Whether it is a European, American, Chinese or an Indian, one thing that is common in all human beings is that they all have blood that is red in colour. And this colour has no relationship with the colour of the skin. Every human normally has 4 to 5 litres of blood in his body. This blood circulates continuously in the arteries, veins and capillaries. Do you why the blood is red? To answer this question we have to first understand the composition of blood. Our blood has four major constituents — plasma, white blood corpuscles, red blood corpuscles, and platelets. More than half of the blood in human body is plasma only. It is a thick fluid of yellow colour. It contains proteins, antibodies, fibrinogen, carbohydrates, fats, salts, etc. Proteins help in the growth of the body. Antibodies kill the harmful bacteria and neutralise the toxins secreted by them. Fibrinogen helps to stop bleeding from cuts by helping in the formation of clot. The white blood corpuscles are much less in number as compared to the red cells. They have a size of about 0.01 millimetre. There is one white blood corpuscle for every 700 red blood corpuscles. They protect the body from germs of diseases by fighting against them. Platelets are very small in size — about 0.002 millimetres. One cubic millimetre of blood contains 150,000 to 400,000 platelets. They are specially helpful in the clotting of blood. The red blood corpuscles give the blood its colour. They are saucer-shaped and have a size of approximately 0.008 millimetres. These cells carry oxygen form one point to another in the body. They contain a pigment called haemoglobin whose colour is red and which, in fact, makes the blood red. Haemoglobin is made up of iron and proteins. Each cubic millimetre of blood in a healthy woman has about 4.5 million red blood corpuscles. On the other hand, one cubic millimetre blood of man has 5.5 million red cells. Shortage of the red blood corpuscles leads to anaemia. As soon as blood passes through lungs, oxygen is absorbed by these particles and carried throughout the body. Every red cell lives for about 4 months and then breaks up, mostly in the spleen. Old red cells are destroyed in the liver and elsewhere at the rate of 2 million cells per second. New red cells are always being formed to replace the worn out cells.