THE GREAT ESSENTIAL TO LIFE—AIR
If the air, supplied to the diver through the tube, is cut off for three minutes, or even less, the diver cannot live.
Part of the waste that comes from our body cells is either watery, or easily dissolved in water; and this is carried in the blood to a special set of filter organs—the liver and the kidneys—and poured out of the body as the urine. Another part of it, when circulating through the skin, is passed off in the form of that watery vapor which we call perspiration, or sweat. But part of the waste can be got rid of only by burning, and what we call burning is another name for combining with oxygen, or to use one word—oxidation; and this is precisely the purpose of the carrying of oxygen by the little red blood cells from the lungs to the deeper parts of the body—to burn up, or oxidize, these waste materials which would otherwise poison our cells. When they are burnt, or oxidized, they become almost harmless.
Why the Red Cells Carry only Oxygen to the Body. But why do not the red cells carry air instead of just oxygen? This is simply a clever little economy of space on nature's part. As a chemist will tell you the air which we breathe is a mixture of two gases—one called nitrogen and the other oxygen; just as syrup, for instance, is a mixture of sugar and water. Then too, as in syrup, there are different amounts of the two substances in the mixture: as syrup is made up of about one-quarter sugar and three-quarters water, so air is made up of one-fifth oxygen and four-fifths nitrogen. Now the interesting thing about this mixture, which we call air, is that the only really "live" and vital part of it for breathing purposes is the one-fifth of oxygen, the four-fifths of nitrogen being of no use to our lungs. In fact, if you split up the air with an electric current, or by some other means, and thus divide it into a small portion of pure oxygen (one-fifth), and a very much larger portion (four-fifths) of nitrogen, the latter would as promptly suffocate the animal that tried to breathe it as if he were plunged under water.[18]
It may perhaps be difficult to think of anything burning inside of your bodies where everything is moist, especially as you do not see any flame; but you do find there one thing which always goes with burning, and that is warmth, or heat. This slow but steady and never-ceasing burning, or oxidation, of the waste and dirt inside your bodies is what keeps them warm. 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. In the simplest forms of life, any part of the soft and delicate surface will do for the blood to reach, in order to throw off its load of carbon "smoke" and take on its supply of oxygen. In fact, animals like jellyfish and worms are lungs all over. But as bodies begin to get bigger, and the skin begins to toughen and harden, this becomes more and more difficult, although even the highest and biggest animals like ourselves still throw off a certain amount of this carbon dioxid and other gases through the skin. Accordingly, certain parts of the surface of the body are set apart specially for this business of breathing; and as we already have an opening into the body provided by the mouth and food tube, the simplest thing to do is to use the mouth for taking in air, when it is not being used for taking in food, and to set aside some part of the food tube for breathing purposes.
