Platelets

The blood platelets (thrombocytes) are fragments of larger cells formed in the bone marrow. They number from 200,000 to 400,000 per μL of blood. Platelets are important in hemostasis, the prevention of blood loss, a component of which is the process of blood clotting, also known as coagulation. When a vessel is injured, platelets stick together to form a plug at the site. Substances released from the platelets and from damaged tissue then interact with clotting factors in the plasma to produce a woundsealing clot. Clotting factors are inactive in the blood until an injury occurs. To protect against unwanted clot formation, 12 different factors must interact before blood coagulates. The final reaction is the conversion of fibrinogen to threads of fibrin that trap blood cells and plasma to produce the clot. What remains of the plasma after blood coagulates is serum.