Platelets form the initial plug at a damaged human blood vessel.
Also called thrombocytes, platelets are small, membrane-bound fragments released from megakaryocytes in the bone marrow. When a vessel is injured, they adhere to exposed structures, become activated, and recruit additional platelets. Their aggregation creates a temporary platelet plug, an early step in hemostasis.
The coagulation system then strengthens this plug with a mesh of fibrin. Fibrin is produced through a chain of reactions involving clotting factors. Platelets also provide a surface that supports several coagulation reactions and release substances involved in repair.
Platelets are not complete cells with nuclei, unlike most white blood cells and red blood cells during their development. They circulate for roughly 7 to 10 days before being removed mainly by the spleen and liver. Too few platelets can impair clotting, while excessive activation can contribute to thrombosis.