Changes

==Description==

==Description==

Normally, haemostastis is maintained by three key events. The first stage, primary haemostasis, involves platelets and the vessels. It is triggered by injury to a vessel, and involves platelet activation, adhesion to endothelial connective tissue and aggregation to other platelets. This forms a fragile plug that helps to stop the bleeding. Platelets also release various substances during primary haemostasis. Vasoactive compounds cause vasoconstriction to limit haemorrhage, and other mediators cause continued platelet activation and aggregation as well as contraction of the plug formed. Primary haemostasis ceases once defects in the vessels are sealed and bleeding stops.

Normally, haemostastis is maintained by three key events. The first stage, primary haemostasis, involves platelets and the blood vessels themselves. It is triggered by injury to a vessel, and platelets become activated, adhere to endothelial connective tissue and aggregate with other platelets. A fragile plug is thus formed which helps to stem haemorrhage from the vessel. Substances are released from platelets during primary haemostasis. Vasoactive compounds give vasoconstriction, and other mediators cause continued platelet activation and aggregation, as well as contraction of the platelet plug. Primary haemostasis ceases once defects in the vessels are sealed and bleeding stops.

 

The platelet plug formed by primary haemostasis is fragile, and gives only temporary benefit unless is it reinforced with fibrin. Secondary haemostasis involves the interaction of proteinaceous clotting factors in a cascade that ultimately results in fibrin formation.Two simultaneous cascades are activated to achieve coagulation: the intrinsic and extrinsic pathways. The intrinsic pathway is activated by contact with collagen due to blood vessel injury and involves the clotting factors XII, XI, IX and VIII. The extrinsic pathway is triggered by tissue injury and is effected via factor VII. These pathways progress independently before converging at the common pathway, which involves the factors X, V, II and I and ultimately results in the formation of fibrin from fibrinogen.  Secondary haemostasis is dependent on the interactions of a number of proteins (clotting factors) within the intrinsic, extrinsic and common pathways of the cascade. The clotting factors aer synthesisde in the liver. The factors circulate in the plasma in an inactive from. Fatos I, VII, IX and X are dependent upon vitamin K to become active.