When you cut your skin, you bleed(流血). If a person loses a lot of blood, he will become ill and may die. Blood is very important. People have always known that. At one time, some people even drank blood to make them strong!
When doctors understand how blood goes around inside the body, they try ways of giving blood to people who need it. They take blood from the healthy people and give it to the people who need it. This is called "blood transfusion". The blood goes from the arm of the healthy person into the arm of the sick person.
But there are two problems. First, it does not always work. Sometimes people die when they have blood transfusion. Later, doctors find that we do not all have the same kind of blood. There are four groups—O, A, B and AB. We all have blood of one of these groups. They should give people the right kind of blood. I have O group blood and the doctor told me I could give blood to anyone else safely.
There is another problem. To give blood of the right kind, doctors have to find a person of the right blood group. Often they cannot find a person in time. If they have a way to keep the blood until someone needs it, they can always have the right kind of blood. At first, they find they can keep it in bottles for fifteen to twenty days. Then they find how to keep it longer. In the end, they find a way of keeping blood for a very long time.
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