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TR3T
Humans usually breathe from sixteen to twenty times each minute. If you analyzed
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the air you breathe, you would find it is a mixture of different gases. Most of it is
*nitrogen
about four-fifths. One-fifth is oxygen. There is also a tiny amount of
carbon dioxide, a little "water vapor (which gives air its humidity), and some "traces of
05 what are called "rare gases.
If you were to put a bag over your nose and mouth to catch the air you breathe out, i図
you would find
(1)Some strange changes. There would still be the same amount of
nitrogen. There would also be the same traces of rare gases. But there would be much
less oxygen and a hundred times more carbon dioxide than in the air you breathe in.
10 There would also be considerably more water vapor.
TR33
,What happens is that each time you breathe, an exchange takes place. You keep
Some oxygen; you breathe out much more carbon dioxide and water vapor than you
breathed in.
、The reason is that every moment of the day and night your body is using
up energy. Your heart uses up energy as it beats. Your muscles use up energy. So
15 does your brain, and so does every other part of you. All this energy is produced by the
work of the millions and millions of cells that make up your body. Every one of these
cells needs Oxygen in order to do its work. As the cells use up oxygen, they form
carbon dioxide, which is a “waste product.
So your body carries out these two processes at the same time. You breathe in the m3
20 OXygen that cells need to produce energy. You breathe out the carbon dioxide that is
harmful. It sounds so simple. Yet your life depends on these processes happening dav
and night without interruption.
