In the cell-based model of coagulation, the intrinsic pathway operates on which surface to generate the burst of thrombin?

In the cell-based model of coagulation, the intrinsic pathway operates on which surface to generate the burst of thrombin?

• A. Tissue factor bearing cell to initiate and amplify coagulation
• B. Proteolytic digestion of fibrinogen
• C. Activation of the contact pathway
• D. Activated platelet surface to produce the burst of thrombin

Answer: (D)

In the cell-based model, the large burst of thrombin comes from assembly of coagulation complexes on the surface of activated platelets. When platelets are activated, their membranes expose phospholipids (like phosphatidylserine) that provide a rich, negatively charged surface for coagulation factors to assemble. On this platelet surface, intrinsic tenase (IXa with VIIIa) and prothrombinase (Xa with Va) complexes form and efficiently convert factors X and II (prothrombin) into Xa and thrombin, respectively. This localized, surface-supported amplification drives a rapid, large amount of thrombin production, which then converts fibrinogen to fibrin to form a stable clot. Tissue factor–bearing surfaces initiate coagulation outside the blood and kick off the process, but the major thrombin surge—the burst—occurs on the activated platelet surface during propagation. The proteolytic conversion of fibrinogen by thrombin occurs after thrombin is generated, not on the surface that generates the burst. Activation of the contact pathway can help start coagulation, but the key site for the burst is the activated platelet surface.

In the cell-based model, the large burst of thrombin comes from assembly of coagulation complexes on the surface of activated platelets. When platelets are activated, their membranes expose phospholipids (like phosphatidylserine) that provide a rich, negatively charged surface for coagulation factors to assemble. On this platelet surface, intrinsic tenase (IXa with VIIIa) and prothrombinase (Xa with Va) complexes form and efficiently convert factors X and II (prothrombin) into Xa and thrombin, respectively. This localized, surface-supported amplification drives a rapid, large amount of thrombin production, which then converts fibrinogen to fibrin to form a stable clot.

Tissue factor–bearing surfaces initiate coagulation outside the blood and kick off the process, but the major thrombin surge—the burst—occurs on the activated platelet surface during propagation. The proteolytic conversion of fibrinogen by thrombin occurs after thrombin is generated, not on the surface that generates the burst. Activation of the contact pathway can help start coagulation, but the key site for the burst is the activated platelet surface.