Platelets are only about 20% of the diameter of red blood cells, the most numerous cell of the blood. The normal platelet count is 150,000-350,000 per microliter 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.
Red blood cells are the most numerous blood cell, about 5,000,000 per microliter. Red blood cells make up about 40% of our total blood volume, a measure called the hematocrit. Their color is caused by hemoglobin, which accounts for nearly all of the red cell volume. Hemoglobin is the critical protein that transports oxygen from our lungs to the tissues. Red cells are normally shaped as round, biconcave discs. With microscopic examination, they look like a red or orange tire with a thin, almost transparent center.
White blood cells are the largest of the blood cells but also the fewest. There are only 5000 to 10,000 white blood cells per microliter. There are several different types of white cells but all are related to immunity and fighting infection.
Platelet Production
Platelets are produced in the bone marrow, the same as the red cells and most of the white blood cells. Platelets are produced from very large bone marrow cells called megakaryocytes. As megakaryocytes develop into giant cells, they undergo a process of fragmentation that results in the release of over 1,000 platelets per megakaryocyte. The dominant hormone controlling megakaryocyte development is thrombopoietin (often abbreviated as TPO).
Platelet Structure
Platelets are actually not true cells but merely circulating fragments of cells. But even though platelets are merely cell fragments, they contain many structures that are critical to stop bleeding. They contain proteins on their surface that allow them to stick to breaks in the blood vessel wall and also to stick to each other. They contain granules that can secrete other proteins required for creating a firm plug to seal blood vessel breaks. Also platelets contain proteins similar to muscle proteins that allow them to change shape when they become sticky.
Platelet Function
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 envelop, attract platelets like a magnet, stimulate the shape change that is shown in the pictures above, 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.
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