Extrinsic Pathway of Coagulation - Bolo

Trigger: Vascular Injury → Tissue Factor (TF) Exposure

• Tissue factor (TF) is constitutively expressed on subendothelial cells (smooth muscle cells, fibroblasts) and is also found on circulating microparticles shed from monocytes and platelets.
• Under normal conditions, the intact vascular endothelium acts as a barrier, preventing plasma coagulation factors from contacting TF.
• After vascular injury, this barrier breaks down and TF is exposed to circulating plasma.

Step-by-Step Mechanism

1. TF + Factor VIIa Complex Formation

• Small amounts of Factor VIIa normally circulate in plasma (Factor VII is the only coagulation factor that circulates in its active form to a small extent).
• Upon vascular injury, Factor VIIa binds to the exposed TF on subendothelial cells.
• This forms the TF/VIIa complex (also called the extrinsic tenase complex), assembled on a phospholipid surface with Ca²⁺.

2. Activation of Factor X (Direct Route)

• The TF/VIIa complex directly cleaves and activates Factor X → Factor Xa.

3. Activation of Factor IX (Indirect Route - Cross-talk with intrinsic pathway)

• The same TF/VIIa complex also activates Factor IX → Factor IXa.
• Factor IXa then joins Factor VIIIa on the platelet phospholipid surface to form the intrinsic tenase complex, which further activates Factor X → Xa.
• This is a key amplification link between the extrinsic and intrinsic pathways.

4. Prothrombinase Complex Formation (Common Pathway)

• Factor Xa assembles with Factor Va (cofactor), Ca²⁺, and prothrombin (Factor II) on the platelet phospholipid membrane to form the prothrombinase complex.
• This complex converts Prothrombin (II) → Thrombin (IIa) - the central protease of coagulation.

5. Thrombin Actions

6. Clot Stabilization

• Thrombin activates Factor XIII → Factor XIIIa (transglutaminase).
• Factor XIIIa covalently cross-links α and γ chains of fibrin strands → insoluble, stable fibrin clot resistant to fibrinolysis.

Natural Inhibitor: TFPI (Tissue Factor Pathway Inhibitor)

• Once the TF/VIIa/Xa complex is formed, TFPI rapidly inhibits this complex, shutting down the extrinsic pathway.
• This makes sustained coagulation dependent on the amplification loop through Factor IXa/VIIIa (intrinsic pathway).
• This explains why patients with hemophilia A (Factor VIII deficiency) or B (Factor IX deficiency) bleed despite an intact extrinsic pathway - the amplification loop is broken.

Modern (Cell-Based) Concept

• Extrinsic pathway (TF/VIIa) = Initiation phase
• Intrinsic pathway (IXa/VIIIa) = Amplification & Propagation phase
• Both pathways cross-activate each other:
  • TF/VIIa complex activates Factor IX (extrinsic → intrinsic)
  • Factors IXa and Xa can activate Factor VII (intrinsic → extrinsic)

Lab Correlation

• A prolonged PT with normal aPTT specifically localizes a defect to the extrinsic pathway (Factor VII deficiency).

• Miller's Anesthesia, 10e, pp. 6715-6717
• Harrison's Principles of Internal Medicine, 22e, Chapter 69
• Medical Physiology (Boron & Boulpaep), p. 646