Final answer:
Tissue factor, also known as thromboplastin, leaks into the blood from cells outside blood vessels during tissue damage and initiates the formation of prothrombinase in the extrinsic pathway of blood coagulation.
Step-by-step explanation:
The tissue protein that leaks into blood from cells outside blood vessels and initiates the formation of prothrombinase is known as tissue factor or thromboplastin. Tissue factor is crucial in the extrinsic pathway of blood coagulation. When there is damage to extravascular tissues, tissue factor binds with factor VII in the blood, leading to a cascade of reactions that result in the production of prothrombinase, which then converts prothrombin to thrombin. Thrombin is essential for the final stages of forming a fibrin clot, which is a complex agglomeration of fibrin, platelets, and erythrocytes that stabilizes at the site of injury to prevent blood loss.