Which of the following substances, not normally present in the circulation, trigger the initiating events in the hemostatic process?

c. Tissue factor

d. Collagen

The initiating agents for hemostasis involve two substances that are not normally present in the circulation—collagen and tissue factor. Tissue factor is released from injured cells, beginning the activation of the extrinsic pathway of coagulation, while disruption of the protective endothelial barrier of blood vessels exposes the underlying collagen to the activation of platelets. In the bloodstream, tissue factor complexes with factor VII which then activates factor X to factor Xa. At the same time, activated platelets change from their discoid shape with their procoagulant phospholipid (termed platelet factor 3) buried on the inner side of the surface membrane to a spreading shape to allow for the externalization of platelet factor 3 activity. Activated factor X, activated factor V, ionized calcium and factor II (prothrombin) then assemble on the platelet phospholipid surface to form the so-called prothrombinase complex which catalyzes the formation of thrombin.