Chapter 55: Hemostasis & Thrombosis

Hemostasis & Thrombosis from Harper’s Illustrated Biochemistry provides a comprehensive analysis of hemostasis and thrombosis, detailing the biochemical processes responsible for the cessation of bleeding and the pathological formation of intravascular clots. The text elucidates the three shared phases of these phenomena: the initial formation of a loose platelet aggregate, the stabilization of this plug by a fibrin mesh, and the eventual dissolution of the clot through fibrinolysis. Significant attention is given to platelet biology, describing how platelets adhere to subendothelial collagen via glycoprotein receptors and von Willebrand factor, subsequently undergoing activation and shape change driven by agonists such as thrombin, ADP, and thromboxane A2. The narrative explains the intricate signal transduction pathways involved, including the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the mobilization of intracellular calcium, which leads to the conformational activation of integrins required for aggregation. The chapter further dissects the coagulation cascade, distinguishing between the extrinsic pathway initiated by tissue factor and the intrinsic pathway, both of which converge on the activation of factor X and the assembly of the prothrombinase complex. This complex catalyzes the conversion of prothrombin to thrombin, a serine protease that cleaves fibrinogen into fibrin monomers that polymerize and are cross-linked by factor XIIIa to form a stable clot. Regulatory mechanisms are thoroughly explored, including the roles of antithrombin, heparin, and the protein C and S system in limiting coagulation. Additionally, the text covers the vitamin K-dependent carboxylation of specific clotting factors, the mechanism of action for anticoagulants like warfarin and aspirin, and the clinical implications of genetic disorders such as hemophilia and von Willebrand disease. Finally, the process of fibrinolysis is detailed, highlighting how plasminogen is activated to plasmin by tissue plasminogen activator to degrade fibrin, a concept utilized in thrombolytic therapies for coronary thrombosis and stroke.