It refers to the spontaneous arrest of the flow of blood from a vessel which is damaged.
Vessels become spasmodic suddenly after the vessel is damaged.
Formation of platelet plug:
After the endothelium is damaged the platelets get stick to collagen as well as to each other resulting in the formation of platelet plug.
Several factors are released by platelets such as ADP, TXA2, and serotonin which results in further aggregation of platelets as well as vasoconstriction.
Next in the process, aggregated plug of platelets make it possible for the platelet factor 3 availability leading to the sequence of coagulation process further to take place.
Fibrin reinforcement of platelet plug:
A series of reactions are activated after the stimulation of coagulation system. Coagulation system works through two interrelated processes – Intrinsic and extrinsic processes.
Extrinsic denotes something that comes from outside. Extrinsic pathway denotes a series of reactions which is activated by tissue factor that comes from outside of blood.
It activates Factor VII by a tissue factor i.e. thromboplastin
Intrinsic denotes something which is found in body part. In the intrinsic pathway proteins for the activation of the process of blood coagulation are present in the blood.
Intrinsic process activates Factor XII.
Both of these processes mainly activate the formation of thrombin.
And in the final result fibrin molecules are released which are helpful for giving firmness to the platelet plug.