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Blood platelets are not real cells, but fragments of other cells called megakaryocytes, which are produced in the bone marrow. Their main function is to ‘look after’ the circulatory system, by triggering blood clotting and repair of damaged tissue immediately after injury.
Platelets normally circulate in the blood as disc-shaped structures. In this form, they are called ‘smooth platelets’, and are inactive. If blood vessel walls are ruptured, for example when someone suffers a cut, molecules are released that trigger platelets in the area to change their shape (‘spiky’ platelets). Along with other molecules from the blood the platelets form a clot over the injury. Over time, this clot is reinforced to form a semi-permanent barrier behind which the injured tissue can mend. Once the tissue is repaired, the platelet plug slowly dissolves.