22. We breathe once for each four heart-beats. Small children breathe more rapidly than grown persons. We usually breathe about eighteen or twenty times in a minute.
23. How Much the Lungs Hold.—Every time we breathe, we take into our lungs about two thirds of a pint of air and breathe out the same quantity. 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. The Two Breaths.—We have learned that the air which we breathe out contains something which is not found in the air which we breathe in. This is carbonic-acid gas. How many of you remember how we found this out? We can also tell this in another way. If we put a candle down in a wide jar it will burn for some time. If we breathe into the jar first, however, the candle will go out as soon as we put it into the jar. This shows that the air which we breathe out contains something which will put a candle out. This is carbonic-acid gas, which is a poison and will destroy life.
26. Other Poisons.—The air which we breathe out also contains other invisible poisons which are very much worse than the carbonic-acid gas. These poisons make the air of a crowded or unventilated room smell very unpleasant to one who has just come in from the fresh air. Such air is unfit to breathe.
27. The Lungs Purify the Blood.—We have learned that the blood becomes dark in its journey through the body. This is because it loses its oxygen and receives carbonic-acid gas. While passing through the capillaries of the lungs, the blood gives out the carbonic-acid gas which it has gathered up in the tissues, and takes up a new supply of oxygen, which restores its scarlet hue.
28. How the Air is Purified.—Perhaps it occurs to you that with so many people and animals breathing all the while, the air would after a time become so filled with carbonic-acid gas that it would be unfit to breathe. This is prevented by a wonderful arrangement of Nature. The carbonic-acid gas which is so poisonous to us is one of the most necessary foods for plants. Plants take in carbonic-acid gas through their leaves, and send the oxygen back into the air ready for us to use again.
29. We have already learned that the oxygen taken in by the lungs is carried to the various parts of the body by the little blood corpuscles. The effect of strong liquors is to injure these corpuscles so that they cannot carry so much oxygen as they ought to do. For this reason, the blood of a drunkard is darker in color than that of a temperate person, and contains more carbonic-acid gas. The drunkard's lungs may supply all the air he needs, but his blood has been so damaged that he cannot use it. Excessive smoking has a similar effect.
SUMMARY.
1. Our bodies need air, just as a candle or a fire does.