Coagulation and Hemostasis

Plasma vs Serum

  • Definitions:

    • Plasma: A fluid rich in nutrients and cells transported throughout the body. It can be separated from blood components including:

    • Water

    • Proteins

    • Antibodies

    • Lipids

    • Carbohydrates (CHO)

    • Electrolytes

    • Clotting factors

    • Enzymes

    • Vitamins

    • Hormones

    • Serum: Fluid obtained when coagulated blood (collected in a red top tube) has been centrifuged. Contains fibrinogen which is absent in serum.

  • Separation of Blood Components:

    • Blood can be separated into several layers:

    • Plasma: Represents 55% of total blood volume.

    • Buffy Coat: Comprising leukocytes and platelets, accounting for less than 1% of total volume.

    • Erythrocytes (Red Blood Cells): Represents 45% of total blood volume.

Primary Hemostasis

  • Definition: The arrest of bleeding via spontaneous formation of a hemostatic platelet plug. It involves:

    • Platelet adhesion: Attachment of platelets to the injured vessel wall.

    • Platelet aggregation: Clumping of platelets together to form a plug.

    • Platelet shape change: Platelets change shape to facilitate aggregation.

    • Platelet secretion: Release of granular contents that promote further platelet aggregation and vasoconstriction.

  • Steps in Primary Hemostasis:

    1. Breach of Endothelium: Injury to the blood vessel.

    2. Vasoconstriction: Reduces blood flow at the site of injury.

    3. Platelet adhesion: Platelets adhere to the exposed collagen at the site.

    4. Secretion of Aggregators: Release of signaling molecules that enhance aggregation.

    5. Platelet Aggregation: Formation of a physical plug with the help of fibrinogen.

Secondary Hemostasis

  • Role: Involves the activation of the coagulation cascade which leads to fibrin mesh stabilization over the platelet plug. It can be described through:

    • Cascade Model: Sequential activation of procoagulants resulting in the formation of a stable fibrin clot.

    • Cell Theory Model: Suggests coagulation occurs primarily on cell surfaces;

    • It includes an initiation phase on the tissue factor (TF) bearing cells.

    • An amplification phase where platelets are activated enhancing coagulation factors.

    • A propagation phase where thrombin generation occurs, stabilizing the clot.

Fibrinolysis

  • Definition: The process of clot removal post-hemostasis, which involves:

    • Fibrinolytic Proteins: Incorporated into the fibrin clot.

    • TPA (Tissue Plasminogen Activator) and UPA (Urokinase Plasminogen Activator): Convert plasminogen to plasmin, leading to degradation of fibrin, thus restoring normal blood flow.

  • Fibrin Degradation Products: Result from plasmin activity include:

    • X, Y, D, E, and D-dimer (D-D).

Coagulation Factors and Their Functions

  • Procoagulants: Plasma transports about 16 procoagulants that include inactive zymogens:

    • When activated become signaling factors leading to thrombus formation.

  • Classification of Coagulation Factors: Grouped into several classifications based on their function:

    • Contact Group: Factors XI, XII, Prekallikrein, and HMWK.

    • Prothrombin Group: Factors II (prothrombin), VII, IX, X.

    • Fibrinogen Group: Factors I (fibrinogen), V, VIII, XIII.

Specific Coagulation Factors and Pathways

  • Factor I: Fibrinogen, main structural protein of the fibrin clot.

  • Factor II: Prothrombin, converted to thrombin by Factor Xa, Va in the presence of Calcium.

  • Factor VII: Activated by tissue factor (TF) and operates in extrinsic pathway.

  • Factor VIII: Antihemophilic factor, cleaved by thrombin which frees it from vWF.

  • Factor IX: Christmas factor, participates in intrinsic pathway.

  • Factor X: Stuart-Prower factor, integration into both intrinsic and extrinsic pathways.

  • Factor XI: Plasma thromboplastin antecedent (PTA), activated by FXIIa.

  • Factor XII: Hageman factor, activates FXI, prekallikrein in the intrinsic pathway.

Anticoagulant Mechanisms and Inhibitors

  • Antithrombin (AT): A serine protease inhibitor that neutralizes enzymes like thrombin, activated by heparin, enhances activity.

  • TFPI (Tissue Factor Pathway Inhibitor): Inhibits factor Xa and VIIa, synthesized by endothelial cells.

  • Protein C Pathway: Activated by thrombin, regulates coagulation to prevent excessive clotting, working with Protein S.

Clinical Implications

  • Aspirin's Role: Inhibits cyclooxygenase, preventing platelet aggregation, thereby used therapeutically in conditions requiring anti-platelet effects.

  • Deficiencies in Coagulation Factors: May lead to conditions like:

    • Hemophilia A: Low Factor VIII.

    • Hemophilia B: Low Factor IX.

    • Factor V Leiden mutation: Involved in thrombosis risk.

Testing for Hemostasis

  • Coagulation Tests Include:

    • Prothrombin Time (PT): Measures how long it takes blood to clot, affected/detecting deficiencies in the extrinsic pathway.

    • Activated Partial Thromboplastin Time (APTT): Measures clotting time for intrinsic pathway deficiencies.

Conclusion

  • Understanding the mechanisms of hemostasis, the coagulation cascade, and regulatory proteins provide insight into treating and managing bleeding disorders, thrombotic conditions, and the underlying risk factors associated with coagulation abnormalities.