Hematology | Hemostasis: Coagulation Cascade

Summary

Key Points

• Hemostasis is the localized stoppage of blood flow, typically initiated by damage to blood vessels, and involves a sequence of five steps. 0:19
• Various pharmacological agents, such as Heparin, Aspirin, Clopidogrel, and Warfarin, target different stages of hemostasis to prevent or treat clotting disorders. 2:56
• The first step, vascular spasm, involves immediate vasoconstriction of the injured blood vessel, reducing blood loss through mechanisms like endothelin release, myogenic response, and nociceptor activation. 9:20
• Fibrin strands are cross-linked by Factor 13 to form a stable fibrin mesh, reinforcing the platelet plug and creating a robust clot to prevent further blood loss. 24:55
• Platelet plug formation, the second step, begins when platelets adhere to exposed collagen via Von Willebrand Factor and glycoprotein 1B, then become activated and release chemicals like ADP and thromboxane A2 to recruit more platelets and promote aggregation. 32:56
• The coagulation cascade, a complex third step, involves intrinsic and extrinsic pathways converging on a common pathway to convert prothrombin into thrombin, which then transforms soluble fibrinogen into insoluble fibrin strands. 33:14
• Clot retraction and repair, the fourth step, involves platelets contracting to pull the ruptured vessel edges closer and secreting growth factors like Platelet-Derived Growth Factor (PDGF) and Vascular Endothelial Growth Factor (VEGF) to stimulate tissue regeneration. 36:04
• Fibrinolysis, the final step, is the process of dissolving the clot once the vessel is repaired, primarily through tissue plasminogen activator (tPA) converting plasminogen into plasmin, which degrades fibrin and produces D-dimers. 37:20
• The body naturally prevents unwanted clotting through mechanisms like endothelial cells secreting nitric oxide and prostacyclin, Heparin sulfate activating antithrombin III, and thrombomodulin activating protein C. 39:44