At the end of each branch is a tiny air sac called an alveolus.

每个分支的末端都有一个叫做肺的小气囊。

These outpouchings are called alveoli, and there are about 500 million of them within the lungs.

这些突起称为肺，肺部大约有5亿个肺。

So, if you wonder why we have alveoli, well, now you know.

所以，如果想道为们有肺，好吧，现道了。

Now, multiply what you see by hundreds of millions, as each of our lungs contain over 100 million alveoli.

现，把看到的乘以几亿，因为们每个肺都有超过1亿个肺。

But in healthy people, alveoli stay properly inflated because they're full of chemicals called pulmonary surfactants that make the small alveoli more stretchy and less squeezy, balancing the pressure between air sacs.

但健康的人体内，肺会适当的保持充气状态，因为肺中充满了一种叫做肺表面活性剂的化学物质，这种物质能使小肺更有弹性，挤压力度更小，从而平衡气囊之间的压力。

Human lungs have essentially the same setup, but in healthy people, alveoli stay properly inflated because they're full of chemicals called pulmonary surfactants that make the small alveoli more stretchy and less squeezy, balancing the pressure between air sacs.

人类的肺基本上具有相同的设置， 但健康人中，肺保持适当的膨胀， 因为它们充满了称为肺表面活性剂的化学物质， 使小肺更有弹性， 更少挤压，平衡气囊之间的压力。