What triggers the extrinsic pathway of coagulation?

The extrinsic pathway of coagulation is primarily triggered by external trauma to tissues, which results in the exposure of tissue factor (TF), also known as thromboplastin. When there is an injury to the vascular system, tissue factor is released from the damaged tissues and their cells into the circulation. This tissue factor then binds with factor VII, leading to the activation of factor VII to VIIa, which subsequently activates factor X to Xa. This pathway operates rapidly and is crucial for the initial stages of hemostasis, especially in response to external injuries.

Other options such as internal bleeding, blood vessel constriction, and circulating antibodies do not specifically initiate the extrinsic pathway. Internal bleeding typically involves the intrinsic pathway as it deals with factors already present in circulating blood. Blood vessel constriction is a response to injury that helps minimize blood loss but is not a direct trigger for the extrinsic pathway. Circulating antibodies are part of the immune response and do not play a direct role in coagulation pathways. Thus, external trauma to tissues is the key factor that activates the extrinsic pathway and initiates the coagulation process.