Intrinsic

Overview of Coagulation Pathways

  • The intrinsic pathway and extrinsic pathway of coagulation lead to the production of the same end-product, the enzyme prothrombinase.

  • Differences between intrinsic and extrinsic pathway:

    • The intrinsic pathway involves a greater number of steps compared to the extrinsic pathway.

    • As a result, the intrinsic pathway takes longer to produce prothrombinase, typically taking a few minutes.

Key Components of the Intrinsic Pathway

  • Definition of Intrinsic Pathway:

    • Defined by the fact that all chemicals involved are contained within the blood and blood vessels.

    • No external chemicals contribute to the intrinsic pathway.

  • Starting Component:

    • Begins with a plasma protein known as inactive factor 12 (Factor XII).

    • Plasma proteins are predominantly produced by liver cells.

  • Activation of Factor XII:

    • Damage to the endothelium exposes subendothelial collagen fibers.

    • Inactive factor XII is sensitive to contact with these exposed collagen fibers.

    • Upon contact, inactive factor XII is activated to become active factor XII (XIIa).

Process of Activation

  1. Active Factor XII:

    • Once activated, factor XII is present with calcium ions (clotting factor IV).

    • Active XII initiates the activation of factor X (factor Xa).

  2. Catalyst Requirement:

    • The activation of factor X by active XII proceeds slowly. Therefore, a catalyst is needed to enhance this reaction.

    • Platelet phospholipids released from damaged platelets serve as this catalyst.

  3. Role of Platelet Phospholipid:

    • These phospholipids:

      • Speed up the activation of factor X by active XII.

      • Facilitate the subsequent binding of active factor X with factor V and calcium ions to form prothrombinase.

Formation of Prothrombinase

  • Final Steps:

    • Active factor X binds to factor V and calcium ions to construct the enzyme prothrombinase.

    • Platelet phospholipids play a dual role in accelerating both reactions:

    • The activation of factor X by active factor XII.

    • The binding of active factor X with factor V and calcium ions.

  • Conclusion:

    • The formation of the prothrombinase molecule signifies the conclusion of the intrinsic pathway, with prothrombinase being crucial for the subsequent steps in the coagulation cascade necessary for blood clotting.