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The human body normally contains 5.5 litres of blood, and the prime function of blood, as it gets circulated throughout the body, is to supply every cell in every part of the body with oxygen and nutrients as well as to remove waste from them. Besides, it also carries hormones and antibodies that help develop resistance against infections.

Transport system
In resting state, the heart pumps about five litres of blood every minute. Along the way, it picks up nutrients absorbed by the stomach from food and oxygen from the lungs, and carries them to various cells in all parts of the body. Not only that, it also serves as the purifying agent by removing wastes in the form of chemicals like urea and lactic acid and carries them to the liver and kidneys from where they are discharged out of the body. It also carries the carbon dioxide from the cells and takes it back to the lungs from where it is exhaled while breathing. It also acts as the agent for carrying hormones produced by various glands to the respective cells that they are meant for. It also carries cells that have healing properties, but these are activated only when the body needs them.

Plasma, or the liquid component of the blood, is 92 percent water, but it also has enzymes, minerals, glucose, waste products like carbon dioxide, urea and lactic acid, and hormones. While carbon dioxide is dissolved in the plasma, minerals are attached to plasma proteins. Apart from plasma, the blood contains white blood cells (WBCs) and platelets that are crucial for immunity and also help in preventing clotting, and red blood cells (RBCs), which also play a major role in providing the body immunity against infections.

Cell production
It is in the bone marrow that the white blood cells, red blood cells and platelets are produced, and from there they pass into the circulating blood. Non-nucleated RBCs can remain in the system for about 120 days, and when they are tired, they are broken down by the WBCs, and while the wastes are expelled, the useful parts are returned to the bone marrow.

Clotting of blood
It is the platelets which come to the rescue when a blood vessel is damaged. As they rush to the damaged area, they release chemicals which start off the clotting process. Strands of protein called fibrin are produced which interlink to form a clot, trapping RBCs and platelets within it. This plugs the damaged portion of the vessel which stops bleeding.

Blood groups
A person’s blood group is determined by heredity. The defining factor is the antigens found on the surface of the RBCs. Antigens are chiefly of A and B types (called blood groups A and B). They can be found together (group AB) or they may not be present at all (group O). The antigens stimulate the production of antibodies, which ignore antigens of their own group, but destroy antigens of foreign cells, or some other group.


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