Coagulation Cascade Steps

When blood vessels are injured, a complex process called the coagulation cascade kicks in to prevent excessive bleeding. Understanding these steps is crucial in Anatomy and Physiology II, as they highlight how our body responds to vascular damage.

1. Vascular injury occurs

   • Damage to blood vessels exposes the underlying collagen and tissue factor.
   • This initiates the coagulation cascade to prevent blood loss.
   • Types of vascular injury can include cuts, abrasions, or trauma.

2. Platelets adhere to exposed collagen

   • Platelets are attracted to the site of injury by von Willebrand factor (vWF).
   • Adherence to collagen activates platelets, causing them to change shape and release chemical signals.
   • This aggregation forms a temporary "platelet plug" at the injury site.

3. Tissue factor released

   • Tissue factor (TF) is a glycoprotein released from damaged tissues.
   • It acts as a key initiator of the extrinsic pathway of coagulation.
   • TF binds with factor VII, leading to the activation of the coagulation cascade.

4. Extrinsic pathway activation

   • The binding of tissue factor and factor VII activates factor VIIa.
   • This complex activates factor X, leading to the common pathway.
   • The extrinsic pathway is rapid and is the first response to vascular injury.

5. Intrinsic pathway activation

   • The intrinsic pathway is activated by damage to the blood vessel and exposure of collagen.
   • It involves a series of factors (XII, XI, IX, and VIII) that amplify the coagulation response.
   • This pathway is slower but provides a sustained response to injury.

6. Common pathway begins

   • Both extrinsic and intrinsic pathways converge at the activation of factor X.
   • Activated factor X (Xa) leads to the conversion of prothrombin to thrombin.
   • This pathway is crucial for the final steps of clot formation.

7. Prothrombin converted to thrombin

   • Thrombin is a key enzyme in the coagulation cascade.
   • It converts fibrinogen into fibrin, which is essential for clot formation.
   • Thrombin also activates additional platelets, amplifying the clotting process.

8. Fibrinogen converted to fibrin

   • Fibrinogen is a soluble plasma protein that is converted to insoluble fibrin strands.
   • Fibrin forms a mesh that stabilizes the platelet plug and creates a more durable clot.
   • This conversion is essential for the structural integrity of the clot.

9. Cross-linking of fibrin

   • Fibrin strands are cross-linked by factor XIII, which stabilizes the clot.
   • This cross-linking enhances the strength and durability of the clot.
   • It ensures that the clot remains intact until the vessel is healed.

10. Clot formation and stabilization

    • The final clot serves to seal the injury and prevent further blood loss.
    • Clot retraction occurs as platelets contract, pulling the edges of the wound together.
    • Eventually, the clot is dissolved through fibrinolysis once healing is complete.