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Blood Coagulation Cascade



  • Coagulation converts primary platelet plug into clot.

  • It may start with intrinsic pathway and/or exterinsic pathway โ†’ both the pathways merge at the common pathway.

Intrinsic Pathway

  • Activated when blood comes in contact with negative surface.

  • Takes place on the surface of activated platelet.

  • starts with factors that are intrinsic to the blood โ†’ called intrinsic pathway.

High Molecular Weight Kiniogen anchors factor XII to platelet surface

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Activation of factor XII to XIIa

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XIIa activates prekallikrein into kallikrein โ†’ kallikrein speeds up conversion of XII to XIIa

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XIIa activates XI into XIa

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ฮณ Carboxy glutamic acid residue of factor IX attaches it to platelet surface via calcium โ†’ XIa activates IX into IXa

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IXa associates with VIIIa, Calcium, and phospholipids โ†’ this complex is called Tenase

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Tenase activates X into Xa.

Extrinsic Pathway

  • Starts with factors outside the blood โ†’ called extrinsic pathway.

Under healthy conditions, endothelial cells prevent exposure of tissue factor to the blood.

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Injury to endothelium

โ†“ Tissue factor is exposed to blood

โ†“ Factor VII binds to tissue factor โ†’ tissue factor activates factor VII into VIIa

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Tissue factor and VIIa associate with calcium and form "Tissue factor + VIIa + Ca" complex

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This complex activates X into Xa.

Common Pathway

Factor Xa associates with factor Va, calcium, and phospholipids

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This complex is called prothrombinase

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It converts prothrombin into thrombin

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Thrombin converts fibrinogen into fibrin monomer

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Fibrin monomers polymerize to form fibrin polymers

โ†“ Fibrin polymers form loose mesh around platelets

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Thrombin converts Factor XIII (Fibrin stabilizing factor) into XIIIa

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XIIIa cause covalent cross-linking between fibrin polymers โ†’ formation of stable fibrin.

Important Points

  • Along with platelets, RBCs and WBCs also get trapped in clot when it is being formed.

  • Arterial clots have high proportion of platelets.

  • Venous clots have high proportion of fibrin.

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