Dr. Jack Feldman

So on the mechanical side, which is obvious to everyone, we want to have air flow in, inhale, and we need to have air flow out.

And the reason we need to do this is because for body metabolism, we need oxygen.

And when oxygen is utilized through the aerobic metabolic process, we produce carbon dioxide.

And so we have to get rid of the carbon dioxide that we produce

in particular because the carbon dioxide affects the acid-base balance of the blood, the pH, and all living cells are very sensitive to what the pH value is.

So your body is very interested in regulating that pH.

So how do we generate this airflow?

We have to expand the lungs, and as the lungs expand,

Basically, it's like a balloon that you would pull apart.

The pressure inside that balloon drops, and air will flow into the balloon.

That lowers the pressure in the air sacs called alveoli, and air will flow in because pressure outside the body is higher than pressure inside the body when you're doing this expansion, when you're inhaling.

What produces that?

Well, the principal muscle is the diaphragm, which is sitting inside the body just below the lung.

And when you want to inhale, you basically contract the diaphragm and it pulls it down.

And as it pulls it down, it's inserting pressure forces on the lung.

The lung wants to expand.

At the same time, the rib cage is going to rotate up and out.

and therefore expanding the cavity, the thoracic cavity.

At the end of inspiration,

Under normal conditions, when you're at rest, you just relax.