Gas Exchange in Humans
IGCSE Biology Topic 11 5:54 English narration · English + 中文 subtitles burned in
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Your lungs would cover half a tennis court if you spread them flat.
如果把你的肺摊平，它能盖住半个网球场。
They are not one big bag — they are millions of tiny sacs, and that is the whole point.
它们不是一个大袋子—— 而是数百万个微小的气囊，这正是关键所在。
Everything about this topic comes back to one idea: making diffusion as fast as possible.
这个专题的所有内容都归结为一个想法：让扩散尽可能快。
Welcome to gas exchange in humans.
欢迎来到人体的气体交换。
Gas exchange surfaces, the breathing system, how you breathe, and what exercise changes.
气体交换表面、呼吸系统、你如何呼吸， 以及运动会改变什么。
Let's begin.
让我们开始吧。
Start with the definition.
先看定义。
Gas exchange is how oxygen gets into the blood and carbon dioxide gets out, by diffusion.
气体交换是氧气如何进入血液、二氧化碳如何出去，通过扩散。
Notice that word — diffusion.
注意这个词——扩散。
Nothing here uses energy.
这里没有任何东西消耗能量。
So a good gas exchange surface is simply one that makes diffusion fast, and it has four features.
所以好的气体交换表面就是让扩散变快的表面，它有四个特征。
A large surface area, so more gas can cross at once.
大的表面积，让更多气体同时通过。
A thin surface, so gases have only a short distance to travel.
薄的表面，让气体只需移动很短的距离。
A good blood supply, and good ventilation with air.
良好的血液供应，还有和空气的良好通气。
Those last two do the same job: they keep the difference steep — fresh blood carries oxygen away and fresh air brings more in, so the concentration gradient never runs down.
最后两个做的是同一件事： 它们让差别保持陡峭——新鲜的血液把氧气带走，新鲜的空气带来更多氧气， 所以浓度梯度永远不会耗尽。
Learn the path air takes, in order.
按顺序记住空气走的路径。
The larynx, which is the voice box at the top of the windpipe.
喉，也就是气管顶端的发声器官。
Then the trachea, the windpipe itself.
然后是气管，也就是风管本身。
Then the bronchi — two tubes, one going to each lung.
然后是支气管——两根管子，一根通向每一侧肺。
Then the bronchioles, smaller and smaller branching tubes inside the lung.
然后是细支气管，肺内部越来越小的分支管道。
And finally the alveoli, the tiny air sacs where gas exchange actually happens.
最后是肺泡，气体交换真正发生的微小气囊。
Around all of this: the ribs protect the lungs, the intercostal muscles sit between the ribs, and the diaphragm is a sheet of muscle below.
围绕着这一切：肋骨保护肺，肋间肌位于肋骨之间，膈肌是下方的一片肌肉。
For the supplement paper, note that the trachea is held open by rings of cartilage, so it cannot collapse when you breathe in.
补充部分要注意，气管由软骨环撑开，所以你吸气时它不会塌陷。
Here is the swap itself.
这就是交换本身。
Oxygen crosses one way into the blood, and carbon dioxide crosses the other way out.
氧气朝一个方向进入血液，二氧化碳朝另一个方向出去。
Now match the alveolus to those four features, because that is exactly how the question is marked.
现在把肺泡和那四个特征对应起来，因为题目正是这样评分的。
There are millions of them, which gives an enormous surface area.
肺泡有数百万个，这提供了巨大的表面积。
Each has a wall only one cell thick, so the distance is tiny.
每一个的壁只有一个细胞厚，所以距离极小。
And each is surrounded by capillaries, giving the good blood supply.
每一个都被毛细血管包围，提供了良好的血液供应。
Add the air you breathe in and out, and you have ventilation.
再加上你吸进呼出的空气，就有了通气。
Every single feature of a good gas exchange surface is there in one tiny sac.
好的气体交换表面的每一个特征，都在这一个小小的气囊里。
Watch the chest itself.
看胸腔本身。
The ribs lift and the diaphragm flattens — then both go back.
肋骨抬起，膈肌变平——然后两者都回去。
For the supplement paper, you must explain breathing through volume and pressure — never say the lungs "suck".
补充部分要求你用体积和压力来解释呼吸——绝不要说肺在"吸"。
Breathing in — inhaling — has four steps.
吸气有四个步骤。
The external intercostal muscles contract, pulling the ribs up and out.
外肋间肌收缩，把肋骨向上向外拉。
The diaphragm contracts and flattens, moving down.
膈肌收缩变平，向下移动。
The thorax therefore becomes bigger, so the pressure inside drops below the outside air pressure.
因此胸腔变大，里面的压力降到低于外面的气压。
And because the outside pressure is now higher, air is pushed in.
因为外面的压力现在更高，空气就被压进来。
Breathing out — exhaling — is the opposite: the muscles relax, the thorax becomes smaller, the pressure rises, and air is pushed out.
呼气正相反：肌肉放松，胸腔变小，压力升高，空气被压出去。
Bigger means lower pressure; smaller means higher.
变大意味着压力更低；变小意味着压力更高。
Compare inspired and expired air — what goes in against what comes out.
比较进去的空气和出来的空气。
Expired air has less oxygen: it falls from about twenty-one percent to about sixteen percent.
氧气从大约百分之二十一降到大约百分之十六。
Notice that you do not use all the oxygen — most of it comes straight back out, which is why rescue breathing works.
注意你并没有用掉全部氧气——大部分又直接出来了， 这就是为什么人工呼吸有效。
Carbon dioxide rises from about nought point nought four percent to about four percent, a hundred times more.
二氧化碳从大约百分之零点零四升到大约百分之四， 多了一百倍。
And water vapour rises too, from a little to a lot.
水蒸气也上升了，从很少到很多。
You can test for carbon dioxide with limewater, which turns cloudy.
你可以用石灰水检验二氧化碳，它会变浑浊。
Expired air turns limewater cloudy much faster, showing it holds more carbon dioxide.
呼出的空气让石灰水浑浊得快得多，说明它含有更多二氧化碳。
During exercise your muscles respire faster and make more carbon dioxide, so the concentration of carbon dioxide in the blood rises.
运动时你的肌肉呼吸更快，产生更多二氧化碳，所以血液中二氧化碳的浓度上升。
For the supplement paper, say what happens next: your brain detects the rise, and makes you breathe at a faster rate and a greater depth.
补充部分要说出接下来发生什么：你的大脑察觉到上升， 让你以更快的频率和更大的深度呼吸。
Now the calculation.
现在是计算。
At rest a person takes twelve breaths per minute at five hundred cubic centimetres per breath.
静息时一个人每分钟呼吸十二次，每次五百立方厘米。
During exercise it becomes twenty breaths per minute at nine hundred cubic centimetres.
运动时变成每分钟二十次，每次九百立方厘米。
Multiply the rate by the depth.
用频率乘以深度。
At rest, twelve times five hundred is six thousand cubic centimetres per minute.
静息时，十二乘五百等于每分钟六千立方厘米。
During exercise, twenty times nine hundred is eighteen thousand cubic centimetres per minute — three times as much.
运动时，二十乘九百等于一万八千——是三倍。
Exercise raises both the rate and the depth.
运动同时提高频率和深度。
An answer that changes only the rate throws away half the marks.
只改变频率的答案会丢掉一半的分。
Last, for the supplement paper: how the airways stay clean.
最后是补充部分：呼吸道如何保持清洁。
Two kinds of cell work as a team.
两种细胞像团队一样工作。
Goblet cells make mucus, which traps pathogens and dust particles.
杯状细胞制造黏液，黏液粘住病原体和灰尘颗粒。
Ciliated cells have tiny hairs that sweep that mucus, with the dirt caught in it, up to the throat — where it is swallowed.
纤毛细胞有细小的毛，把这些黏液连同粘住的脏东西一起扫到喉咙——在那里被吞下去。
One cell makes the trap, the other moves it.
一种细胞做陷阱，另一种把它移走。
Name both, and say the mucus goes upwards.
两个都要说出名字，并说明黏液是向上走的。
Three marks to lock in.
锁住三个分。
First: a gas exchange surface is large, thin, good blood supply, good ventilation — four features, four marks.
第一：气体交换表面要大、要薄、血液供应好、通气好—— 四个特征，四分。
Second: explain breathing with volume and pressure, and never say the lungs suck; bigger thorax means lower pressure, so air is pushed in.
第二：用体积和压力解释呼吸，绝不要说肺在"吸"； 胸腔变大意味着压力更低，所以空气被压进来。
Third: exercise raises both rate and depth of breathing.
第三：运动同时提高呼吸的频率和深度。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。