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| Components of Circulatory System (Contd.) |
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| The WBCs are larger in size than the RBCs but much lesser in number. They are different from the erythrocytes in the following aspects. |
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| a) They have no haemoglobin |
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| b) They are bigger in size |
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| c) They are nucleated and amoeboid |
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| d) They have a longer life span |
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| e) There are several varieties with different functions. |
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| A blood smear showing red blood cells and three types of white cells also called as leucocytes (leucos = colourless, cyton = cell) |
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| The total number of leucocytes per microlitre is called the total leucocyte count (TLC) and is of diagnostic value in many diseases. The number of WBCs in a healthy person ranges from 5,000 to 10,000 per cubic millimetre. The rise in WBC count is called leucocytosis while a fall in WBC count is called leucopoenia, Leukemia is a pathological condition (blood cancer). The formation of WBC occurs in bone marrow, lymph nodes, thymus, spleen and is called leucopoiesis. |
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| There are several varieties of leucocytes. They are of 2 main types. |
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| a) granulocytes |
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| b) agranulocytes |
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| These have a granular cytoplasm and lobed nucleus. These are of 3 types on the basis of the shapes of their nuclei and the staining reactions of their granules. |
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| Neutrophils |
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| These are also called as scavenger cells. They form about 79% of the total leucocyte count. It has granular cytoplasm and a multilobed nucleus (2-7 lobed). They show amoeboid movement Their life span is about 10-12 hours. They engulf pathogens by phagocytosis. When neutrophils die it is known as pus. A localized accumulation of pus is called an abscess. |
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| Basophils |
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| They have a s shape or 2-3 lobed nucleus and a coarse cytoplasmic granules which take up basic stains like methylene blue. They secrete heparin and histamine and so play an important role in anticoagulation and formation of ground substance. |
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| Eosinophils |
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| These are slightly larger in size. The cytoplasm contains coarse granules which stain with acid dyes like eosin. The nucleus is 2 or 3 lobed. They increase in number during allergic reactions. They bring about distraction and detoxification of toxins. |
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| These are non-granular white blood cells that contain non - lobulated nuclei. They are produced in the lymph nodes and spleen and are of 2 types lymphocytes and monocytes. |
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| Lymphocytes |
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| These form about 25-30% of the total leucocyte count. They have a large nucleus. The cytoplasm is basophilic without any granules. They are divided into 2 groups namely small lymphocytes and large lymphocytes. These are formed in the thymus and lymphoid tissues like spleen, tonsils, lymph nodes from the precursor cells called lymphoblasts. |
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| Monocytes |
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| These are the largest sized leucocytes forming about 5% of the total leucocyte count. They have a oval, kidney or horse shaped nucleus. They are motile and have the power of engulfing bacteria. These also differentiate into macrophages or scavenger cells which remove damaged and dead cells to clean the body. They are usually formed in the lymph nodes and the spleen from precursor cells called monoblasts. |
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| Different Blood Corpuscles |
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| These are colourless, oval shaped and discoidal cytoplasmic fragments formed from the giant cells called megakaryocytes of bone marrow. The average life span of platelets is about 5-9 days. These play a major role in the process of blood clotting. At the site of injury, the platelets release a number of platelet factors and an enzyme thromboplastin which cause the coagulation of blood and clot formation to prevent excessive bleeding. Blood platelets also secrete a vasoconstrictor called serotonin which causes constriction of blood vessels to reduce the blood loss. |
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