The Circulatory System
A-Level Biology Topic 8 9:17 English narration · English + 中文 subtitles burned in
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Your heart beats about a hundred thousand times a day, and it never gets a day off.
你的心脏每天大约跳动十万次，而且从不休息。
But here is the clever part. It is not one loop. It is two.
但巧妙之处在于：它不是一个循环，而是两个。
Blood leaves the heart, goes to the lungs to pick up oxygen, and comes straight back to the heart.
血液离开心脏，去肺里取氧，然后直接回到心脏。
Only then is it pushed out to the rest of your body.
之后才被推送到身体其他部位。
So every drop of blood passes through the heart twice on each full trip.
所以每一滴血在一次完整的旅程中都要两次经过心脏。
In this lesson we follow that blood all the way round.
这节课我们就跟着血液走完全程。
The circulatory system of a mammal is a closed double circulation.
哺乳动物有封闭式双循环。
Closed means the blood always stays inside blood vessels.
封闭指血液始终留在血管内。
Double means there are two loops.
双循环指有两个回路。
The pulmonary loop carries blood from the heart to the lungs and back.
肺循环把血液从心脏送到肺再送回。
The systemic loop carries it from the heart to the body and back.
体循环把血液从心脏送到身体再送回。
Now the named vessels.
再看有名字的血管。
The pulmonary artery takes blood low in oxygen to the lungs.
肺动脉把含氧低的血送到肺。
The pulmonary vein brings oxygen rich blood back.
肺静脉把富氧血送回来。
The aorta is the big artery to the body, and the vena cava is the big vein returning from it.
主动脉是通向身体的大动脉，腔静脉是从身体返回的大静脉。
Remember: arteries carry blood away from the heart, veins carry it back.
注意这条规则： 动脉总是把血送离心脏，静脉总是把血送回心脏。
Blood travels in this order.
这说的是方向，不是含氧量。
Artery, arteriole, capillary, venule, and then vein.
血液按这个顺序流动：动脉、小动脉、毛细血管、小静脉，然后是静脉。
Each vessel is built for the pressure it meets.
每种血管都为它所承受的压力而建。
Start with the artery.
先看动脉。
It has a thick wall of muscle and elastic fibres, and a narrow lumen — the space inside.
它有厚的肌肉和弹性纤维管壁，内腔狭窄。
Blood leaves the heart at high pressure, so the elastic wall stretches and then springs back, which smooths the flow between beats.
血液以高压离开心脏，所以弹性壁先撑开再回弹，让两次心跳之间的血流变得平稳。
Now the capillary.
再看毛细血管。
Its wall is just one cell thick, so the distance substances have to travel is tiny.
它的壁只有一个细胞厚，物质要走的距离极短。
That is what makes exchange fast.
这就是交换快的原因。
And the vein.
还有静脉。
Its wall is thin, the lumen is wide, and it carries valves.
它壁薄、内腔宽，并带有瓣膜。
By the time blood gets here the pressure is low, so the valves are what stop it sliding backwards.
血液到这里时压力已经很低， 所以是瓣膜阻止血液倒流。
Watch what happens along a single capillary.
看看在一条毛细血管里发生了什么。
At the start, where the blood arrives from an artery, the pressure is high.
开始处，血液从动脉流来，压力很高。
That pressure squeezes liquid out through the thin wall and into the spaces between the cells.
这个压力把液体挤过薄壁，进入细胞之间的空隙。
Oxygen and glucose go with it.
氧和葡萄糖也跟着出去。
Further along, the pressure has dropped, and most of that liquid drains back in, carrying waste away with it.
再往前走，压力已经下降，大部分液体又流回来，同时把废物带走。
This slow leak and return is happening in every tissue of your body, right now.
这种缓慢的渗出与回收，此刻正在你身体的每一处组织里发生。
That liquid has a name. It is tissue fluid.
那种液体有个名字，叫组织液。
Water is the main part of blood, and water is very good at this job for two reasons. It is an excellent solvent, so it can carry dissolved substances, and it has a high specific heat capacity, so the temperature of the blood stays steady. At the arterial end of a capillary the blood pressure is high, and it pushes plasma out through the wall.
水是血液的主要成分，而水非常适合这项工作，有两个原因： 它是极好的溶剂，能携带溶解的物质；它保温能力强，所以血液温度保持稳定。
The cells and the large proteins are too big to follow, so they stay inside.
在毛细血管的动脉端，血压很高，把血浆推出管壁。
The fluid that escapes bathes the body cells, delivering oxygen and glucose and taking waste away.
细胞和大蛋白太大跟不出去，就留在里面。 逸出的液体浸润着体细胞，送去氧和葡萄糖，带走废物。
At the venous end the pressure has fallen, so most of that fluid returns to the blood.
在静脉端压力已经下降，所以大部分液体又回到血液中。
Oxygen is carried by haemoglobin inside the red blood cells.
氧由红细胞里的血红蛋白携带。
In a stained blood smear you can also pick out the three white cells the syllabus names — monocytes, neutrophils and lymphocytes — all larger than a red cell, and all with a nucleus.
在染色的血涂片上，你还能认出考纲点名的 三种白细胞——单核细胞、中性粒细胞和淋巴细胞——它们都比红细胞大，而且都有细胞核。
To show how tightly haemoglobin holds on, we plot a graph.
为了显示血红蛋白抓得有多紧，我们画一张图。
Along the bottom is the partial pressure of oxygen — how much oxygen is around.
横轴是氧分压，也就是周围有多少氧。
Up the side is saturation — how full of oxygen the haemoglobin is.
纵轴是饱和度，也就是血红蛋白装了多满的氧。
The graph that comes out is the oxygen dissociation curve, and it is S-shaped.
画出来的这条曲线叫氧解离曲线，呈 S 形。
Read it in two places.
分两处来读它。
In the lungs, where there is plenty of oxygen, haemoglobin loads up and is almost completely full.
在肺里氧很充足，血红蛋白装满，几乎完全饱和。
In a respiring tissue, where oxygen is scarce, haemoglobin lets go and unloads its oxygen for the cells.
在呼吸作用旺盛的组织里氧很少，血红蛋白就松手，把氧卸给细胞。
Now the clever part.
接下来是巧妙的地方。
An active tissue makes a lot of carbon dioxide, and that lowers the pH.
活跃的组织产生大量二氧化碳，这会降低 pH。
Lower pH makes haemoglobin release its oxygen more easily, so the whole curve moves to the right.
pH 降低让血红蛋白更容易释放氧，于是整条曲线向右移动。
That is the Bohr shift, and it means oxygen is given up exactly where it is needed most.
这就是波尔位移，意思是氧恰好在最需要的地方被交出来。
Let us use the curve.
我们来用一下这条曲线。
In the lungs the partial pressure of oxygen is about twelve kilopascals, and haemoglobin is about ninety eight percent saturated.
在肺里氧分压约为十二千帕，血红蛋白约百分之九十八饱和。
In an exercising muscle it is only about three kilopascals, and saturation falls to about forty percent.
在运动中的肌肉里只有约三千帕，饱和度降到约百分之四十。
Read both points off the graph first. Then subtract one from the other.
先在图上读出这两个点， 然后相减。
Ninety eight take away forty is fifty eight, so fifty eight percent of the oxygen is unloaded in that muscle.
九十八减四十等于五十八，所以有百分之五十八的氧在那块肌肉里被卸下。
And why is the curve S-shaped? The answer is cooperative binding.
那么曲线为什么一开始就是 S 形的呢？
The first oxygen to attach changes the shape of the haemoglobin and makes it easier for the next ones to attach.
答案是协同结合。 第一个氧结合上去会改变血红蛋白的形状，让后面的氧更容易结合。
That is why the middle is so steep — and a small drop in oxygen then releases a large amount of it.
这就是曲线中段如此陡峭的原因，而陡峭正是关键：氧只要略微下降，就会释放出大量的氧。
Carbon dioxide travels a different way.
二氧化碳走的是另一条路。
A little of it simply dissolves in the plasma, but most is changed first, inside the red blood cell.
一小部分直接溶解在血浆里，但大部分先在红细胞内被转化。
The enzyme carbonic anhydrase speeds up a reaction between carbon dioxide and water, making carbonic acid.
碳酸酐酶这种酶加快二氧化碳与水反应，生成碳酸。
That acid then splits in two, giving hydrogen ions and hydrogencarbonate ions.
碳酸接着一分为二， 给出氢离子和碳酸氢根离子。
The hydrogencarbonate moves out into the plasma, and that is the main way carbon dioxide is carried around the body.
碳酸氢根移到血浆中，这就是二氧化碳在体内运输的主要方式。
But moving a negative ion out would leave the cell unbalanced, so a chloride ion moves in to take its place.
但把一个负离子移出去会让细胞电荷失衡，所以一个氯离子移进来顶替它。
That swap is the chloride shift.
这次交换就是氯转移。
Meanwhile the hydrogen ions left behind join onto haemoglobin, forming haemoglobinic acid, which mops them up and keeps the pH steady.
同时留下的氢离子与血红蛋白结合，形成血红蛋白酸， 把氢离子收拾掉，让 pH 保持稳定。
A little carbon dioxide also joins haemoglobin directly, forming carbaminohaemoglobin.
还有少量二氧化碳直接与血红蛋白结合。
Now the pump itself.
现在看泵本身。
The heart has four chambers.
心脏有四个腔。
The two upper ones are the atria.
上面两个是心房。
Their walls are thin, because all they have to do is push blood downwards into the chamber below.
它们的壁很薄， 因为它们要做的只是把血往下推进下面的腔。
The two lower ones are the ventricles.
下面两个是心室。
Their walls are thick and muscular, because they push blood right out of the heart.
它们的壁厚而多肌肉， 因为它们要把血推出心脏。
Look carefully and you will see the left ventricle has the thickest wall of all.
仔细看你会发现左心室的壁最厚。
That is because the left side has to send blood all the way round the body, while the right side only has to reach the lungs, which are very close by.
这是因为左侧要把血送到全身各处，而右侧只需送到很近的肺。
Between the chambers sit valves, and they make sure blood can only move one way.
各腔之间有瓣膜，保证血液只能朝一个方向流动。
One heartbeat is called the cardiac cycle, and it has two halves.
一次心跳叫做心动周期，它有两个阶段。
When the heart muscle contracts, we call systole.
心肌收缩时，我们称为收缩期。
When it relaxes and fills again, we call diastole.
心肌舒张并重新充盈时，我们称为舒张期。
Watch what the pressure does.
看看压力的变化。
As a chamber contracts, the pressure inside it shoots up. As it relaxes, the pressure falls away.
腔室收缩时，里面的压力猛升；舒张时，压力落下去。
Those changing pressures are what open and close the valves, with no nerve or muscle needed.
正是这些变化的压力开启和关闭瓣膜，不需要神经或肌肉。
And this gives you the exam trick for reading the graph. Wherever two pressure curves cross over, a valve has just opened or just closed.
这就给了你读图的考试窍门：凡是两条压力曲线交叉的地方，都刚好有一个瓣膜开或关。
When ventricle pressure rises above the pressure in the aorta, the valve to the aorta opens and blood is pushed out.
当心室压力升到高于主动脉压力时，通往主动脉的瓣膜打开，血被推出去。
When it falls back below, it snaps shut.
当它又落到下面时，那个瓣膜就啪地关上。
Nothing outside the heart tells it when to beat. It sets its own rhythm.
没有任何来自心脏外部的东西告诉它何时跳动。
In the wall of the right atrium sits a small patch of tissue called the sinoatrial node, and it is the pacemaker.
它自己设定节律。 在右心房壁上有一小片组织，叫窦房结，它就是起搏点。
It sends out a wave of electrical excitation that spreads across both atria and makes them contract.
它发出一波电兴奋，扩散到两个心房，使它们收缩。
The wave then reaches the atrioventricular node, which does something useful. It holds it back for a fraction of a second, so the atria have time to finish emptying before the ventricles start.
这波兴奋随后到达房室结， 房室结做了件很有用的事：它把兴奋拖住几分之一秒，让心房有时间排空，心室才开始收缩。
Then the wave is passed on to the Purkyne tissue, which carries it down to the base of the heart and up through the ventricle walls.
然后兴奋被传给浦肯野组织，由它带到心脏底部，再沿心室壁向上传。
So the ventricles squeeze from the bottom upwards, pushing the blood neatly out of the top.
于是心室由下往上挤压，血液就被利落地从顶部推出去。
Four things the examiner keeps asking for.
考官反复要的四点。
First, the left ventricle wall is always thicker than the right, and the reason is the distance it must pump.
第一，左心室壁总是比右心室厚，原因是它要把血泵得更远。
Say the reason, not just the fact.
要说出原因，而不只是说出事实。
Second, on a cardiac cycle graph, a valve moves wherever two curves cross.
第二，在心动周期图上，凡是两条曲线交叉处就有瓣膜动作。
Find the crossing point and you have the answer.
找到交点，答案就有了。
Third, extra carbon dioxide shifts it right, and that means more oxygen is unloaded to the tissue that is working hardest.
第三，多余的二氧化碳把曲线向右移， 意思是有更多的氧被卸给工作最辛苦的组织。
Fourth, most carbon dioxide is carried as hydrogencarbonate, not dissolved and not stuck to haemoglobin, and the chloride shift is what balances the charge.
第四，大部分二氧化碳以碳酸氢根的形式运输， 不是溶解的，也不是结合在血红蛋白上的，而氯转移负责平衡电荷。
So there it is.
就是这样了。
Two loops, three kinds of vessel, an S-shaped curve that gives oxygen up where it is needed, and a pump that keeps its own time.
两个回路、三种血管、一条在最需要处交出氧的 S 形曲线， 还有一个自己掌握节奏的泵。
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。