Overview
This lecture explains the steps of hemostasis—the process that stops bleeding—including the formation of clots, the roles of clotting factors, and disorders affecting coagulation.
Mechanisms of Hemostasis
- Hemostasis prevents blood loss from ruptured vessels through three steps: vascular spasm, platelet plug formation, and coagulation.
- Vascular spasm involves vessel smooth muscle contraction to reduce blood flow, triggered by endothelins and pain receptors.
- Platelet plug formation occurs when platelets stick to exposed collagen, aided by von Willebrand factor, and release chemicals (ADP, serotonin) to reinforce the plug.
- Coagulation is the formation of a stable blood clot as a cascade of reactions leads to fibrin strand formation.
Coagulation Pathways and Factors
- The extrinsic pathway triggers coagulation after external tissue injury and releases tissue factor (factor III) to activate factor VII, then factor X.
- The intrinsic pathway starts within the blood after vessel wall damage or contact with certain surfaces, activating factors XII, XI, IX, and VIII before factor X.
- Both pathways converge at the common pathway, where factor X activates conversion of prothrombin (factor II) to thrombin, which converts fibrinogen (factor I) to fibrin.
- There are 12 clotting factors, mainly produced by the liver and platelets, many requiring vitamin K and calcium ions.
Fibrinolysis and Anticoagulants
- Fibrinolysis gradually dissolves clots as plasmin degrades fibrin, restoring vessel flow.
- Plasma anticoagulants (protein C system, TFPI, antithrombin, heparin) limit clotting to injury sites and prevent systemic clot formation.
Clotting Disorders
- Thrombocytopenia (low platelets) leads to excessive bleeding; thrombocytosis (high platelets) increases abnormal clot (thrombus) risk.
- Hemophilia is a genetic disorder causing clotting factor deficiencies, mostly factor VIII (hemophilia A), IX (hemophilia B), or XI (hemophilia C).
- Thrombosis is abnormal clot formation; embolism occurs when part of a clot breaks loose and blocks critical vessels, potentially causing heart attacks or strokes.
- Aspirin inhibits platelet aggregation and is used as a preventive anticoagulant but carries bleeding risk.
- Thrombolytic agents, such as tissue plasminogen activator, help dissolve existing clots in emergencies.
Key Terms & Definitions
- Hemostasis — process stopping bleeding via vessel constriction, platelet plug, and clot formation.
- Vascular Spasm — rapid constriction of a blood vessel after injury.
- Platelet Plug — temporary seal formed by aggregated platelets at a wound.
- Coagulation — blood clotting via a cascade of reactions forming fibrin.
- Clotting Factors — proteins enabling coagulation, named I–XIII.
- Fibrinolysis — breakdown and removal of a blood clot.
- Anticoagulant — substance that prevents or limits clotting.
- Thrombocytopenia — abnormally low platelet count.
- Thrombosis — formation of an abnormal blood clot.
- Embolus/Embolism — clot fragment traveling and blocking a vessel.
- Hemophilia — genetic disorder causing clotting factor deficiency.
Action Items / Next Steps
- Review animations of coagulation pathways at the provided OpenStax link.
- Study Table 18.1 for roles and sources of clotting factors.
- Prepare for questions on the differences between intrinsic, extrinsic, and common coagulation pathways.