What prevents the formation of massive clots throughout the cardiovascular system?

This is because the clotting factors are usually in an inactive state during circulation of blood. However, when there is a cut and the blood encounters other tissues during bleeding, the prothrombin is activated. It is proteolytically cleaved to form thrombin. Thrombin then converts soluble fibrinogen into insoluble strands of fibrin.

if you know your abc's you should be able to get your answer or just use google