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Information On Blood

Updated on August 17, 2014

Blood is the thick fluid that flows throughout the body servicing its cells to keep them alive. On the one hand it delivers oxygen collected from the air we breathe and nutrients extracted from the food we eat, and on the other it gathers up waste products such as carbon dioxide and uric acid and transports them to appropriate organs for eventual disposal. Blood also protects us against infection and regulates our temperature and fluid levels. The channels through which blood flows are the arteries and veins. The main pumping mechanism forcing the blood along is the heart. Veins carry the blood to the heart, and arteries carry it away from the heart.

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About 55% of blood is a straw-colored fluid called plasma. The rest of the blood is made up of cells, or corpuscles, of which there are three main types with three different functions. Red blood cells transport oxygen and carbon dioxide, white blood cells help to fight infection, and platelets assist in blood clotting.

Plasma itself is over 90% water. The remaining 10% contains just about everything the body needs to function - glucose, protein, salts, vitamins, chemical hormones and infection-fighting antibodies - as well as the waste products to be discarded. The proportions vary according to the particular needs of the body. It is because blood is so liquid that it can flow easily through the body.

Blood serum is the clear yellowish watery fluid that remains if the chemicals responsible for clotting are removed from plasma.

Red Blood Cells

Not surprisingly, the red color of blood comes from its red cells. The color is produced by an iron containing pigment called hemoglobin. Hemoglobin acts as a carrier for oxygen.

Red blood cells are shaped like a disc, hollowed out on both sides so that they have as large a surface as possible. As it passes through the lungs, one cell can attract as many as four oxygen molecules from the air that has been breathed in.

Hemoglobin also attracts carbon dioxide so that, as blood travels through the body, the red blood cells operate a sort of exchange system, dropping off life-giving oxygen and picking up used and toxic carbon dioxide to be discharged in the lungs on the return journey.

The more oxygen the blood is carrying, the redder it is. Blood in the arteries flowing from the lungs, full of oxygen, is bright red, whereas blood in the veins, which has been depleted of oxygen on its journey throughout the body, is more bluish in color. Interestingly, the red color can only be seen if there is a lot of blood. If you were to look at a tiny smear under a microscope, it would be yellow.

Every cubic millimeter of blood contains about five million red blood cells. Women have slightly fewer than men. One red blood cell lives for about four months, during which time it will circuit the system about 300 000 times. At the end of this period it will die and eventually be discarded through the body's elimination system. New cells to replace the discarded ones are manufactured in the bone marrow.

White Blood Cells

The main task of white blood cells is to fight infection.

There are about 700 times fewer white blood cells than red ones. Scientifically, white cells are known as leukocytes (hence leukemia is the name for a disease in which they multiply out of control).

Despite their name, white blood cells are in fact colorless. There are different types of white cells, but the main task of all of them is to attack harmful substances that invade the body. They may do this directly, literally by gobbling up bacteria, or by producing antibodies which work through the body's immune system. When an invader enters the body's tissues, the white cells hurry to the affected area and swing into action to destroy the interloper.

At the same time, emergency supplies are produced so that an abnormally large number of white cells in the blood is frequently an indication of infection. Pus is simply the combination of white cells, together with dead germs, destroyed tissue and other waste matter.

Like red cells, white cells eventually die. Their life span depends on what they are called on to do in the body and varies from a few hours to days, months or even years. Some replacement cells are manufactured in the bone marrow, others in the lymphatic system, including the spleen.


Blood clotting occurs when the blood turns from liquid to solid. A blood clot can be fatal if it forms in an artery and blocks it, but clotting is also an important part of our survival mechanism.

If the blood did not clot, every time we sustained even a minor cut we would bleed to death - and indeed someone suffering from the disease hemophilia has precisely that problem. The clotting in blood is provided by the third type of blood constituent, platelets. When we are injured so that our blood comes into contact with something that is different from the lining of the blood vessels it is accustomed to, the platelets stick together and seal the wound. If the wound is serious, a further series of chemical reactions is set off which eventually cause a clot further sealing the hole.

Platelets are also manufactured in the bone marrow, and live for a relatively short period compared with red and white cells - about ten days.

The need for constant replacement of the blood cells means that the bone marrow is absolutely essential to life.

An analysis of blood by the taking of a so-called blood count, which measures the numbers of different blood cells in a particular volume of blood, is often used by doctors to detect the presence of disease.

Blood Groups

Not everyone's blood is the same. Different people have blood that behaves in different ways under certain circumstances.

One of the factors that dictates this behavior is the type of antigens contained in the blood. Antigens are proteins that stimulate the production of antibodies to fight infection. If a person with antigens of one kind receives blood with antigens of a different kind, the antibodies produced will behave differently and may be fatal.

The antigens likely to cause problems have been given the names A and B. The blood group you are in depends on the presence of these antigens in your blood. The four blood types are A, B, AB, and O.

People in the A-group have A antigens in their blood, people in the B- group have B antigens, and people in AB have both. O denotes people who have neither A nor B antigen. If you need a blood transfusion, you must be given blood from the same group as your own.

About 45% of Europeans have group O blood. Group O blood can also be given in an emergency because it does not have either the A or B antigen. Type B blood is the most common type in Asian races.

Another antigen contained in blood is called the Rhesus (Rh) factor, but not everyone has it. People who do have it (about 85% of the population) are said to be Rh positive. People who do not have it are Rh negative. If Rh positive blood is given to an Rh negative person, they may produce antibodies to fight off the foreign invader and so destroy the transfused blood. Once the antibodies have been produced and are in the system, a further transfusion of Rh positive blood can be fatal.

If an Rh negative woman is pregnant with an Rh positive child (who has inherited its blood group from its father), some of the baby's blood cells may pass into the mother during the final weeks of pregnancy or when the baby is being born or following a miscarriage. Since these cells are foreign to the mother, she will produce antibodies to fight them. If the woman becomes pregnant again, these antibodies may enter the bloodstream of the fetus and destroy its red blood cells, with the result that the baby may be born jaundiced or, in extreme cases, stillborn. These days, there is an injection called Anti-D which, if given to a woman within 72 hours, totally prevents any antibody formation and so protects future pregnancies and prevents these problems. This injection is given routinely to Rh negative mothers.

The blood group we are in is inherited from our parents. Sometimes blood tests are used in cases where paternity is disputed.

These blood tests, however, cannot establish that a man is the father although they may establish that he is not. The inheritance patterns are complex, but basically if the child has a different blood group from its mother and the man's is different again, the man could not possibly be the father since the child must have inherited its blood group from one parent.

If the man's blood is the same as the child's, it means he is possibly but not necessarily the father - the father could be some other man with the same blood group. So called genetic fingerprinting is also available nowadays.

This is a blood test which checks more than the blood group, and it can tell accurately whether a man is the father or not.


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