What generally triggers the extrinsic pathway of coagulation?

The extrinsic pathway of coagulation is primarily initiated by exposure to tissue factor, also known as thromboplastin. Tissue factor is a protein that is not normally present in blood but is expressed by cells outside the blood vessels, particularly in response to injury. When there is damage to the vascular endothelial layer, tissue factor is released and comes into contact with circulating factor VII, leading to its activation. This complex then activates additional coagulation factors, ultimately resulting in the formation of a fibrin clot.

Other options, while related to coagulation in different contexts, do not initiate the extrinsic pathway. Blood vessel injury can lead to the exposure of tissue factor, but it is the tissue factor itself that acts as the crucial trigger. Prolonged immobility and low platelet levels are associated with different aspects of the hemostatic process and may affect clot stability or formation, but they do not directly activate the extrinsic pathway. Understanding this distinction clarifies why the exposure to tissue factor is essential for the initiation of the extrinsic coagulation pathway.