Initiation

Coagulation is initiated when tissue factor (TF) binds to activated factor VII (VIIa) in plasma. TF is present in the subendothelium of blood vessels and in the lipid core of atherosclerotic plaques or it can be expressed by circulating white blood cells (monocytes) when they attach to damaged endothelial cells (Weitz & Bates, 2005) TF and factor VIIa together activate factors IX and X. Activated factor X (Xa) converts a small amount of prothrombin to thrombin.

Amplification

Once there is a small amount of thrombin present, coagulation is intensified. More factor IX is activated along with factor VIII. Together these two further trigger factor X. Thrombin also activates platelets (Tanaka et al., 2009).

Propagation

Activated platelets express factor XI and factor V on their membranes. The combination of factors Xa and Va plus calcium and platelet phospholipid, produces a burst of thrombin. Thrombin converts fibrinogen to fibrin on the platelet surface and the clot, now containing fibrin, platelets and trapped blood cells, is stabilised with factor XIIIa (Hoffman & Monroe, 2007).

Clot Dissolution

Tissue plasminogen activator (tPA) is released from endothelium as it heals, and in response to the presence of thrombin. tPA converts plasminogen to plasmin, an enzyme that breaks down fibrin and the clot dissolves.