Final answer:
The intrinsic pathway is initiated when factor XII comes into contact with negatively charged molecules, leading to a series of reactions that activate factors XI, IX, and X. The extrinsic pathway is triggered by damage to surrounding tissues, leading to the activation of factors III, VII, and X. Both pathways lead to the common pathway, which leads to the formation of fibrin to seal off the vessel.
Step-by-step explanation:
Intrinsic Pathway
The intrinsic pathway (also known as the contact activation pathway) is longer and more complex. It is typically initiated when factor XII comes into contact with negatively charged molecules, setting off a series of reactions that activate factor XI, then factor IX. Chemicals released by platelets increase the rate of these activation reactions. Finally, factor VIII combines with factor IX to form an enzyme complex that activates factor X, leading to the common pathway. The events in the intrinsic pathway are completed in a few minutes.
Extrinsic Pathway
The extrinsic pathway is quicker and more direct. It begins when damage occurs to surrounding tissues. The damaged cells release factor III, which sequentially activates factor VII. This enzyme complex then leads to activation of factor X, which activates the common pathway. The events in the extrinsic pathway are completed in a matter of seconds.
Common Pathway
Both the intrinsic and extrinsic pathways lead to the common pathway. Once factor X is activated, prothrombinase converts prothrombin into thrombin. Thrombin then converts fibrinogen into fibrin, which is stabilized by factor XIII. The stabilized clot can be acted upon by contractile proteins within the platelets to tighten the clot and expel serum.