When you run fast, or wrestle, or work hard, your muscle-cells work faster, and make more waste, and you breathe faster to get in the oxygen to burn this up--in other words, you fan the body fires, and in consequence you get a great deal hotter, and perhaps perspire in order to get rid of your surplus heat.
The Ocean of Air.

Where does the blood in the body go in order to get this oxygen, which is so vital to it?
Naturally, somewhere upon the surface of the body, because we are surrounded by air wherever we sit, or stand, or move, just as fishes are by water.

All outdoors, as we say, is full of air.

We are walking, just as fishes swim, at the bottom of an ocean of air some thirty miles deep; and the nearer we get up toward the surface of that ocean, as, for instance, when we climb a high mountain, the lighter and thinner the air becomes.

Above ten thousand feet we often have great difficulty in breathing properly, because the air is so thin and weak in oxygen.
How the Lungs Grew Up.