Final answer:
The intrinsic pathway is activated when factor XII encounters negatively charged molecules, setting off a cascade of reactions that activate factors XI, IX, and VIII and lead to the common pathway in blood coagulation.
Step-by-step explanation:
The intrinsic pathway is initiated by exposure to negatively charged substances, which activate a series of clotting factors leading to the common final coagulation pathway and the formation of a blood clot.
The intrinsic pathway of blood coagulation is usually initiated when factor XII (also known as Hageman factor) comes into contact with negatively charged surfaces such as inorganic polymers or phosphates. This can happen inside the body due to tissue damage from internal factors like arterial disease. Factor XII activation leads to a cascade of reactions activating factors XI, IX, and VIII, all contributing to the formation of an enzyme complex that activates factor X (Stuart-Prower factor or thrombokinase), most commonly through contact with negatively charged molecules. This cascade, accelerated by chemicals released by platelets, culminates in the common pathway, where prothrombin is converted to thrombin, which then converts fibrinogen into the fibrin strands that make up the stable clot. It's worth noting that clotting factors, including factor X, require Vitamin K for their synthesis, which is primarily done by the liver. This complex process is essential for wound healing and preventing excessive bleeding.