Our lessons thus far have shown us two kinds of blood coursing through the body, and we have learned to associate the impure, purple blood with the waste, poisonous matter from the worn-out tissues which that blood is carrying away. But how does it carry these impurities away? This is what we have to learn next. Think of the dark purple blood as it is being collected up by the veins. It was, just before, flowing through the arteries as bright red, arterial blood. The blood carried by the veins up to the heart, and thence to the lungs, is the same dark purple fluid. It returns to the heart by the pulmonary veins pure, bright- red blood. It has been cleansed from its impurities while passing through the lungs. But it has not only given up its poisonous impurities, it has taken in something in the lungs which is very necessary to our existence. You know that one-fifth of the bulk of all the air around us consists of oxygen, and that the chief property of this gas is its power of burning or oxidizing other substances. Suppose I placed a piece of burning candle in a jar of oxygen; what would happen? It would burn rapidly, with a very brilliant glow, and it would continue to burn till all theoxygen was consumed. In the meantime the substance of the candle would disappear. But how does it disappear? The oxygen, you know, burns the carbon and hydrogen of which the candle is composed, and changes these into the form of gases-carbonic acid and water-vapor. Think ofthe coal burning in the fire-grate. The smoke which risesup the chimney is very different in appearance, and yet it contains all the substance of the coal itself, converted into another form by the oxygen of the air; for burning is merely a change; nothing is lost; there is no destruction.
Now let us see what all this has to do with getting rid of waste matters from the body. The solid particles of brain, muscle, and other tissues, although they have by their work become worn-out and waste matter, cannot be carried away out of the body in that state. They mustundergo a complete change into new and totally different substances. This change is brought about by the gas- oxygen. Throughout life the work of breathing never ceases, and the work is two fold. We exhale-or send out-and we inhale, or breathe in. That which we inhale is pure air, one-fifth of it being oxygen. The oxygen, thus taken in as we breathe, is absorbed into the blood in the lungs, and carried into all parts of the body.
Wherever the oxygen meets with tissues which have been used up, or worn out by their work, it seizes upon them, and a slow burning or combustion at once commences. The harder we work the faster our tissues aredestroyed, and the quicker must we breathe, in order to take in sufficient oxygen to carry on the burning. It is this constant burning up of the waste matters by oxygen in all parts of the body that keeps us warm.
When we are in bad health, and the blood does not circulate freely, we are always cold. There is not enough oxygen sent through the body at such times to do the necessary work of burning or oxidizing the waste matters.
What happens when we have been undergoing some unusual exercise? We breathe more rapidly; the heart beats at a quicker rate; and we feel the blood coursing through the body.
Why is this? More of the tissues than usual have been used up and become waste matter, and more oxygen must be taken in and carried to them by the blood to burn them up. The increased burning makes us feel warmer. If you notice the surface of the skin at such a time, you will find it covered with drops of moisture. This moisture oozes out from the skin-we sweat or perspire. If Ibreathe, moreover, on a slate or some polished surface, Ifind that my breath contains moisture too.
Now our lessons in chemistry have shown us that water is one of the products of burning. It is always produced when hydrogen burns. Hence we learn that hydrogen is one of the materials of which our bodies are made.
You remember, of course, our now familiar experiment of breathing into lime-water, and what it teaches us.
Carbonic acid is another of the products of the burning, for it is given off with our breath.
These two substances, water and carbonic acid, are being constantly formed by the oxidation of the hydrogen and carbon, which enter largely into the constituent materials of the body. There are other products of the burning-one in particular, ammonia-but carbonic acidand water are the chief.
The blood, as it courses through the body, absorbs these products of oxidation, and they change its character from a life-giving to a poisonous stream. It becomes dark purple in color, and is carried in this state by the veins back to the heart, and so on to the lungs. There it gives up carbonic acid and water, and takes in oxygen in exchange. It is this exchange which reconverts the venous into arterial blood once more.
