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，会适当的保持充气状态，因为中充满了一种叫做表面活性剂的化学物质，这种物质能使小有弹性，挤压力度小，从而平衡气囊之间的压力。

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，部大约有5亿个。

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

所以，如果你想知道为什么我们有，好吧，现你知道了。

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

现，把你看到的乘以几亿，因为我们每个都有超过1亿个。