Final answer:
The extrinsic pathway is initiated by the release of tissue factors from damaged tissues, which leads to the activation of the common pathway and the formation of a blood clot.
Step-by-step explanation:
The introduction of a substance into the bloodstream from the tissues when a blood vessel is damaged initiates the extrinsic pathway to fibrin formation. This pathway is quicker to respond and is more direct, beginning upon damage to the surrounding tissues. The damaged extravascular cells that are extrinsic to the bloodstream release factor III, known as tissue factor or thromboplastin.
Following the release of thromboplastin, Ca²+² and other factors sequentially get involved, leading to the activation of factor X which joins the common pathway. The common pathway culminates in the conversion of prothrombin to thrombin by the enzyme prothrombinase, and then the conversion of fibinogen to insoluble fibrin strands by thrombin, eventually forming a blood clot.