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Here is a picture of a man's lungs.

They are soft like sponges with thousands of little pipes going through them.

The pipes keep branching like the branches of a tree so that they go through every part of each lung.

Our lungs are in the upper part of our chest.

we have two of them.

The air we breathe goes right in to every part of the lungs through these branching pipes.

They take it to the blood which is moving all the time through the lungs and round to every part of our bodies.

The blood makes a journey round body and back to the lungs in a very short time.

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What is blood?

It is the red liquid which comes out of your finger when you cut it.

There are about thirteen pints of blood in a person's body.

We can give a pint of blood at a time to a blood bank for the use of others who may need it.

A healthy body makes up the pint of lood quickly.

What does our blood do for us?

It takes food to all parts of our bodies and takes waste away from them.

All the parts of our bodies are made up of cells.

These cells, which are very small, all need foot all the time.

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Here are some cells, thousands of times the size they are in the body.

Each different sort of cell has its own work to do, different from the work other sorts of cells do.

No one had seen cells before the invention of the microscope and its development in the sixteenth and seventeeth centuries.

Before then no one could make pictures of cells because no one could see them.

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Cells are like litter flames.

A flame needs food.

We get a quick flame-for a fire or a cigarette- by lighting a match or using a lighter.

The flame of the match burns the match and the flame of the lighter burns the liquid in the lighter, if there is air for them to burn in.

All fires burn something.

What they burn is their fuel.

Fuel is food for fire.

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The blood is like a stream.

The cells take what they need, their fuel, out of the blood stream, as plants and fishes take their food out of water.

The blood stream carries food and the oxygen which it has taken up in our lungs to all the cells in the body.

Old cells die and give place to new cells in the body as plants and fish and other living things in the world about us die and give place to others.

Three million of your red blood cells die every second and other cells take their place.

The red cell population of your body changes completely in about three months.

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What makes the blood go on moving round body in a stream?

The heart sends it round.

The heart is between the lungs.

A person's heart is the size of his shut hand.

The heart is a pump.

If you put your hand under water like this, and keep letting a little water into it and sending it out again, you are pumping the water.

The heart pumps blood in a way a little like this.

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The heart has four rooms in it with doors (valves) between them.

It pumps blood in and out through these doors by changing the size of the rooms so that the doors open and shut.

It can do this because it is made of muscle.

The heart keeps a stream of blood going all round the body and back again to itself.

The pipes which take blood from the heart are named arteries;

the pipes through which it comes back to the heart are named veins.

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A solid line represents an artery and a broken line a vein.

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The first man to discover that the blood goes all parts of the body,

out through the valves of the heart through one system of pipes and back again through another,

was the seventeenth-century doctor William Harvey.

The journey of our blood all round the body is the circulation of the blood.

AS you see in the picture opposite,the branches of the arteries are like the branches of a tree

which get smaller and smaller the farther they are from the roots.

The small branches go to all parts of the body.

They go to the ends of your fingers and toes, to all the muscles-

those parts by which you move your arms and legs and head and other parts of the body.

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Everywhere the blood stream does two things:

supplies the cells with food and oxygen and takes away waste.

It is as if the blood kept the litter fires in the cells burning and took away the ashes.

The fuel for the fires in the cells is given us by the food we eat.

It cannot burn without oxygen.

This gas is as necessary to all living things as it is necessary to the bruning of fires made of wood or coal.

Burning is the change which takes place when oxygen and fuel unite.

Coal, wood and other fuels (cooking gas, for example) burn by taking oxygen from the air.