The coagulation cascade is a complex series of events that lead to the formation of a blood clot, ensuring the cessation of bleeding. It involves three main pathways:
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Intrinsic Pathway: Initiated by damage to the blood vessel, activating factors XII, XI, IX, and VIII.
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Extrinsic Pathway: Triggered by external trauma, involving tissue factor (TF) and factor VII.
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Common Pathway: Where both intrinsic and extrinsic pathways converge, leading to the activation of factor X.
🔁 Common Pathway and Fibrinogen’s Role
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Factor Xa, in combination with Factor Va, converts prothrombin (Factor II) into thrombin (Factor IIa).
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Thrombin then converts fibrinogen (Factor I) into fibrin, forming a mesh that stabilizes the platelet plug and constitutes the final clot.
🧪 Key Clotting Factors and Their Functions
| Factor | Name | Role in Clotting |
|---|---|---|
| I | Fibrinogen | Converted to fibrin by thrombin to form the clot |
| II | Prothrombin | Converted to thrombin, which activates fibrinogen |
| III | Tissue Factor (TF) | Initiates the extrinsic pathway by activating Factor VII |
| IV | Calcium (Ca²⁺) | Essential for all steps in the coagulation cascade |
| V | Proaccelerin | Co-factor for Factor Xa in the common pathway |
| VII | Stable Factor | Initiates the extrinsic pathway by activating Factor X |
| VIII | Anti-hemophilic Factor A | Co-factor for Factor IXa in the intrinsic pathway |
| IX | Christmas Factor | Activated by Factor XIa, activates Factor X in the intrinsic pathway |
| X | Stuart-Prower Factor | Activated by Factor IXa/VIIIa or TF/VIIa, converts prothrombin to thrombin |
| XI | Plasma Thromboplastin Antecedent | Activated by Factor XIIa, activates Factor IX in the intrinsic pathway |
| XII | Hageman Factor | Activated by contact with damaged surfaces, activates Factor XI |
| XIII | Fibrin-Stabilizing Factor | Cross-links fibrin strands to stabilize the clot |