Dr. Jack Feldman

At some point, at the end of those branches, you put a little sphere, which is an alveolus, and that determines what the surface area is going to be.

you then have a mechanical problem.

You have the surface area, you have to be able to pull it apart.

So imagine you have a little square of elastic membrane.

It doesn't take a lot of force to pull it apart.

But now if you increase it by 50 times, you need a lot more force to pull it apart.

So amphibians who were breathing, not by compressing the lungs and then just passively expanding it,

weren't able to generate a lot of force.

So they have relatively few branches.

So if you look at the surface area that they pack in their lungs relative to their body size, it's not very impressive.

Whereas when you get to mammals,

the amount of branching that you have is you have four to 500 million alveoli.

So you have a membrane inside of you, a third the size of a tennis court, that you actually have to expand every breath.

And you do that without exerting much of a... You don't feel it.

And that's because you have this amazing muscle, the diaphragm, which because of its positioning, just by moving...

two thirds of an inch down is able to expand that membrane enough to move air into the lungs.

At rest, the volume of air in your lungs is about two and a half liters.

When you take a breath, you're taking another 500 milliliters or half a liter.

That's the size maybe of my fist.

So you're increasing the volume by 20%.