top of page

Blood Coagulation Cascade



  • Coagulation converts primary platelet plug into clot.

  • It may start with intrinsic pathway and/or exterinsic pathway → 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 → called intrinsic pathway.

High Molecular Weight Kiniogen anchors factor XII to platelet surface

↓

Activation of factor XII to XIIa

↓

XIIa activates prekallikrein into kallikrein → kallikrein speeds up conversion of XII to XIIa

↓

XIIa activates XI into XIa

↓

γ Carboxy glutamic acid residue of factor IX attaches it to platelet surface via calcium → XIa activates IX into IXa

↓

IXa associates with VIIIa, Calcium, and phospholipids → this complex is called Tenase

↓

Tenase activates X into Xa.

Extrinsic Pathway

  • Starts with factors outside the blood → called extrinsic pathway.

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

↓

Injury to endothelium

↓ Tissue factor is exposed to blood

↓ Factor VII binds to tissue factor → tissue factor activates factor VII into VIIa

↓

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

↓

This complex activates X into Xa.

Common Pathway

Factor Xa associates with factor Va, calcium, and phospholipids

↓

This complex is called prothrombinase

↓

It converts prothrombin into thrombin

↓

Thrombin converts fibrinogen into fibrin monomer

↓

Fibrin monomers polymerize to form fibrin polymers

↓ Fibrin polymers form loose mesh around platelets

↓

Thrombin converts Factor XIII (Fibrin stabilizing factor) into XIIIa

↓

XIIIa cause covalent cross-linking between fibrin polymers → 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.

 

Next >>

Comments

bottom of page