Common Pathway

Common Pathway of Coagulation

  • Overview

    • The common pathway of coagulation begins when prothrombinase is formed.

    • Prothrombinase can be derived from either the extrinsic pathway or the intrinsic pathway.

    • Regardless of the pathway, the ultimate outcome is the same, leading to coagulation through the common pathway.

Formation of Prothrombinase

  • Prothrombinase is produced by:

    • Extrinsic pathway

    • Intrinsic pathway

  • Both pathways lead to the same product: prothrombinase.

Role of Prothrombinase

  • Prothrombinase targets an inactive enzyme plasma protein known as prothrombin.

  • Prothrombin:

    • Function: An inactive enzyme that is converted into thrombin.

    • Source: Synthesized by liver cells.

  • In the presence of calcium ions (Ca^{2+}) (Clotting Factor IV), prothrombinase activates prothrombin, resulting in the enzyme thrombin.

Functions of Thrombin

  • Thrombin (active enzyme) is crucial and has multiple roles, some of which are summarized in separate discussions.

  • Key effects of thrombin include:

    • Conversion of fibrinogen to fibrin in the presence of calcium ions.

Difference Between Fibrinogen and Fibrin

  • Fibrinogen:

    • Nature: Soluble plasma protein.

    • Status: Blood remains liquid.

  • Fibrin:

    • Nature: Insoluble.

    • Status: Formation of a blood clot.

    • Process: As fibrinogen transforms into fibrin, it leads to the creation of a network essential for sealing the damaged area of a blood vessel.

Importance of the Fibrin Network

  • Prior to fibrin formation, a platelet plug may obstruct the area; however, it is not robust enough to completely stop bleeding.

  • The fibrin network acts as a structural foundation to reinforce this blockage:

    • The formation of a secure barrier is critical for hemostasis.

    • Thrombin's additional role: It activates clotting factor XIII, known as fibrin stabilizing factor.

Sources of Fibrin Stabilizing Factor (Factor XIII)

  • Two primary sources of Factor XIII:

    1. Available in blood plasma.

    2. Derived from activated platelets.

    • Note: Most plasma proteins, including fibrin stabilizing factor, are produced by liver cells.

Activation of Fibrin Stabilizing Factor

  • Upon activation by thrombin, fibrin stabilizing factor changes the loose and unstable fibrin molecules into strong and stable fibrin.

    • Result: The formation of a robust fibrin network which:

    • Serves as a scaffold for other blood components.

    • Supports the closure of the vascular injury.

    • As blood is pushed out, cells become trapped in this network leading to the solidification of the blood clot as it sheds excess plasma.

Conclusion of the Common Pathway

  • The common pathway concludes with stable fibrin strands that form a cohesive clot, effectively sealing the damaged blood vessel and preventing further bleeding.

  • The entire sequence begins with the formation of prothrombinase and concludes with the creation of the fibrin network essential for hemostasis.