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What is blood?

Unlike most life-sustaining parts of the human body, blood is easy to lose.

One slip of the carving knife and it's flowing freely. So it's just as well we can lose quite a lot of it before suffering fatal consequences. The human adult has about 5 litres (8.8 pints) of blood and we can lose a third (almost 3 pints) and still survive. But lose 50 per cent and you'll be staring death in the face. So what is it about blood that is so essential for life?

The red blood cells, which make up around 40-45 per cent of blood, give blood its distinctive scarlet hue. More specifically, it is the iron in the protein haemoglobin that makes the cells red.

Haemoglobin takes oxygen from the lungs and exchanges it for carbon dioxide and other waste products from cells as the blood passes round the body.

At high altitudes, where the air is described as "thin", because it contains lower levels of oxygen, people need mechanisms to ensure they get sufficient oxygen. In the high Andes of South America, locals have higher concentrations of haemoglobin in their blood, making their bodies more efficient at using the oxygen that is available.

About 55 per cent of blood is made up of plasma - a pale-coloured clear liquid containing sugar, fats, proteins, hormones, vitamins and minerals such as sodium, potassium and calcium.

Plasma also carries platelets and fibrinogen, essential for the clotting process. Platelets have a sticky surface, and they clump together to form a plug, or clot, to stop bleeding, while fibrinogen makes a scab on the surface of the clot to keep germs out of the wound. Clotting disorders, such as haemophilia, result in uncontrolled bleeding, both internally and from external injuries.

Haemophilia is an inherited disorder that results from a deficiency of an essential blood protein. It is sometimes referred to as "the royal disease" because it affected many of the royal families of 19th-century Europe, including some of Queen Victoria's children.

Clotting is not always a good thing. Sometimes a clot forms inside a blood vessel and travels around the body where it can cause a heart attack if it blocks the coronary artery, or a stroke if it blocks the blood supply to the brain.

Also present in the blood are white blood cells, essential for fighting disease, producing antibodies to destroy bacteria, viruses and parasites.

White blood cells are normally only a tiny fraction of a healthy person's blood. Increased numbers of white cells can indicate the presence of infection. It could also be a sign of a disease of the blood itself, such as leukaemia.

Some diseases of the blood are inherited, for example sickle-cell anaemia.

In sickle-cell anaemia, abnormal protein present in the haemoglobin forms crystals, and these change the shape of the red blood cells, which should be round.

The distorted sickle-shaped cells block the capillaries and prevent blood from flowing normally, causing periodic painful attacks. The trait is carried on a single gene by up to one in 10 people of African or Caribbean descent, and around 6,000 people in the UK are known to have the disease itself.

Diet also affects the health of blood cells and the arteries, veins and capillaries through which they flow. Too much fat in the blood can clog up the arteries, causing an increase in blood pressure, and a lack of iron - or vitamin C to help the body to absorb it - can cause anaemia.

As well as keeping cells supplied with oxygen and bolstering the body's immune response, the blood has a role in temperature control. Muscles, for example, generate heat. The blood cells around them get hotter and as the blood circulates it transports the heat to cooler areas of the body.

If there is excess heat, such as is generated by vigorous exercise - or an anxious moment - there is a rush of blood to the capillaries near the skin, causing a blush - or a flush - as excess heat is released through the skin.

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