Coagulation and Fibrinolysis

Hemostasis & Clotting Factors - Body's Plug Plan

• Hemostasis: Physiological process stopping blood loss.
  • Primary: Immediate vascular spasm. Platelet plug formation:
    • Adhesion (von Willebrand Factor (vWF) binds GpIb).
    • Activation (platelets release ADP, TXA2).
    • Aggregation (GpIIb/IIIa binds fibrinogen, linking platelets).
  • Secondary: Coagulation cascade (Intrinsic, Extrinsic, Common pathways) activation → Fibrin mesh formation, stabilizing the plug.
• Key Clotting Factors: Series of plasma proteins (mostly zymogens from liver).
  • I: Fibrinogen (precursor to fibrin).
  • II: Prothrombin (precursor to thrombin).
  • III: Tissue Factor (TF) / Thromboplastin (activates extrinsic pathway).
  • IV: Calcium ($Ca^{2+}$) (essential cofactor).
  • Vitamin K-dependent factors: II, VII, IX, X, Protein C, Protein S. 📌 Mnemonic: "1972" (IX, X, VII, II) + C&S.

⭐ Factor VII has the shortest half-life among all clotting factors, making it the first to decrease with warfarin therapy or liver disease affecting Vitamin K-dependent factor synthesis.

Coagulation Cascade - Clotting Dominoes

• Intrinsic Pathway (aPTT): Activated by contact with negatively charged surfaces (e.g., collagen).
  • Factors: XII → XI → IX (+VIIIa, Phospholipids (PL), $Ca^{2+}$) → X.
  • Monitored by: Activated Partial Thromboplastin Time (aPTT).
  • 📌 Mnemonic: PTT - Play Table Tennis (Inside). Factors 12, 11, 9, 8.
• Extrinsic Pathway (PT): Activated by Tissue Factor (TF/Factor III) release from damaged tissue.
  • Factors: VII (+TF, PL, $Ca^{2+}$) → X.
  • Monitored by: Prothrombin Time (PT).
  • 📌 Mnemonic: PT - Play Tennis (Outside). Factor 7 (and 3).
• Common Pathway: Convergence of intrinsic and extrinsic pathways.
  • Factors: Xa (+Va, PL, $Ca^{2+}$) converts Prothrombin (II) → Thrombin (IIa).
  • Thrombin (IIa): Converts Fibrinogen (I) → Fibrin (Ia); Activates Factor XIII.
  • Factor XIIIa: Cross-links fibrin monomers → Stable clot.
  • 📌 Mnemonic: Factors 10, 5, 2, 1 (e.g., $10 bill, $5 bill, $2 coin, $1 coin).

⭐ Vitamin K is crucial for the $\gamma$-carboxylation and activation of clotting factors II, VII, IX, X, and anticoagulant proteins C & S. Warfarin acts by inhibiting Vitamin K epoxide reductase.

Fibrinolysis & Regulation - Clot Dissolution Crew

• Core Process: Breakdown of fibrin clot.
• Key Enzyme: Plasmin (active form of Plasminogen).
• Activation: Plasminogen → Plasmin by:
  • Tissue Plasminogen Activator (t-PA): Endothelial cells; major physiological.
  • Urokinase Plasminogen Activator (u-PA).
• Plasmin Action: Degrades fibrin mesh → FDPs (incl. D-dimer).
  • $Fibrin \xrightarrow{Plasmin} FDPs + D-dimer$
• Regulation (Inhibitors):
  • α2-Antiplasmin (α2-AP): Primary inhibitor of free plasmin.
  • PAI-1 (Plasminogen Activator Inhibitor-1): Inhibits t-PA & u-PA.
  • TAFI (Thrombin Activatable Fibrinolysis Inhibitor): ↓ fibrinolysis (modifies fibrin, ↓ plasminogen/t-PA binding).

⭐ D-dimer: Specific marker for active fibrinolysis (cross-linked fibrin breakdown); elevated in Deep Vein Thrombosis (DVT), Pulmonary Embolism (PE), and Disseminated Intravascular Coagulation (DIC).

Lab Tests & Disorders - When Clotting Goes Wrong

• Common Lab Tests:
  • PT/INR: Extrinsic/Common (FVII). Warfarin (INR 2-3).
  • aPTT: Intrinsic/Common (FVIII, IX, XI, XII). Heparin.
  • Bleeding Time/PFA-100: Platelet function, vWD.
  • Fibrinogen: ↓ in DIC.
  • D-dimer: ↑ Fibrin Degradation Product (DVT, PE, DIC).
  • Mixing studies: Factor deficiency (corrects) vs. inhibitor (no correction).
• Bleeding Disorders:
  • Hemophilia A (↓FVIII), B (↓FIX): X-linked. ↑aPTT.
  • Von Willebrand Disease (vWD): Most common inherited. ↑BT/PFA.
  • Vitamin K Deficiency: ↑PT, ↑aPTT (Factors II, VII, IX, X, Protein C, S).
  • DIC: ↑PT, ↑aPTT, ↓Fibrinogen, ↑D-dimer, ↓Platelets.
• Thrombotic Disorders (Hypercoagulable):
  • Factor V Leiden (APC resistance) - Most common inherited.
  • Antiphospholipid Syndrome (APS).

⭐ Failure of aPTT to correct with a 1:1 mixing study (plasma mixing test) suggests the presence of a coagulation factor inhibitor (e.g., lupus anticoagulant or specific factor inhibitor).

High‑Yield Points - ⚡ Biggest Takeaways

• Vitamin K is essential for synthesis of factors II, VII, IX, X, Protein C & S.
• Hemophilia A (Factor VIII deficiency) and B (Factor IX deficiency) are key X-linked recessive disorders.
• von Willebrand Disease, most common inherited bleeding disorder, impacts vWF and Factor VIII activity.
• PT/INR measures extrinsic/common pathways (Warfarin); aPTT measures intrinsic/common pathways (Heparin).
• Plasmin, from plasminogen via tPA/uPA, mediates fibrinolysis; D-dimer indicates active clot breakdown.