Chapter 22: Disorders of Hemostasis

Normal hemostasis is divided into three sequential steps: instantaneous vessel constriction to reduce blood flow, the formation of a primary platelet plug involving platelet adhesion (mediated by von Willebrand factor, vWF) and aggregation (stimulated by ADP and TXA2), and finally, blood coagulation, which converts soluble fibrinogen into a stable, insoluble fibrin clot via the stepwise activation of the intrinsic and extrinsic pathways. The coagulation cascade requires specific clotting factors synthesized primarily in the liver, many of which are dependent on Vitamin K, and is carefully controlled by natural anticoagulants like Antithrombin III and Protein C. After clot formation, the process concludes with clot retraction, where the serum is squeezed out, and fibrinolysis, the dissolution of the clot by the enzyme plasmin to allow for tissue repair. Disorders of hemostasis fall into two major categories: hypercoagulability (excessive clotting) and bleeding disorders (insufficient clotting). Hypercoagulability states predispose to thrombosis, arising from increased platelet function (seen in conditions like atherosclerosis or thrombocytosis) or accelerated clotting activity, such as inherited factor V Leiden or acquired conditions like malignancy and antiphospholipid syndrome. Conversely, bleeding disorders result from defects in platelet number (thrombocytopenia, caused by decreased production or increased destruction like in ITP or HIT) or impaired platelet function (thrombocytopathia). Coagulation factor deficiencies, including inherited disorders like Hemophilia A and von Willebrand disease, or acquired conditions such as liver disease, lead to extensive hemorrhage. The most severe acquired disorder discussed is Disseminated Intravascular Coagulation (DIC), a paradoxical condition triggered by massive, unregulated coagulation that simultaneously leads to widespread microvascular thrombosis and dangerous consumptive bleeding.