When you cut your skin, you bleed . 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 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 also find that they can give any kind of blood to people of group AB. But they find that they must give A-group blood to A-group people and B-group blood to B-group people. I have O-group blood and the doctor told me that I could give blood to anyone else safely. ks5u There is another problem. To give blood of the right kind, doctors have to find a person of the right blood group. Often they can not 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. They do this by making it very cold. Then they find how to keep it longer. In the end they find a way of keeping blood for a very long time. We call a place where we keep money a "bank". We call a place where we keep blood a "blood bank". One day, when you grow up, you may decide to give blood to a "blood bank". In this way you may stop someone from dying. Or perhaps one day you may become ill. You may need blood. The "blood bank" will give it to you. The writer doesn't talk about  _  in the passage.
how important blood is to us the four groups of blood where the blood bank is what "blood transfusion" is