The heart is not the only thing that goes faster and harder when you
run about in the morning and play hard. You are breathing faster and
deeper as well, as if there were something in the air outside that you
needed in your body as much as food.
But, of course, you know that air is not good to eat. It has no
strength in it, as food has; it isn't even a liquid like milk or
coffee or tea. It is so thin and light that we call it a _gas_.
Indeed, I suppose it is pretty hard for you to believe that air is a
real thing at all. But all outdoors is full of the gas called air, and
everything that seems to be empty, like a room or an empty box, is
full of it.
You cannot even smell it, as you can that other gas which comes
through pipes into our houses and burns at the gas jets; nor can you
see it like the gas that comes out of a boiling kettle or from the
whistle of a locomotive, and which we call _steam_. This is simply
because air is so pure that it has no smell, and is so perfectly clear
that we can see right through it. Almost the only way that we can
recognize it is by feeling it when it is moving. But it is a very real
thing for all that; and, like sunshine and food, is one of the most
important things in the world for us.
What is it that air does in the body? We must need it very much, for
we die quickly when we cannot get it: it takes us only about three
minutes to suffocate, or choke to death, if we can't get it.
You remember that the blood is pumped out from the heart, all through
the body. Everywhere it goes,--to the feet and the hands and the
head,--it is carrying two things: food that it has sucked up from the
food tube, and hundreds and hundreds of tiny red sponges called red
_corpuscles_. These little sponges are full of air which they sucked up
as the blood passed through the lungs. When we stop breathing,--that
is, taking in air,--the little red sponges of course can't get any air
to carry to the different parts of the body.
The body is made up of millions of tiny, tiny animals, called
_cells_,--so tiny that they can be seen only under a microscope. Each
of these cells must have food and air, just like any other animal.
They eat the food the blood brings to them, and they take the air from
the red corpuscles in the blood. With the air as a "draft," they burn
up the waste scraps, as we burn scraps from the kitchen, in the back
of the stove.
Suppose you light a candle and place it under a glass jar and watch
what will happen. The flame will become weaker and weaker, and at last
it will quite go out. You might think at first that the wind blew it
out; but how could the wind get through or under the jar? No, the
glass keeps all the outside air away from the flame; and that is just
the reason why it does go out. Unless it has fresh air, it cannot
burn. There is something--a gas--in the air that makes the flame burn,
and when it has used up all this gas inside the glass, and can't get
any more, it stops burning.
Now you will want to know what this gas in the air is. When we write
about it, we use its nickname, the large capital letter _O_; but its
whole name is _Oxygen_.
Just as the candle flame must have oxygen to keep it burning, so our
cells must have oxygen to burn their impurities, or waste; and if they
don't get the oxygen, and can't burn their impurities, they are
poisoned by them and "go out," or die.
You can see the flame when the candle is burning, but you can't see
the fires that burn in our bodies; there are no real flames at all. I
know it is hard for you to believe that there can be any burning when
our bodies are so wet and damp. But if you can't see it, you can
easily feel it. Blow on your hand. How warm your breath is! Touch your
hand to your cheek. It is quite warm, too. If you run or play hard,
you sometimes become so hot that you want to take off your coat. That
is because your fires are burning faster. The muscles are using more
food and making more scraps to be burned. You breathe faster and
faster till at last you are "out of breath" and feel as if you would
smother or choke. The blood has hard work to bring oxygen enough to
keep the fires going.
After the cells have burned the food scraps, they turn the "ashes" and
"smoke" back into the blood-stream that is always flowing past them.
If the cells did not do this, they would soon smother to death, just
as you could not possibly live in a house without chimneys to carry
off the smoke. And, of course, the blood wants to get rid of this
waste just as quickly as possible.
Part of the waste in the body is liquid, like water, and can flow away
through the blood pipes without needing to be burned. Some of this
watery waste comes out through the skin and stands in beads or drops
upon it. That is the part we call perspiration, or sweat. The rest of
it goes in the blood to another strainer called the _kidneys_, passes
through this as _urine_, and is carried away from the body as the
waste water from the bathtub and the sink is carried away from a
house.
For the "smoke" Mother Nature has still another beautiful plan. She
sends the blood-stream flowing through the _lungs_, where it can send
off its "smoke" and then get fresh air to carry to the cells in the
muscles. When you breathe out, you are sending out the "smoke"; and
when you breathe in, you are taking in fresh air.
Our body "smoke" is not brown or blue, like the smoke from a fire; it
is a clear, odorless gas, called _carbon dioxid_. This is the same gas
that makes the choke-damp of coal mines, which suffocates the miners
if the mine is not well ventilated; and the same gas that sometimes
gathers at the bottom of a well, making it dangerous for anyone to go
down into the well to clean it. And this gas is poisonous in our
bodies just as it is in the mine or the well.
You see, then, how important it is that we should live much of our
lives in the clear pure air out of doors, and should bring the fresh
air into our houses and schools and shops. "Fill up" with it all you
can on your way to school, for the best of air indoors is never half
so good as the free-blowing breezes outside.