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Platelets

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Platelets, along with red cells and plasma, form a major proportion of both human blood.
Platelets are fragments of the cells in bone marrow, called megakaryocytes. Stimulated by the hormone thrombopoietin, platelets break off the megakaryocytes and enter the blood stream, where they circulate for about 10 days before ending their cycle in the spleen. In the healthy body, thrombopoietin will help to maintain the count of platelets at a normal level. Platelets provide the necessary hormones and proteins for coagulation. Collagen is released when the lining of a blood vessel is damaged. The platelet recognizes collagen and begins to work on coagulating the blood by forming a stopper, so further damage to the blood vessel is prevented. A higher than normal count of platelets, known as thrombocytosis, can cause serious health risks. Too much clotting of
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Conversely, lower than normal counts can lead to extensive bleeding. The body has several defences against pathogens so we do not fall ill with the diseases they cause. In addition to being the smallest blood cell, platelets are also the lightest. Therefore they are pushed out from the centre of flowing blood to the wall of the blood vessel. There they roll along the surface of the vessel wall, which is lined by cells called endothelium. The endothelium surface prevents anything from sticking to it. However when there is an injury or cut, and the endothelial layer is broken, the tough fibers that surround a blood vessel are exposed to the liquid flowing blood. It is the platelets that react first to injury. The tough fibers surrounding the vessel wall, attract platelets and platelets then clump onto these fibers, providing the initial seal to prevent bleeding, the leak of red blood cells and plasma through the vessel
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