Damage to the blood vessel: when an injury occurs, damage to the walls of blood vessels in the skin or tissue releases a substance called tissue thromboplastin, almost like an SOS signal.
This substance is composed mainly of phospholipids from the damaged tissues.
The thromboplastin then triggers a cascade of enzymatic reactions among multiple blood plasma proteins called blood coagulation factors.
These reactions eventually form prothrombin activator(also called prothrombinase), which causes the prothrombin to change into the enzyme thrombin.
Thrombin initiates coagulation by converting the soluble fibrinogen into insoluble fibrin threads that will bind the clot.