Describe the blood clotting process.

Platelets come into contact with an area of an artery wall which has become damaged, changing their shape from flattened disks to long thin projections. The platelets stick together, forming a temporary plug. Thromboplastin, released from damaged tissue and platelets, as well as Ca2+ and vitamin K from the plasma, are released which changes the soluble prothrombin into thrombin. As this is an enzyme, it catalyses soluble fibrinogen to change into the insoluble fibrin. The fibrin strands form a tangled mesh which trap a bundle of blood cells, forming a clot