The Blood

The blood is a collection of cells floating around in plasma. There are erythrocytes (red blood cells) and leucocytes (white blood cells).

Red blood cells (of which there are millions in one drop of blood) carry oxygen and carbon-dioxide around the body. They have a distinct shape similar to a car tyre. This shape is known as a bi-concave disc. They have no nucleus. A red pigment called haemoglobin is present, and it is this that carries the oxygen from the lungs to the tissues.

The haemoglobin takes up oxygen in the lungs and readily gives it up at each cell. This is a process of diffusion. When the haemoglobin takes in oxygen, it turns into oxyhaemoglobin. Haemoglobin contains iron which plays an important part in the diffusion process. Carbon dioxide combines with the water in the red blood cell to form carbonic acid, this process is speeded up by an enzyme in the red blood cells. Once the carbonic acid is formed, most of it leaks out into the blood plasma whilst about 30% remains in the blood cell. Carbon monoxide poisoning is a very real threat. Carbon monoxide combines with the haemoglobin 300 times more readily than oxygen. This has the effect of blocking oxygen cells from reaching the tissues.

Red blood cells live only for about four months. After this time they are destroyed. Red blood cells are manufactured at a rate of 2 million per second in bone marrow. Often the bone marrow has to produce more blood cells than this, so that the number of red blood cells in the blood increases. This is necessary for people living at high altitudes when oxygen is less concentrated so that the tissues still get the correct amount of oxygen.

White blood cells do not contain haemoglobin and they do have a nucleus. There is roughly 1:700 white cells to red cells in the blood. Their job is to help to fight disease. There are two types of White blood cell; phagocytes and lymph cells. They are both produced in bone marrow.

Phagocytes live like Amoeba. When a phagocyte comes into contact with a germ cell, it engulfs it. The germ is killed and digested. Phagocytes roam around the body looking for germs. When there is an 'invasion', there is a 'battle'. The area becomes inflamed and pus (casualties of war) is formed.

Lymph cells produce a chemical substance to kill viruses when they are detected. Not all viruses are killed instantly, some set out to damage your body making you feel ill. Germs contain antigens which are detected by the lymph cells and make them produce antibodies. The antibodies attack the germ cells in different ways, some make them burst, others make the germ easier for a phagocyte to ingest.

Plasma is the fluid part of the blood. It is mostly water but substances such as salt, urea, food and hormones and dissolved in it. It also contains plasma protein. There are three types of protein:

• Albumen - makes the blood thick and viscous
• Globulin - contains antibodies and certain types are required for clotting
• Fibrinogen - plays an important part in the clotting of blood.

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