Normal coagulation
Three processes halt bleeding:
[edit | edit source]- Gap-plugging by platelets
- Platelets bind to collagen (found in exposed vessel wall) and aggregate
- Vasoconstriction
- Occurs due to serotonin and other vasoconstrictors, which come from platelets adherent to the walls of the damaged vessel
- Coagulation cascade
- The loose aggregation of platelets is bound together and converted into the definitive clot by fibrin, through a cascade of reactions
- The fundamental reaction is conversion of the soluble plasma protein fibrinogen to insoluble fibrin, catalysed by thrombin
- Thrombin is itself formed by the action of activated factor X
- Factor X can be activated by either intrinsic or extrinsic systems
- Intrinsic pathway involves factors XII, XI, IX, and VIII
- Extrinsic pathway involves factor VII being activated by tissue thromboplastin
- Stabilisation of the clot into a dense aggregate requires activated factor XIII and calcium ions
Fibrinolysis
[edit | edit source]- Tissue plasminogen activator (t-PA) is released from endothelial cells, a process stimulated by fibrin formation. T-PA converts plasminogen to plasmin
- Plasmin causes fibrin dissolution
- Fibrinolytic agents such as streptokinase (that binds and activates plasminogen) active the fibrinolytic system
- Also tenecteplase and reteplase