A platelet is a cell fragment that circulates in the blood. Platelets are involved in hemostasis through the making of blood clots. A low platelet count (number of platelets in the blood) can cause a person to bleed without their blood clotting (making scabs). A high platelet count can increase the risk of thrombosis (blood clots inside blood vessels), which stops blood from flowing properly.
Platelets do not have a cell nucleus. They are disc-shaped, and are 1.5 to 3 micrometers in diameter. The body does not have a lot of platelets, so they can all be used up quickly. They contain RNA, a canalicular system, and several different types of granules; lysosomes (containing acid hydrolases), dense bodies (containing ADP, ATP serotonin and calcium) and alpha granules (containing fibrinogen, factor V, vitronectin, thrombospondin and von Willebrand factor), the contents of which are released when the platelet is activated.