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Platelets are the smallest of the three major types of blood cells. They are only about 20% of the diameter of red cells. The normal platelet count is 150,000-400,000 per micro liter of blood but since platelets are so small, they make up just a tiny fraction of the blood volume. The principal function of platelets is to prevent bleeding.
Platelets are produced in the bone marrow, the same as the red cells and most of the white blood cells. They are produced from very large cells called megakaryocytics. As megakaryocytics develop into giant cells, they undergo a process of fragmentation that results in the release of over 1,000 platelets per megakaryocytic. The dominant hormone controlling megakaryocytic development is thromboprotein (often abbreviated as TPO).
Platelets are actually not true cells but merely circulating fragments of cells. But even though platelets are cell fragments, they contain many structures that stop the bleeding. They contain proteins on their surface that allow them to stick to breaks in the blood vessel wall and also stick to each other. They contain granules that can secrete other proteins required for creating a firm plug to seal blood vessel breaks.
Platelets are not only the smallest blood cell, they are the lightest. Therefore they are pushed out from the center 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 is a very special surface, like Teflon, that 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 like an envelope, attract platelets like a magnet, stimulate the shape change 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 injury.