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Your amazing bodyBloodBlood, the sticky red fluid three times thicker than water, is the body's transport system, circulating continuously throughout its intricate network of veins, arteries and capillaries. One small drop contains millions of cells. The blood accounts for a sixteenth of our body weight. At rest, the heart pumps the average man's 5-6 litres (9-10.5 pints) of blood or the average woman's 4-5 litres (7-9 pints) to the lungs and other tissues each minute. During exercise, as much as 50 pints (28 litres) or more a minute may course through the vessels. The blood supplies life-sustaining oxygen. It delivers vitamins, minerals and other essential nutrients from digested food. It carries hormones from glands to control many vital functions. It helps fight infections. It cleanses the tissues, exchanging carbon dioxide for oxygen and removing other wastes to the kidneys. It also distributes warmth. New blood cells are constantly being made inside the bones to replace old ones as they die off or are lost through illness or injury. Without blood, the body cannot work, cannot survive. Blood, truly, is the fluid of life itself.
What is blood made of and what does it do?Nearly half the blood (45 percent) consists of blood cells - red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). Most of the rest (55 percent) is plasma, a clear, watery, pale yellow fluid. In one drop of blood, there are five million or more disc-shaped red blood cells, which live for about three months. These cells contain haemoglobin, a red iron-containing protein that gives blood its colour. Haemoglobin carries oxygen from the lungs to the tissues, and also contains enzymes, minerals and sugars. Microscopic differences between red blood cells permit blood group classification, see below. White blood cells are bigger but less in number and do not live as long as red cells. They cleanse the body and are its main defence against infections and cancers. In one drop of blood, there are from 7,000 up to as many as 50,000 white blood cells, depending on the body's needs at that time. Some produce protective antibodies (substances that destroy harmful germs), others surround and devour invading micro-organisms. There are several types of white blood cells. For example, neutrophils which are the main constituent of pus, which also destroys bacteria, and monocytes and lymphocytes which also are important for the immune system. Basophils make heparin, an anti-clotting substance. Platelets are much smaller than red or white cells, with 250,000 in a drop. They clump together to stop bleeding. Without them, you could bleed to death. The presence of both clotting and anti-clotting agents in the blood ensures a balanced system. Blood plasma, which carries the cells and platelets, consists chiefly of water, with about as much salt as sea water, plus thousands of other dissolved constituents - nutrients, wastes, proteins, hormones, salts and minerals. Measurement of the levels of these substances help diagnose illnesses.  How and why does blood clot?In normal clotting, platelets start to clump together within seconds of the skin being cut. They adhere to the damaged wall of the blood vessel and change their shape from discs to spiny spheres, which enmesh with each other and a clot develops. However, these same platelets can also contribute to unwanted clots, such as in deep vein thrombosis (DVT) and add to fatty deposits building up on blood vessel walls (atherosclerosis), which is a major causes of heart attacks and strokes. Calcium, vitamin K and a protein called fibrinogen help the platelets form a clot. They are released by platelets and stimulate coagulation of the blood. Altogether, 13 different coagulation factors, all essential for health, are involved in blood clotting. Without calcium and vitamin K, blood takes longer than normal to clot. In the final step of the clotting process, threads of fibrin, a stringy protein, help to plug the injury with a web-like mesh that traps the blood cells within it. This web hardens as it dries, contracting to form a clot or scab. A bruise, too, is the result of a blood clot. What are blood groups and how do they differ?Blood groups differ according to proteins known as antigens on the surface of red blood cells. Blood group matching is essential for safe blood transfusions. In the early 20th century, two types of antigen, A and B, were discovered. The combination of these types produces four blood groups: type A, type B, type AB (both antigens) and type O (neither A nor B antigen). The frequency of each group varies between races - in Britain, A is the most common, followed by O, B and AB. Another blood group system involves Rhesus factors, discovered in 1940 during studies on Rhesus monkeys to find out more about human anatomy. Rhesus factor D, the most important, is found in the blood of 85 percent of people, who are known as rhesus positive. The remaining 15 percent are rhesus negative. People can thus be classified according to both systems - as, for example, AB-positive or O-negative. The chief importance of Rhesus factor is in pregnancy. A baby's life can be endangered if it inherits a rhesus positive blood type from its father while the mother is rhesus negative, because she may form antibodies against the baby's blood. What can go wrong with your blood?Disorders can affect any of the blood's components. The most common is anaemia, which usually arises from iron deficiency in oxygen-carrying haemoglobin. Anaemia can lead to headaches, tiredness and lethargy and, if more severe, breathing difficulties and heartproblems. In leukaemia, cancer of the bone marrow, excessive numbers of abnormal white cells impair the production of red blood cells and platelets. This may cause many other organs, such as the liver and brain, to malfunction. There is a group of disorders where abnormally prolonged or excessive bleeding may occur anywhere in the body after injury or even in the absence of injury. One well-known, but fortunately rare, example of this is haemophilia, an inherited condition in which even the slightest injury can produce potentially fatal internal or external bleeding. It has affected several royal families. Multiple myeloma is a bone marrow cancer which causes an excess of proteins in the plasma. Sickle cell anaemia and thalassaemia are inherited disorders which make blood cells fragile. What can you do to maintain healthy blood?Blood contains living cells so, just like any other part of the body, they need good nutrition. A balanced diet with ample fresh fruit and vegetables is the key to ensuring the mix of vitamins and other essential nutrients. - - - - - - - - - October 2001 Back to the 'Your amazing body' index. |
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