The coagulation cascade is a complex series of biochemical reactions in the blood that lead to clot formation, which is crucial for stopping bleeding after injury. It involves a sequence of enzymatic activations, where inactive proteins (called clotting factors) are converted into active forms.

The cascade is divided into three pathways:

1. Intrinsic Pathway:

• Activated by damage to the blood vessel.
• Involves clotting factors XII, XI, IX, and VIII.
• This pathway is slower but amplifies the coagulation process.
• It leads to the activation of Factor X, which joins the common pathway.

2. Extrinsic Pathway:

• Activated by external trauma that causes blood to leak out of the vessel.
• It involves tissue factor (Factor III) and Factor VII.
• This pathway is faster and initiates the coagulation process.
• Also leads to the activation of Factor X.

3. Common Pathway:

• Both intrinsic and extrinsic pathways converge here.
• Factor X, once activated (Xa), combines with Factor V to convert prothrombin (Factor II) into thrombin.
• Thrombin is crucial as it converts fibrinogen (Factor I) into fibrin, forming the structural basis of a blood clot.
• Thrombin also activates Factor XIII, which cross-links fibrin, stabilizing the clot.

Key Points:

• Calcium ions (Ca²⁺) and vitamin K are essential for various steps in the coagulation process.
• The balance between pro-coagulant and anti-coagulant factors is crucial to avoid excessive clotting (thrombosis) or bleeding disorders (hemophilia, for example).

This cascade ensures the body can respond to bleeding while maintaining blood flow under normal conditions.