Which of the following is the PRIMARY mechanism by which tissue factor (TF) initiates the extrinsic coagulation pathway in thrombosis?

Explanation

## Tissue Factor and Extrinsic Pathway Initiation **Key Point:** Tissue factor (TF, also called thromboplastin) is a membrane-bound cofactor that initiates the extrinsic coagulation cascade by binding to Factor VII. ### Mechanism of Action 1. Tissue factor is exposed when blood vessel endothelium is damaged 2. TF binds to circulating Factor VII (forming the TF–Factor VIIa complex) 3. This complex then activates Factor X to Factor Xa 4. Factor Xa, in the presence of Factor V, converts prothrombin to thrombin **High-Yield:** The TF–Factor VII complex is the rate-limiting step of the extrinsic pathway and is the primary trigger for coagulation in vivo following vessel injury. ### Comparison of Coagulation Pathways | Pathway | Initiator | Key Complex | Final Step | |---------|-----------|-------------|------------| | Extrinsic | Tissue Factor (TF) | TF–Factor VIIa | Activates Factor X | | Intrinsic | Contact (Factor XII) | Tenase complex (VIII–IX) | Activates Factor X | | Common | Factor X activation | Prothrombinase (V–X) | Prothrombin → Thrombin | **Clinical Pearl:** Tissue factor is constitutively expressed on fibroblasts and smooth muscle cells in the vessel wall, making it the physiologic initiator of hemostasis when endothelium is breached.