flat and it would cover about seventy square metres — roughly half a tennis court.
把你肺的内表面摊平，大约能盖住七十平方米——差不多半个网球场。
That whole area is folded up inside your chest, into millions of tiny air sacs.
这么大的面积全都折叠在你的胸腔里，变成了几百万个小气囊。
Why go to all that trouble?
为什么要这么麻烦？
Because oxygen has to move into your blood by diffusion alone, and diffusion is slow unless you give it a huge area to work across.
因为氧气只能靠扩散进入你的血液， 而除非给它一个巨大的面积，扩散是很慢的。
In this lesson we follow a breath all the way down, and see exactly how the lung is built for speed.
这节课我们跟着一口气一路向下，看看肺是怎样为速度而生的。
Air starts at the trachea, the windpipe running down your neck.
空气从气管开始，气管是沿着你脖子往下的那根管子。
At the bottom it splits into two bronchi, one heading into each lung.
到底部它分成两条支气管， 各通向一侧的肺。
Inside the lung each bronchus branches again and again, into airways that get smaller and smaller.
在肺内每条支气管又一再分支，气道变得越来越小。
These are the bronchioles.
这些就是细支气管。
At the very end of the smallest ones sit clusters of tiny air sacs.
在最细的那些末端，聚集着一簇簇小气囊。
These are the alveoli, and this is where the actual gas exchange happens.
这些就是肺泡，真正的气体交换就发生在这里。
Every alveolus is wrapped in a net of capillaries, so the air you breathed in and the blood in your vessels end up incredibly close together.
每个肺泡外面都裹着一张毛细血管网， 所以你吸进来的空气和血管里的血液，最后靠得极近。
Cut across an airway and you find several tissues, each doing one job.
横切一条气道，你会看到好几种组织，每一种只做一件事。
In the wall of the trachea and the bronchi sit C-shaped rings of cartilage.
在气管和支气管的壁上有 C 形的软骨环。
They are stiff, so the airway cannot collapse when you breathe in hard.
它们很硬，所以你用力吸气时气道不会塌陷。
The lining itself is ciliated epithelium: tiny hairs called cilia beat to sweep mucus up towards the throat, away from the lungs.
衬里本身是纤毛上皮：叫做纤毛的细小毛发不断摆动，把黏液向上扫向咽喉、离开肺部。
Behind it is a band of smooth muscle.
它后面是一圈平滑肌。
When it contracts, the airway gets narrower.
它收缩时，气道就变窄。
Woven through the walls are elastic fibres.
壁里还编织着弹性纤维。
They stretch as you breathe in, then spring back to help push the air out again.
你吸气时它们被拉伸，然后回弹，帮助把空气推出去。
And down in the alveoli the lining is squamous epithelium — flat cells so thin that gases have almost no distance to travel.
而在肺泡里，衬里是扁平上皮——细胞非常薄，气体几乎不需要走什么距离。
The air you breathe is not clean, so the airway has to protect itself.
你吸进的空气并不干净，所以气道必须保护自己。
Scattered through the lining are goblet cells, and behind them mucous glands.
衬里中散布着杯状细胞，后面还有黏液腺。
Both make a sticky layer of mucus that floats on top of the lining.
两者都分泌一层黏黏的黏液，浮在衬里表面。
That mucus traps the dust and the microbes you breathe in, before they can reach the delicate alveoli.
这层黏液把你吸进来的灰尘和微生物粘住， 不让它们到达娇嫩的肺泡。
Then comes the clever part.
接下来是巧妙的部分。
The lining cells carry thousands of tiny hairs called cilia, and the cilia beat together in waves.
衬里细胞上长着成千上万根细毛，叫纤毛， 纤毛一起像波浪一样摆动。
They push the dirty mucus steadily upwards, away from the lungs and towards the throat, where you swallow it without ever noticing.
它们把带脏东西的黏液稳稳地往上推，离开肺，推向喉咙， 你在毫无察觉之间就把它咽了下去。
Zoom in on one alveolus with a capillary pressed against it.
把镜头拉近到一个肺泡，旁边紧贴着一条毛细血管。
Between the air and the blood there are only two barriers.
空气和血液之间只有两道屏障。
The wall of the alveolus is squamous epithelium, one cell thick.
肺泡壁是扁平上皮，一个细胞厚。
The wall of the capillary is endothelium, also one cell thick.
毛细血管壁是内皮，也是一个细胞厚。
Both cells are flattened out, so together they are less than one micrometre — that is a thousandth of a millimetre.
两种细胞都被压得很扁，所以加起来还不到一微米——也就是一毫米的千分之一。
The inside of the alveolus is also moist, which matters, because a gas has to dissolve in that film of liquid before it can cross.
肺泡内表面还是湿润的，这一点很重要，因为气体必须先溶解在那层液膜里才能穿过去。
So why is the alveolus so good at its job?
那么肺泡为什么这么擅长它的工作呢？
There are four reasons, and the examiner wants all four.
有四个原因，考官四个都要。
First, a huge surface area — millions of sacs adding up to about seventy square metres.
第一，巨大的表面积——几百万个气囊加起来约七十平方米。
Second, very thin walls, one cell on the alveolus side and one cell on the capillary side.
第二，非常薄的壁：肺泡这边一个细胞，毛细血管那边一个细胞。
Third, a rich blood supply.
第三，丰富的血液供应。
Every sac is wrapped in a dense net of capillaries, and that blood is always moving, always carrying the oxygen away.
每个气囊都被一张密密的毛细血管网裹着， 而那些血一直在流动，一直把氧带走。
Fourth, a moist lining.
第四，湿润的内衬。
The inside surface is covered by a thin film of liquid, and a gas has to dissolve in it before it can cross.
内表面覆盖着一层薄薄的液膜，气体必须先溶解在里面才能穿过去。
Miss any one of these and the exchange slows down.
少了其中任何一条，交换都会变慢。
Nothing pumps the gases across.
没有任何东西在泵这些气体。
They simply diffuse, each one from where it is more concentrated to where it is less.
它们只是扩散，各自从浓度高的地方走向浓度低的地方。
Start with oxygen.
先看氧气。
The fresh air in the alveolus is rich in oxygen, and the blood arriving has just given its oxygen away to the tissues, so it is low.
肺泡里的新鲜空气富含氧，而刚到达的血液已经把氧交给了组织，所以含氧低。
Oxygen therefore moves from the air into the blood.
于是氧从空气进入血液。
Now carbon dioxide, which does the opposite.
再看二氧化碳，情况正好相反。
The blood arrives loaded with it and the alveolar air has very little, so carbon dioxide moves out of the blood and into the air, ready to be breathed out.
血液带着大量二氧化碳到来，而肺泡空气里几乎没有， 所以二氧化碳从血液走进空气，准备被呼出去。
And here is why it never slows down. Breathing keeps replacing the air, and the flowing blood keeps carrying the gases away, so a steep gradient is maintained the whole time.
这就是它为什么从不减速的原因：呼吸不断更换空气，流动的血液不断把气体带走， 所以两个梯度都保持陡峭。
All of this is captured by one relationship, called Fick's law.
这一切都可以用一个关系式概括，叫菲克定律。
It says the rate of diffusion is proportional to the surface area, multiplied by the concentration difference, divided by the diffusion distance.
它说扩散速率正比于表面积， 乘以浓度差，再除以扩散距离。
Three quantities, and each one matters.
三个量，每一个都重要。
A bigger surface area means a faster rate.
表面积越大，速率越快。
A bigger concentration difference means a faster rate.
浓度差越大，速率越快。
And a shorter diffusion distance also means a faster rate, because that one sits on the bottom of the fraction.
扩散距离越短，速率也越快，因为它在分数的下面。
So to make exchange fast you want a large top and a small bottom.
所以想让交换变快，就要让上面大、下面小。
Now use the law to explain the lung.
现在用这个定律来解释肺。
Take each term in turn.
逐项来看。
Surface area: millions of alveoli give a total of about seventy square metres, so the top of the fraction is huge.
表面积：几百万个肺泡加起来大约七十平方米， 所以分数的上面非常大。
Diffusion distance: two flattened cells, under one micrometre, so the bottom of the fraction is tiny.
扩散距离：两个压扁的细胞，不到一微米，所以分数的下面非常小。
Concentration difference: ventilation and blood flow mean it is kept steep the whole time.
浓度差：通气和血流让它全程保持陡峭。
Every one of the three works in the same direction, which is why gas exchange in the lung is so fast.
三项都朝同一个方向起作用， 这就是肺里气体交换如此之快的原因。
And here is the exam point. Tie each fact to the term it serves.
这里还有一个考试要点： 把每个事实对应到它服务的那一项。
Simply listing big surface area, thin walls and a good blood supply is the weaker answer, because it never says which part of the equation each one helps.
只是罗列表面积大、壁薄、血液供应好，是较弱的答案， 因为它始终没有说清每一点帮的是方程里的哪一部分。
Three things the examiner keeps asking for.
考官反复要的三点。
First, when you explain fast exchange, name the term of Fick's law that each adaptation serves.
第一，解释交换为什么快时，要点明每一项适应对应菲克定律的哪一项。
That single habit turns an average answer into a full-mark one.
光是这一个习惯，就能把一个中等的答案变成满分答案。
Second, do not just write that the wall is thin.
第二，不要只写壁很薄。
Write that it is one cell thick — the marker is looking for that exact phrase.
要写它是一个细胞厚——阅卷人找的就是这个说法。
Third, when you are given an airway tissue, match it to its job: cartilage holds the airway open, cilia and goblet cells trap and sweep the mucus, smooth muscle narrows the airway, and elastic fibres recoil to push air back out.
第三，给你一种气道组织时，要把它对应到它的功能：软骨撑开气道， 纤毛和杯状细胞粘住并清扫黏液，平滑肌使气道变窄，弹性纤维回弹把空气推出去。
So there it is.
就是这样了。
One long branching path, a lining that cleans itself, and at the end a surface enormous in area and thinner than you can imagine.
一条长长的分支通路，一层会自我清洁的衬里， 末端是一个面积巨大、薄得超乎想象的表面。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。