Inside the walls of these tiny air sacs are myriads of tiny capillaries, smallest of all blood vessels. The walls of the capillaries are so thin that the molecules of oxygen in the air readily pass through them and into the red blood cells where they unite with the hemoglobin.

This chemical combination, called oxygenation, changes the color of the blood from bluish-purple to a bright cherry red. At the same time carbon dioxide leaves the red blood cells and is breathed out through the windpipes into air outside the body.

Normally we breathe in and out about fifteen times a minute, the total amount of the air at rest being about one pint each time we breathe. Our lungs hold about six pints of air, so that approximately one sixth of the air is exchanged every time we take a breath. Naturally when we are exercising strenuously a great deal more air will be exchanged.