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Composition Of Human Blood
Whole human blood is made up of four major components; red blood cells (erythrocytes); white blood cells (leukocytes); plasma and platelets. A normal human being has between 4.7 and 5 litres of blood in their body and this typically constitutes 8% or so of their total body weight.
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Nature And Functions Of White Blood Cells
The white blood cells typically contribute about 1% of whole blood volume. They have a role in combating infections which is why white blood cell counts can provide clinicians with valuable information when treating an illness. White blood cells comprise several different types of cell which play specific roles in combating infections: granulocytes (neutrophils, eosinophils and basophils), lymphocytes and monocytes (macrophages).
Lymphocytes are the “first responders" of the immune system. Their function is to recognise and bind to foreign bacteria, viruses and fungi that they encounter. Macrophages and granulocytes surround and destroy these alien cells and so doing, they inhibit the infection. Macrophages and granulocytes also remove dead and dying red blood cells from the body and other foreign matter, such as dust and asbestos that may have been ingested. White blood cells help with wound healing by removing tissue debris and dead cells from the site of the injury.
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White Blood Cell Count
Typically, white blood cell counts will provide a total number for leukocytes and will usually be provided together with information concerning red blood cells and platelet counts, giving the clinician a broader range of information (red blood cells are responsible for carrying oxygen around the body, for instance, and numbers are affected by anaemia – insufficient iron in the bloodstream).
A low white blood cell count is referred to as leukopenia whereas a high white blood cell count is called leukocytosis. Leukopenia can result from infections which damage the immune system, such as HIV or leukemia (a cancer of the blood). Some treatments sch as chemotherapy and radiotherapy can also reduce the white blood count; therefore, the white blood count is monitored during the course of these treatments. Leukocytosis, on the other hand, tends to result from bacterial infections, trauma, stress and intense exercise; however, it can also be caused by leukemia. Leukocytosis is also associated with the last month of pregnancy and also during the labour process itself.
Typically, the white blood cell count will be in the range 4.3 to 10.8 x 106 per ml. White blood counts that become more extreme and continue to deviate from the normal value indicate that a condition is worsening; whereas they will approach the normal range as the illness responds to treatment or spontaneously improves.
Commonly, a differential white blood cell count will be performed which will determine the relative populations of the different types of white blood cell present since this provides greater diagnostic capacity. The results may be given as percentage of total white cells or/and an absolute number.
The significance of finding leukocytes in a urinalysis sample is discussed in a related article.
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- Human Blood, Dr D. O’Neil, Palomar College: http://anthro.palomar.edu/blood/default.htm
- Children’s Hospital Wisconsin, Overview of Blood and Blood Components: http://www.chw.org/display/PPF/DocID/21846/router.asp
- Kimball’s Biology Pages: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Blood.html