The blood is the life stream. It carries food and energy to all parts of the body; it scavenges the waste material of living which would otherwise poison the living creature; it helps to keep body temperature within the narrow limits in which cells can stay alive; it fights off the unceasing attacks of the bacteria of disease and decay. When the blood ceases to flow, life ends if the supply of blood to the brain stops for only a second, the person becomes unconscious.

Blood normally accounts for about 1/13th of a man’s total weight. A man weighing 70 Kg has about 5.4 Kg of blood. Blood is pumped by the heart into the arteries which branch and become smaller and smaller , like the branches and twigs of a tree, into capillaries ( from Latin, ‘capillus’ means a hair) The blood leaves the capillaries for minute veins which become bigger veins and returns to the lungs and heart to start its journey again.