Blood is a fluid that performs the vital functions of transporting oxygen and nutrients to parts of our body when they are needed and delivering wastes where they are excreted. Our human body contains about 5 litres of blood which makes up around 7 percent of total body weight. Other functions of blood are to fight infections and keep the body warm. Blood circulating through vessels exerts pressure. This varies from person to person, and ranges from 120/70 to 140/90. The first figure refers to the pressure when the heart contracts; while the second figure is the pressure when the heart relaxes.
Composition of blood
Blood consists of plasma, blood cells, platelets and chemicals. Plasma is a pale yellow liquid that contains hormones, proteins, clotting factors and other nutrients such as glucose. Blood cells can be divided into white blood cells, red blood cells and platelets. They each perform a different function. White blood cells fight infections and foreign invaders. Red blood cells carry oxygen and carbon dioxide to and from the lungs to various organs. Platelets are sticky cells that form a web to prevent bleeding if a blood vessel is broken.
Red blood cells are so coloured because they contain a chemical called haemoglobin. They are produced in the bone marrow and have an average life span of four months in the blood stream. In a cubic millilitre of blood, there are about 4.6 to 5.2 million red cells. On the other hand, there are around 5,000 to 7,000 white cells per micro litre of blood; however, they may multiply to 10,000 if the body is under attack from bacteria or virus. White cells can be divided into five types but how they function are beyond the scope of this article.
White blood cells do not attack red blood cells in the body because they recognise them through markers called antigens. If cells from another body are placed in our body, our immune system recognises that they are not ours, and destroys them.
Generally, blood can be divided into four types:
(1) A (with only A antigen)
(2) B (with only B antigen)
(3) AB (with both A and B)
(4) O (no A or B)
However, some people have another antibody on their red blood cells call the Rhesus factor while others do not. Those having the Rhesus factor will be Rh+. Knowing what type of blood you have is important because if you suffer from excessive blood loss, you need matching blood to replace the shortage. The general rules of compatibility in blood transfusions are as follows:
(1) Type A blood can be given to anyone who has type A or AB.
(2) Type B blood can be given to anyone with B or AB.
(3) AB blood can only be given to a person with AB blood.
(4) O can be given to anyone.
(5) A person with type A can only take blood from someone with type A or O.
(6) Type B can only take blood from someone who is B or O.
(7) AB can receive blood from anyone.
People who wish to donate blood need to be screened for diseases such as hepatitis B, hepatitis C, HIV and other diseases. Sometimes people who are scheduled to have an operation will have their own blood extracted several weeks before the operation, so that they can get their own blood back if they need a transfusion.