Our lungs hold, however, very much more than this amount.

A man, after he has taken a full breath, can breathe out a gallon of air, or more than ten times the usual amount.

After he has breathed out all he can, there is still almost half a gallon of air in his lungs which he cannot breathe out.

So you see the lungs hold almost a gallon and a half of air.
~24.~ Do you think you can tell why Nature has given us so much more room in the lungs than we ordinarily use in breathing?
If you will run up and down stairs three or four times you will see why we need this extra lung-room.

It is because when we exercise vigorously the heart works very much faster and beats harder, and we must breathe much faster and fuller to enable the lungs to purify the blood as fast as the heart pumps it into them.
~25.