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Platelets are cells that stick together to form blood clots. Normally they don't stick together, but when there is a disturbance in the blood vessels (such as a broken vessel) they stick together at that point and prevent blood from coming out of the vein or artery. They are not cells in the conventional sense, but are broken-off pieces of megakaryocyte[?] cytoplasm released from the bone marrow into the blood stream.

Platelets are separated, by centrifugation, from the rest of donated blood and given to patients who need them. A (see-through) bag of them is pale orange. They are separated because they don't survive the normal storage used for red blood cells, and must be stored separately.

A normal platelet count in a healthy person is between 150 and 400 (x 109/L of blood). People can live independently with a count as low as 20. People can live in hospital with a count as low as 5, but spontaneous bleeding gets to be a problem. That's when platelet transfusions have to be done. A low platelet count is called thrombocytopenia, having too many platelets is called thrombocytosis.

See Also: Haemostasis[?]

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