Dr. Jack Feldman

And so part of these many different sub-nuclei now seems to be in mammals to be involved in the control of expiratory muscles.

But we have to remember that mammals are very special when it comes to breathing because we're the only class of vertebrates that have a diaphragm.

If you look at amphibians and reptiles, they don't have a diaphragm.

And the way they breathe is not by actively inspiring and passively expiring.

They breathe by actively expiring and passively inspiring because they don't have a powerful inspiratory muscle.

And somewhere along the line, the diaphragm developed.

The amazing thing about the diaphragm is that it's mechanically extremely efficient.

If you look at how oxygen gets from outside the body into the bloodstream,

The critical passage is across the membrane in the lung.

It's called the alveolar capillary membrane.

The alveolus is part of the lung, and the blood runs through capillaries, which are the smallest tubes in the circulatory system.

And at that point, oxygen can go from the air-filled alveolus

into the blood.

The key element is the surface area.

The bigger the surface area, the more oxygen that can pass through.

It's entirely a passive process.

There's no magic about making oxygen go in.

Now, how do you get a pack, a large surface area in a small chest?

When you start out with one tube, which is the trachea, the trachea expands.

Now you have two tubes, then you have four tubes, and it keeps branching.