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When you see someone opening the window in a room full of people,you know that is to let in air from outside.

As good air comes into through the window,bad,used air,with more carbon dioxide and water in it and less oxygen,goes out.

We say good air is fresh air. Fresh air is clean and good to breathe and has enough oxygen in it for our needs.

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These boys are outside in the fresh air,breathing deeply while their teacher is saying: IN OUT for every breath which they take.

If you could watch them you would see their chests becoming first larger,then smaller in size, as their lungs breathed air in and out.

Breathing goes on when we are awake and when we are sleeping.

Most of time we are not conscious of our breathing.

The motion of our lungs as we breathe is automatic.

It goes on by itself, the lungs taking fresh air in and letting used air out about eighteen times a minute.

This is the common rate of breathing.

We become conscious of our breathing if anything shuts the air off from us,so that we do not get enough oxygen for our needs.

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Keeping your mouth shut, take your nose between your thumb and one fingers,so that you shut the air out and shut your breath in.

How long can you hold your breath?

You will be wise if you do not try to hold it more than a minute.

If oxygen is kept from a person for long he will become unconscious.

When people become unconscious through getting water or smoke in their lungs it is very important to start them breathing again.

This is done by turning them face down (putting something between their teeth to keep their mouths open) and then working their lungs to start them breathing.

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The higher up we go the less is the pressure of the air,because the weight of the air above us is less.

As the pressure becomes less (air gets thinner) the amount of oxygen we get in one breath becomes less.

We must take in more air to get the same amount of oxygen.

The instrument we use to measure the pressure of the air is the barometer.

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Men have been able to get to the top of the highest mountain in the world,Mount Everest.

They had to use oxygen when they got up high.

They had to keep control over amount of oxygen they used.

They could get no more supplies from those below.

This man who is going up a high mountain, is using a supply of oxygen which is stored in those cans he is carrying on his back.

By opening and shutting a valve,he controls the amount of oxygen he breathes in.

The gas has been pumped into the small space inside the cans.

It is under pressure in there and comes out when the valve is opened.

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It was more than a century after Harvey had discovered the circulation of the blood that two other scientists,one English and the other French,discovered oxygen.

These men say that when a flame burns it takes something out of the air.

That is why a candle goes out when it has been burning for a little time in a small amount of air.

It has taken all the oxygen out of the air.

You may see this by putting a lighted candle under a glass cover,so:

When the candle has used the oxygen form the air it can not go on burning.

It cannot burn without oxygen.

Its light goes out.

So with the burning that goes on inside the millions of cells of our bodies.

If the blood stops carrying oxygen to the cells,they go out.

They die.

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When we are running, our hearts are working much harder than when are sitting down.

Our blood is being pumped more quickly through our bodies.

The blood must carry a greater supply of oxygen to the muscles all over the body.

Our hearts work for us automatically.

A heart can pump as much as three thousand gallons of blood a day.

The amount pumped at any time is controlled automatically.

After running hard we breath very quickly because while running we couldn't take in enough air for oxygen needs.

We are out of breath,as we say.

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Our bodies control themselves in many other ways.

For example, the temperature of our bodies (and those of all warm-blooded animals) is controlled;

our blood temperature is kept at 98.6 degrees Fahrenheit,or near it.

When a person is very ill some of the automatic controls of the body stop working.

The temperature,measured by Fahrenheit thermometer,may go as high as 103°F or 104°F

but if goes much higher, or keeps as high as that for very long, one cannot go on living,but will die.

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If you have a high temperature you will feel unwell.

Then it is good to go to bed and send for your doctor.

You may have to go to a hospital.

A nurse is taking this patient's temperature.

He has had a bad cold and has been in bed for a day.

He felt unwell.

His temperature went up to 102°F but it is down again.

The nurse is looking at the thermometer,which she put under the man's tongue.

He has kept it there with his mouth shut for two minutes.

The man's temperature is 98.6°F.

That temperature is right for a healthy person.

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The body has many different ways of keeping itself warm enough and not too warm.

It uses up more food,more fuel for the cells,in cold weather.

It shivers-that is, its muscles go on making quick little motions to keep it warm.

We cannot stop ourselves from shivering.

The body's heating system is not under our conscious control; it is automatic.

To stop shivering,we have to warm ourselves some other way:

we may run or walk quickly, or taking hot drinks, or cover ourselves up warmly,

or use hot water bottles to warm our beds.