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Blood composition

On average, a person has 5.5 litres of blood . Blood is a liquid, however, 45% of its composition is made up of solid particles. The fluid which carries these particles is a yellow liquid called plasma. The solid matter consists of:-

1. Red blood cells - erythrocytes

2. White blood cells - leucocytes

3. Platelets
   

The plasma consists of water (90%), blood proteins, food, vitamins and minerals, urea and hormones.

Red blood cells - Erythrocytes

These are tiny disc like cells which don’t have any nucleus. Because of this, they only likve for about four months, after which they are broken down in the spleen and the liver. Some of their components are used again to make new cells.

Red blood cells are made up of a spongy cytoplasm covered by an elastic cell membrane. In their cytoplasm they have a red pigment called haemoglobin, which is a protein combined with iron. In high oxygen concentrations, it forms oxyhaemoglobin. This breaks down and releases oxygen when the concentration of oxygen is low.

Blood which contains a lot of oxyhaemoglobin is called oxygenated blood and is bright red in colour. Blood with little oxyhaemoglogin is called deoxygenated blood and looks a deep red-purple.

White blood cells - Leucocytes

Unlike the red cells, white blood cells come in many types. The two most common are the phagocytes and the lymphocytes.

1. Lymphocytes produce antibodies in response to the bacteria or viruses (the antigens) which begin the process of destruction.

2. Phagocytes finish the job by engulfing the partially destroyed bacteria
   

There is one white cell to every 600 red cells. Most are larger then the red cells and have a nucleus.

The phagocytes can move about by a flowing action of their cytoplasm and can escape from blood capillaries into the tissues by squeezing between the cells. In this way they can reach any infected part of the body. They can eat or engulf other cells particularly bacteria, in the same way amoebas feed. Phagocytes are the body’s main form of defence against disease-causing bacteria. They gather in wounds and destroy bacteria before they can enter and spread the infection throughout the rest of the body.

Platelets

Platelets are irregularly shaped objects about 0.003mm in diameter. There are about 25 000 platelets per mm3 of blood. They are pieces of special blood cells budded off in the red bone marrow. Their function is to help clot the blood at wounds and so stop the bleeding.

A clot begins to form when platelets are damaged by the rough surface caused by a cut or torn tissue. They release fibrinogen which changes to fibrin that forms a mesh of fibres blocking the wound. This prevents further blood loss taking place.