Thrombocytes, or platelets, are the smallest cellular component of blood. They circulate inactivated, about 250,000 per cubic mm of blood, until they come into contact with a damaged blood vessel.
At this point, the platelets form a clump, adhering to each other and to the blood vessel wall.
They secrete chemicals that alter a blood-borne protein, fibrinogen, so that it forms a mesh of fibers at the damage site. A clot forms when platelets and red and white blood cells become trapped in the fibers.
Blood clotting begins within seconds of injury.