Intrinsic and Common pt2

Coagulation Process: Intrinsic Pathway and Formation of Prothrombinase

Overview of Coagulation

  • Coagulation is a vital process that prevents blood loss due to injury and involves complex interactions between various proteins and cells in the bloodstream.

  • The coagulation process can be divided into two main pathways: intrinsic pathway and extrinsic pathway.

  • Both pathways ultimately lead to the formation of a common enzyme called prothrombinase (PTA) that is crucial for blood clot formation.

Intrinsic Pathway Overview

  • Focus of discussion in part two is the intrinsic pathway of coagulation.

  • Formation of prothrombinase is a key event in this pathway.

Key Components in Intrinsic Pathway

  • Inactive Factor XII:

    • A plasma protein produced by liver cells.

    • Activates in response to damaged endothelial cells and exposed collagen fibers.

  • Activation of Factor XII:

    • Process: When Factor XII touches exposed collagen fibers, it becomes activated.

    • Comparison: Activation of Factor XII is similar to how platelets become active after blood vessel injury.

  • Activation of Factor X:

    • Once activated, Factor XII (active XII) begins to activate Factor X (FX).

    • Calcium Ions: Required for the activation of Factor X, sourced from active platelets and plasma proteins.

Speed of Reaction

  • Initial Finding: Active Factor XII is noted as slow in activating Factor X compared to the extrinsic pathway where the tissue factor activates Factor X more rapidly.

  • Catalyst Introduction:

    • Because reactions happening slowly may require a catalyst, a catalyst must be introduced to speed up the reaction.

  • Phospholipid Release from Platelets:

    • Damaged platelets release a specific phospholipid known as platelet phospholipid.

    • This phospholipid acts as the catalyst speeding up Factor X's activation process.

Formation of Prothrombinase

  • Mechanism:

    • Active Factor XII, in the presence of calcium ions and platelet phospholipid, activates Factor X (FX).

    • Once Factor X becomes active, it binds to Factor V and calcium ions, leading to the formation of the prothrombinase complex.

  • Time Frame: The formation of prothrombinase through the intrinsic pathway takes a few minutes.

Common Pathway Initiation

  • Both Pathways to Prothrombinase:

    • Both intrinsic and extrinsic pathways converge by producing prothrombinase, which is essential for the subsequent steps of coagulation.

  • Definition: The rest of the coagulation process that follows the formation of prothrombinase is referred to as the common pathway.

Common Pathway Process

  • Activation of Prothrombin:

    • The enzyme prothrombinase acts on an inactive protein in the blood known as prothrombin, which is also dependent on calcium ions for activation.

  • Formation of Thrombin: Prothrombinase converts prothrombin into the active form called thrombin.

  • Conversion of Fibrinogen:

    • Thrombin then acts on fibrinogen, a soluble plasma protein, converting it into insoluble fibrin strands.

    • Clarification that the process is about transformation from soluble to insoluble rather than merely activating fibrinogen.

Formation of Stable Fibrin

  • Initial Formation of Fibrin:

    • The early fibrin strands formed are referred to as loose fibrin which is not strongly stable.

  • Activation of Fibrin Stabilizing Factor (Factor XIII):

    • While some thrombin converts fibrinogen into loose fibrin, others activate inactive fibrin stabilizing factor (Factor XIII) from plasma and damaged platelets.

  • Stabilization Process:

    • Once activated, Factor XIII stabilizes loose fibrin into strong stable fibrin strands.

Final Outcome of Coagulation

  • Blood Clot Formation:

    • Strong fibrin molecules create a network over the damage site, forming an effective blood clot that stops bleeding.

  • Role of Macrophages and Phagocytes:

    • The clot will be eventually removed by macrophages and phagocytes as tissue repair begins.

  • Duration of Clot Stability: The fibrin molecules need to remain in place until healing processes initiate.

Conclusion

  • The intrinsic pathway, while slower than the extrinsic, plays a fundamental role in the coagulation process. The interplay between key factors and the conversion from prothrombin to thrombin underscores the intricate regulation of blood clotting needed for hemostasis.