Haemostasis is the process of blood clot formation at the site of vessel injury. When a blood vessel wall is disrupted, the haemostatic response must be quick, localized, and carefully regulated. Haemostasis is maintained in the body via three mechanisms: Vascular spasm (Damaged blood vessels constrict),Platelet plug formation(Platelets adhere to damaged endothelium to form platelet plug i.e,primary Haemostasis) and then degranulate and Blood coagulation (Clots form upon the conversion of fibrinogen to fibrin, and its addition to the platelet plug i.e,secondary Haemostasis). Various steps involved are: The first step is immediate constriction of damaged vessels caused by vasoconstrictive paracrine released by the endothelium. Vasoconstriction temporarily decreases blood flow and pressure within the vessel. When you put pressure on a bleeding wound, you also decrease flow within the damaged vessel. Vasoconstriction is rapidly followed by the second step, mechanical blockage of the hole by a platelet plug. The plug forms as platelets stick to the exposed collagen (platelet adhesion) and become activated, releasing cytokines into the area around the injury. Platelet factors reinforce local vasoconstriction and activate more platelets which stick to one another (platelet aggregation) to form a loose platelet plug. Simultaneously, exposed collagen and tissue factor (a protein-phospholipid mixture) initiate the third step, a series of reactions known as the coagulation cascade. The cascade is a series of enzymatic reactions that ends in the formation of a fibrin protein fiber mesh that stabilizes the platelet plug. The reinforced platelet plug is called a clot. Some chemical factors involved in the coagulation cascade also promote platelet adhesion and aggregation in the damaged region.
