Mechanisms of coagulation

• Blood clotting, also known as mechanisms of coagulation, is a crucial process that prevents excessive bleeding when a blood vessel is injured.

• Here’s a detailed overview of how it works:

1. Vascular Spasm

• Immediately after a blood vessel is injured, its walls constrict to reduce blood flow.

• This initial response is known as a vascular spasm.

2. Platelet Plug Formation

• Platelets, a type of blood cell, adhere to the site of injury and to each other, forming a temporary "platelet plug."

• Platelets release chemicals that attract more platelets to the site, reinforcing the plug.

3. Coagulation Cascade

• This step involves a series of complex biochemical reactions involving clotting factors (proteins in blood plasma).

• The cascade is initiated by the exposed tissue at the injury site and proceeds through a series of steps involving these clotting factors.

• The cascade has two pathways—the intrinsic and extrinsic pathways—which converge on a common pathway.

Mechanisms of coagulation

A. Intrinsic Pathway

1. Initiated by damage inside the blood vessel:

   1. Activation of Factor XII: When blood comes into contact with negatively charged surfaces.

   2. Factor XI Activation: Activated by factor XIIa.

   3. Factor IX Activation: Activated by factor XIa, with the help of calcium.

   4. Factor VIII Activation: Works as a cofactor with activated factor IX (IXa) to activate factor X.

B. Extrinsic Pathway

• Triggered by external trauma that causes blood to escape from the vessel:

  1. Release of Tissue Factor (Factor III): From damaged tissue.

  2. Activation of Factor VII: By tissue factor, forming a complex with factor III.

  3. Activation of Factor X: By the tissue factor-Factor VIIa complex.

C. Common Pathway

• Both intrinsic and extrinsic pathways converge here:

  1. Activation of Factor X: Leads to the conversion of prothrombin (Factor II) to thrombin (Factor IIa).

  2. Thrombin’s Role: Converts fibrinogen (Factor I) to fibrin.

  3. Formation of Fibrin Clot: Fibrin threads form a mesh that traps blood cells, forming a clot.

  4. Factor XIII Activation: Stabilizes the fibrin clot by cross-linking fibrin strands.

4. Formation of Fibrin Mesh

• Thrombin converts fibrinogen, another blood protein, into fibrin.

• Fibrin strands weave through the platelet plug, forming a stable mesh that solidifies the plug, creating a stable clot that seals the wound until the blood vessel can be permanently repaired.

5. Clot Retraction and Repair

• The clot contracts to bring the edges of the blood vessel closer together, aiding in healing.

• Eventually, the clot is broken down and absorbed by the body once the vessel is healed.

This process is tightly regulated by the body to ensure that clots form only when needed and that they are dissolved once they are no longer necessary.