Blood clotting is a complex series of events that requires exacting chemical balances. The mechanism itself follows a precise order. The University of Illinois School of Medicine says prior to clotting itself, the body prepares by constricting the injured vessel to slow blood loss. Platelets aggregate at the site of the injury to form a temporary plug but the actual clotting process occurs in three main phases.
Phase 1: Creation of the Prothrombrin Activator
According to Elaine N. Marieb, A series of enzymes are activated in sequence leading to factor X, which is also an enzyme. When factor X has been activated, it combines with calcium ions, platelet factor 3 that has been released by platelets and factor V, a protein, to form prothrombrin activator. Platelet factor 3 shortens the clotting time. All of these chemicals are found in the blood, so this phase begins immediately following the injury. This is the slowest phase in the sequence. The other two phases require only 10 to 15 seconds total once phase 1 is finished.
Phase 2: Formation of Thrombin
Marieb goes on to say that prothrombrin activator from phase 1 combines with a protein found in the plasma called prothrombrin. The result is an enzyme called thrombin. Enzymes are chemicals that are very active. They cause things to happen and continue the cascade of events leading to the clot.
Phase 3: Formation of the Fibrin Mesh
Marieb concludes by saying fibrinogen is a liver protein found in the blood plasma. When exposed to thrombin, fibrinogen turns into fibrin. Fibrin is a molecule that is long and sticky. It adheres the plug of platelets together into an insoluble mass. Factor XIII is also formed that links the fibrin strand together to strengthen the clot. At this point the clot has been formed. After a half hour to an hour, a further process occurs to contract the entire clot to make it stronger.
- University of Illinois at Urbana-Champaign College of Medicine: The Clotting Process (Hemostasis)
- "Human Anatomy and Physiology," Sixth Edition; Elaine N. Marieb; 2004