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Your blood is a living fluid, which nourishes the organs and tissues throughout the body with food and oxygen and plays an essential role in the body's fight against disease

Blood begins to be pumped round the body in the early weeks in the womb and continues until death. Its purpose is to deliver oxygen, food and other vital substances to the tissues and, in return, to extract carbon dioxide and waste products for disposal.

Blood, by virtue of its ability to clot and to destroy the micro-organisms that produce disease, is also a vital part of the body's natural defence mechanism.

Blood is made up of millions of cells floating in a watery, yellowish fluid called plasma. Red blood cells form about 40 per cent of its volume. These take oxygen from the lungs to the tissues and transport carbon dioxide, a waste product of cell function, back to the lungs to be breathed out.

Each red blood cell is a flattened doughnut-shaped disc with a concave centre, about eight microns in diameter (a micron is one thousandth of a millimetre), two microns thick at the edge and one micron thick in the centre. This shape is designed to enable each cell to absorb and release oxygen molecules and to allow the cells to squeeze through the narrowest of blood vessels.

The surface structure of red blood cells varies slightly from one person to the next and this is the basis of classifying blood into various groups. There are four different blood groups - known as A, B, AB and O - which are classified according to the presence or absence of certain antigens, or foreign bodies, on their surface.

Forty per cent of people in the UK are group A, another forty per cent are group O, around 12 per cent are group B, and eight per cent are group AB. Fifteen per cent of people are rhesus negative - lacking a protein found in the blood of most of the population.

The main vehicles of oxygen transportation in the bloodstream are millions of molecules of a substance called haemoglobin. It's haemoglobin which gives red blood cells their bright red colour.

Haemoglobin is a pigmented protein containing iron, which combines with oxygen in the lungs. This 'oxygenated' blood is carried in the arteries and released into the tissues where, with the help of enzymes in the red cells, carbon dioxide and water (another waste product of cell activity) are locked on to the red cells and taken back in the veins to the lungs. This 'deoxygenated' blood is darker in colour and gives the veins their characteristic bluish colour.

Red blood cells begin to be produced in the first few weeks after conception and, for the first three months, manufacture takes place in the liver. After six months, red blood cell production is transferred to the bone marrow. Until adolescence, the marrow in all the bones makes red blood cells, but after the age of about 20, red cell production is confined to the spine, ribs and breastbone.

Red blood cells begin life as irregular, roundish cells with huge nuclei, known as haemocytoblasts. These cells go through a rapid series of divisions during which the nucleus becomes progressively smaller until it is lost altogether. Iron - the major constituent of haemoglobin - vitamin B12, folic acid and proteins are all required for their manufacture.

Red cells have to be renewed constantly because of the amount of wear and tear they undergo during their travels round the bloodstream. The average lifespan of a red cell is just 120 days.

After this, cells from the bone marrow and spleen attack them to destroy them. Some of the chemical remains are then returned to the plasma for later use. Others, including haemoglobin, are sent to the liver to be further broken down.

Our bodies have remarkable mechanisms for controlling the number of red cells in circulation. If a lot of blood is lost, if parts of the bone marrow are destroyed or if the amount of oxygen reaching the tissues is diminished due to heart failure or high altitude, the bone marrow increases red blood cell production to compensate.

Even strenuous daily exercise stimulates extra red cell output because of the body's increased need for oxygen. In fact, athletes can have twice as many red blood corpuscles as sedentary people. White blood cells, also known as white blood corpuscles or leucocytes, are bigger than red blood cells and there are far fewer of them. White cells do not all look alike and are capable of moving with a creeping motion. Their main role is defending the body against bacteria and disease.

White cells are divided into three distinct groups known as polymorphonuclear leucocytes (or granulocytes), lymphocytes and monocytes. Polymorphonuclear leucocytes, or granulocytes, make up 50 to 75 per cent of the white cells. This group, in turn, is subdivided into three different kinds known as neutrophils, basophils and eosinophils.

Neutrophils, also known as phagocytes, are the most numerous and important cells. When infection strikes, these cells 'swim' to the affected area and start to engulf invading bacteria. As they do so, granules inside the cells make chemicals to destroy the trapped bacteria. The pus that collects at the site of an infection is the result of this action and is largely made up of dead white cells.

Eosinophils - so called because their granules are stained pink when blood is mixed with the dye eosin - make up one to four per cent of the white cells.

As well as combating bacterial attack, they also play a role in allergic response. When foreign proteins or antigens get into the blood, substances called antibodies are made to combine with them and neutralize their effects. At the same time, the chemical histamine is released. The eosinophils quell allergic reactions by damping down the effects of this histamine. Once the antibodies and antigens have combined, the eosinophils then remove the chemical remains.

Although basophils make up less than one per cent of white cells, they are vital. Their granules make and release a substance called heparin which stops blood from clotting inside the vessels.

Lymphocytes, which make up about 25 per cent of the blood's white cells, have a vital part to play in natural immunity. Unlike other blood cells, which are created in the bone marrow, many are formed in the lymph nodes.

Lymphocytes make anti-toxins to counteract the potentially damaging effects of the powerful toxins (poisons) or chemicals produced by some bacteria, making antibodies and the chemicals which help prevent body cells from succumbing to bacterial invasion.

With HIV, certain lymphocytes, known as T-lymphocytes, become infected by the HIV virus; this can progress to AIDS, with the result that the immune system is unable to function properly. This leaves the body open to various infections.

Monocytes form up to eight per cent of the white cells. The large monocytes contain large nuclei which engulf bacteria and remove the debris of cell remains resulting from bacterial attack.

Blood also contains millions of minute platelets, the smallest type of blood cell. Their sticky surfaces enable them to stick to the broken ends of damaged capillaries to stop any bleeding that occurs.

Platelets also help trigger off the blood clotting mechanism by combining with a dozen bio-chemical substances called clotting factors, the most important of which is called prothrombin. These factors are found in the fluid part of the blood - the plasma.

The same qualities can also lead to the formation of clots in healthy blood vessels, as in thrombosis, and to the build-up of fatty deposits called atherosclerosis, which block the arteries in heart disease.

Polymorphonuclear leucocytes are made in the bone marrow, from myelocyte cells. Polymorphs live for only 12 hours on average and for only two or three hours when they are fighting bacterial invasion. In such circumstances, the output of all white cells increases to meet the body's demands.

Lymphocytes have an average life of 200 days. They are made in the spleen and in areas such as the tonsils and the lymph glands scattered throughout the body.

Both monocytes and platelets are made in the bone marrow. The lifespan of monocytes, which spend part of their time in the tissues and part in the plasma, is still a mystery. The body replaces all its millions of platelets once every four days.

Ninety per cent of plasma is composed of water which contains vital chemicals including vitamins, minerals, sugars, fats and proteins which are needed for optimum cell function and cell renewal.

This vital fluid also contains hormones, made by endocrine glands, which are carried to particular body organs to regulate functions such as reproduction.

Plasma also transports urea - produced by the breakdown of proteins in the tissues - to the kidneys for excretion in the urine.

Bleeding, whether internal or external, should always be regarded as a serious matter. But the body has survival mechanisms that allow a young, healthy person to lose a quarter of his blood without long-term ill-effects.


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