Two principal mechanisms limit blood loss after vascular injury. Initially, platelets are activated, adhere to the site of the injury, and aggregate into a plug that limits blood loss. Proteins and small molecules released from activated platelets stimulate the plug formation process, and fibrinogen from the plasma forms bridges between activated platelets. These events allow the initiation of the clotting cascade, the second mechanism to limit blood loss. Negatively charged phospholipids exposed on cell surfaces at the site of injury and on activated platelets interact with tissue factor, setting off a cascade of reactions leading to generation of fibrin and the formation of an insoluble fibrin clot that strengthens the platelet plug.
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