For most of history, humans had no idea what purpose the heart served.
长期以来，心脏的功能一直不为人所知。
In fact, the organ so confused Leonardo da Vinci, that he gave up studying it.
实际上，这个器官也让达·芬奇十分困惑，让他不得不放弃对它的研究。
Although everyone could feel their own heart beating, it wasn't always clear what each thump was achieving.
尽管每个人都能感觉到自己心脏的跳动，但对每次跳动起到的作用却并不清楚。
Now we know that the heart pumps blood.
现在，我们已经知道了心脏的功能是泵血。
But that fact wasn't always obvious, because if a heart was exposed or taken out, the body would perish quickly.
但这在以前却不那么显而易见，因为如果把心脏从体内取出来，身体也很快会死去。
It's also impossible to see through the blood vessels, and even if that were possible,
而且想看清血管里面的情况也不现实，即使看到了，
the blood itself is opaque, making it difficult to see the heart valves working.
血液本身不透明，很难观察心脏瓣膜的运作。
Even in the 21st century, only a few people in surgery teams have actually seen a working heart.
即使在21世纪，只有手术人员才能看到工作中的心脏。
Internet searches for heart function, point to crude models, diagrams or animations that don't really show how it works.
如果上网搜一下心脏如何工作，搜到的是粗略模型、图解，即使是动画都不能准确地描述心脏工作的过程。
It's as if there has been a centuries old conspiracy amongst teachers and students
似乎长久以来，学生和老师们
to accept that heart function cannot be demonstrated.
都默认了心脏的工作过程是无法模拟的。
Meaning that the next best thing is simply to cut it open and label the parts.
这意味着只能退而求其次，把心脏切开，注明各部分名称。
That way students might not fully grasp the way it works,
这种方法虽然不能帮助学生完全明白心脏的工作过程，
but can superficially understand it, learning such concepts as the heart is a four-chambered organ,
但起码表面上能理解而且学到一些知识，比如心脏有四个腔，
or potentially misleading statements like, mammals have a dual-circulation:
以及一些本身就错误的论断，如哺乳动物有双循环，
one with blood going to the lungs and back, and another to the body and back.
一个负责把血液输送到肺然后再流回心脏；另外一个把血液输送到全身然后再流回心脏。
In reality, mammals have a figure-eight circulation.
但实际上，哺乳动物的血液循环呈“8”字形。
Blood goes from one heart pump to the lungs,
血液先通过心脏中的第一个泵挤压被输送到肺部。
back to the second heart pump, which sends it to the body, and then back to the first pump.
回到第二个泵再次挤压被输送到身体各部，然后再回到第一个泵。
That's an important difference because it marks two completely different morphologies.
上述两次血液被挤压的循环方式有很大差别，因为这代表着两种不同的生理结构。
This confusion makes many students wary of the heart in biology lessons,
许多学生对此感到困惑，在生物课上学习心脏方面的知识也感到很吃力，
thinking it signals an intimidating subject full of complicated names and diagrams.
认为这个是个高深的领域，名词难记，图解复杂。

Only those who end up studying medicine compeltely understand how it all actually works.
只有那些最终学习了医学的学生才完全弄懂了心脏是如何工作的。
That's when its functions become apparent as medics get to observe the motion of the heart's valves.
为了弄清心脏的功能，医学学生观察了心脏瓣膜的运动。
So, let's imagine you're a medic for a day.
现在，设想一下你是一名医学学生。
What you'll need to get started is a whole fresh heart, like one from a sheep or pig.
首先，拿一个新鲜完整的心脏，可以是羊的心脏，也可以是猪的心脏。
Immerse this heart in water and you'll see that it doesn't pump when squeezed by hand.
把心脏完全浸入水中，你会发现即使用手捏它也不会泵水。
That's because water doesn't enter the heart cleanly enough for the pumping mechanism to work.
那是因为进入心脏的水还不够，导致心脏无法泵水。
We can solve this problem in an extraordinarly simple way.
我们可以用一个很简单的办法来解决这个问题。
Simply identify the two atria and cut them off, trimming them down to the tops of the ventricles.
那就是找到两个心房并整个切除，一直切到心室上部。
This makes the heart look less complicated because the atria have several incoming veins attached.
这样，心脏看起来就不完整了，因为心房上连接着数条静脉。
So without them there, the only vessels remaining are the two major heart arteries:
所以没了心房，剩下的就只有两个主要的心脏动脉了：
the aorta and pulmonary artery, which rise like white columns from between the ventricles.
主动脉和肺动脉，像两个心室间一根上升的圆锥。
It looks -- and really is -- very simple.
这样看起来的确简单多了。
If you run water into the right ventricle from a tap (the left also works, but less spectacularly),
如果用水管给右心室注水（用左心室也可以，只是不那么直观），
you'll see that the ventricular valve tries to close against the incoming stream.
你会看到心室瓣膜在水注进来时会闭合。
And then ventricle inflates with water.
之后整个心室都灌满了水。
Squeeze the ventricle and a stream of water squirts out of the pulmonary artery.
捏右心室，就会看到一股水流从肺动脉涌出。
The ventricular valves, called the tricuspid in the right ventricle and the mitral in the left,
心室瓣膜有左右之分，右心室的称为三尖瓣，左心室的称为两尖瓣，
can be seen through the clear water opening and closing like parachutes as the ventricle is rhythmically squeezed.
在注水时可以看到它们像降落伞一样打开、闭合，但要同时有节奏地捏心室。
This flow of water mimics the flow of blood in life. The valves are completely efficient.
这种水流的循环模拟了生命体中的血液循环。心脏瓣膜起到了至关重要的作用。
You'll notice they don't leak at all when the ventricles are squeezed.
你会发现当捏心室时，并不会有水漏出来。
Over time, they also close against each other with very little wear and tear,
慢慢地它们会自己闭合，这过程几乎不会受损，
which explains how this mechanism continues to work seamlessly for more than 2 billion beats a heart gives in its lifetime.
这就解释了为什么心脏能夜以继日地高效工作，在人的一生中，能跳动达20亿次之多。
Now, anyone studying the heart can hold one in their hands, make it pump for real and watch the action unfold.
现在，可以的话找一个心脏让它跳动，观察整个过程。
So place your hand above your own and feel its rhymic beat.
然后把手放在你自己的心脏上感受一下这种有韵律的跳动。
Understanding how this dependable inner pump works gives new resonance to the feeling you get
弄懂了这个牢靠的内在的泵的工作原理之后，你就能明白
when you run a race, drink too much caffeine or catch the eye of the one you love.
当你跑步时、摄入了太多咖啡因时或者看见你爱的人，是怎样的感觉了。