Compare the difference between intrinsic pathway and extrinsic pathway of blood coagulation for five marks. Give me the proper difference and it should be of exam oriented. Make a table and describe it.
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Intrinsic vs. Extrinsic Pathway of Blood Coagulation
(Source: Harper's Illustrated Biochemistry 32e, Basic Medical Biochemistry 6e, Medical Physiology, Miller's Anesthesia 10e)
Coagulation Cascade Diagram
Comparison Table: Intrinsic vs. Extrinsic Pathway
| Feature | Intrinsic Pathway (Contact Activation Pathway) | Extrinsic Pathway (Tissue Factor Pathway) |
|---|---|---|
| Trigger / Initiating stimulus | Activation by negatively charged surfaces (collagen, glass, dextran sulfate, kaolin) in contact with blood | Exposure of blood to Tissue Factor (TF / Factor III) released from damaged subendothelial tissues |
| Components involved | Factors XII, XI, IX, VIII, prekallikrein (PK), high-molecular-weight kininogen (HMWK), Ca²⁺, platelet phospholipids | Factors VII, X, Tissue Factor (III), Ca²⁺, phospholipids |
| Key initiating factor | Factor XII (Hageman factor) - activated by contact with negatively charged surface | Factor VII (Proconvertin) - activated upon binding to Tissue Factor after vascular injury |
| Speed of reaction | Slower - involves more steps and multiple factors | Faster - fewer steps; rapid initiation of hemostasis |
| Role in physiology | Primarily an amplification system - propagates and sustains thrombin generation initiated by the extrinsic pathway | Represents the initiation phase of plasma-mediated hemostasis; principal pathway in vivo |
| In vitro test | aPTT (Activated Partial Thromboplastin Time) - prolonged when intrinsic factors are deficient | PT (Prothrombin Time) - prolonged when extrinsic factors (especially Factor VII) are deficient |
| Sequence of activation | XII → XIIa → XI → XIa → IX → IXa → (with VIIIa, Ca²⁺, PL) → Xa | VII + TF → VIIa/TF complex → (with Ca²⁺, PL) → Xa |
| Vitamin K-dependent factors | Factors IX (also II and X in common pathway) | Factors VII (also II, IX, X - vitamin K dependent) |
| Associated disorders | Deficiency of Factor VIII → Hemophilia A; Factor IX → Hemophilia B; Factor XI → Hemophilia C | Factor VII deficiency → prolonged PT; tissue factor pathway defects are rare |
| Inhibitors | Contact pathway inhibited by C1-inhibitor, antithrombin III | TFPI (Tissue Factor Pathway Inhibitor) is the key inhibitor; limits the extrinsic pathway |
| Location of reaction | On the phospholipid membrane of activated platelets | At the site of tissue injury on subendothelial tissue factor (membrane-bound) |
Convergence: The Common Pathway
Both pathways converge at Factor X activation (Factor Xa). From here:
- Xa + Va + Ca²⁺ + phospholipid form the Prothrombinase complex
- Prothrombinase converts Prothrombin (Factor II) → Thrombin (Factor IIa)
- Thrombin cleaves Fibrinogen (Factor I) → Fibrin monomers → polymerize into a soft clot
- Factor XIIIa (activated by thrombin) cross-links fibrin strands into an insoluble hard clot
Key Points to Remember for Exams
- "Intrinsic = all components in blood" - no external tissue factor required; activated by internal contact with abnormal surfaces.
- "Extrinsic = external trigger" - tissue factor (extrinsic to blood) is needed.
- TFPI is the physiologic brake on the extrinsic pathway - this is why the intrinsic pathway is essential for sustaining coagulation.
- Both pathways are not truly independent - the TF/VIIa complex of the extrinsic pathway also activates Factor IX of the intrinsic pathway, creating cross-talk.
- Lab correlation: aPTT tests intrinsic; PT/INR tests extrinsic. A patient with prolonged aPTT but normal PT has an intrinsic pathway defect (e.g., Hemophilia A or B).
Sources: Harper's Illustrated Biochemistry 32e, p. 1417-1421 | Basic Medical Biochemistry 6e, p. 1557 | Medical Physiology (Boron & Boulpaep), p. 643 | Miller's Anesthesia 10e, p. 6715-6717
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