Which factor initiates the coagulation cascade via the extrinsic pathway?

The initiation of the coagulation cascade via the extrinsic pathway is primarily triggered by tissue factor, also known as thromboplastin or factor III. Tissue factor is a membrane glycoprotein that is exposed at the site of vascular injury and interacts with plasma factor VII, activating it to factor VIIa. Once activated, factor VIIa forms a complex with tissue factor, which then acts as a catalyst for the activation of factor X, leading to the common pathway of coagulation.

This tissue factor-mediated process is crucial in responding quickly to vascular injuries, as it allows for rapid thrombus formation, essential for hemostasis. Understanding the role of tissue factor is fundamental in clinical settings for managing conditions related to bleeding and thrombotic disorders. Other factors mentioned, such as factor VII, factor X, and calcium, are important in the coagulation process but do not initiate the extrinsic pathway specifically. Factor VII requires tissue factor for activation, factor X is part of the common pathway activated after the extrinsic pathway has begun, and calcium ions serve as a necessary cofactor in several steps of the coagulation cascade but do not trigger it directly.