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

在每分支的末端都有一叫做的小气囊。

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

这些突起称为，在部大约有5。

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，现在你知道了。

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

现在，把你看到的乘以几，因为我们每都有超过1。

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.

但在健康的人体内，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.

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