Changes

Two simultaneous cascades are activated to achieve coagulation: the intrinsic and extrinsic pathways. The intrinsic pathway is activated by contact with collagen and involves the clotting factors XII, XI, IX and VIII. The extrinsic pathway is triggered by trauma and contact with "tissue factor", and involves the 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.  

Two simultaneous cascades are activated to achieve coagulation: the intrinsic and extrinsic pathways. The intrinsic pathway is activated by contact with collagen and involves the clotting factors XII, XI, IX and VIII. The extrinsic pathway is triggered by trauma and contact with "tissue factor", and involves the 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.  

Within each of the three arms of the coagulation cascade, certain clotting factors are dependent on vitamin K for activity. These include factor VII, factor XI and factors II and X in the extrinsic, intrinsic and common pathways respectively. Vitamin K is an essential cofactor and acts by facilitating the proteins' carboxylation to their fuctional forms. This carboxylation reaction oxidises vitamin K, and an enzyme known as vitamin K epoxide reductase is needed to reduce vitamin K back to its original form, enabling it to be recycled for the continued synthesis of factors II, VII, XI and X in the liver.  

Within each of the three arms of the coagulation cascade, certain clotting factors are dependent on vitamin K for activity. These include factor VII, factor XI and factors II and X in the extrinsic, intrinsic and common pathways respectively. Vitamin K carboxylates these factors to their fuctional forms, and in the process itself becomes oxidised. Vitamin K is always required for the production of new II, VII, IX, and X in the liver and levels are tightly regulated. It is therefore essential that vitamin K is recycled after it is oxidised in the carboxylation reaction, and the enzyme vitamin K epoxide reductase is respsonsible for this.