part 8

70 Everybody needs air; we breathe in air from outside our bodies in every breath. When you put your face under water you cannot keep it there long. Swimmers can't swim under water very long. They need air. Good swimmers may swim with their faces in the water; if so, they keep turning their heads to take a breath through their mouths. Swimmers do not take in air through their noses under the water because water would get in through the nose with the air and go to the lungs. With water in the lungs a person can't go on living.

71 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 the body and back to the lungs in a very short time.

72 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 blood 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 food all the time.

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

74 Cells are like little 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.

75 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.