What is blood coagulation? Enumerate the various steps involved in blood coagulation. (IFS 2020, 15 Marks)

Introduction
Blood coagulation, also known as blood clotting, is a complex physiological process that prevents excessive bleeding when a blood vessel is injured. It involves a series of steps that result in the formation of a blood clot to seal the wound and stop the bleeding.
 Blood Coagulation
Please refer to IFS 2017 question.
Steps Involved in Blood Coagulation
•    Vasoconstriction (Vascular Spasm)
o    Immediately after injury, the blood vessels constrict (vasoconstriction) to reduce blood flow to the injured area, limiting blood loss.
o    Role: This process is temporary and helps minimize blood loss until the clotting cascade begins.
•    Platelet Plug Formation (Primary Hemostasis)
o    Platelets (thrombocytes) adhere to the exposed collagen fibers at the site of injury.
o    Role: Platelets aggregate and release chemical signals (such as ADP, thromboxane A2) that recruit more platelets to form a temporary "platelet plug" at the site of injury.
•    Activation of Coagulation Cascade
o    The coagulation cascade involves a series of enzymatic reactions where inactive proteins (clotting factors) are activated in a stepwise fashion.
    These proteins include Factor I (Fibrinogen), Factor II (Prothrombin), Factor III (Tissue Factor), Factor IV (Calcium ions), and others.
    The cascade occurs in three stages: intrinsic pathway, extrinsic pathway, and common pathway.
•    Extrinsic Pathway (Tissue Factor Pathway)
o    The extrinsic pathway is initiated by the release of tissue factor (TF) from the damaged tissue. TF binds to Factor VII, activating it to Factor VIIa.
o    Role: Factor VIIa-Tissue Factor complex activates Factor X to Factor Xa in the common pathway.
•    Intrinsic Pathway (Contact Activation Pathway)
o    The intrinsic pathway involves several clotting factors (Factor XII, XI, IX, and VIII) that are activated in response to damage to the blood vessel.
o    Role: Factor IXa, activated in this pathway, forms a complex with Factor VIIIa, which activates Factor X to Factor Xa in the common pathway.
•    Common Pathway
o    Both the extrinsic and intrinsic pathways lead to the activation of Factor X to Factor Xa.
o    Role: Factor Xa, along with Factor V (activated as Factor Va), activates Prothrombin (Factor II) to Thrombin (Factor IIa).
•    Formation of Fibrin Mesh
o    Thrombin (Factor IIa) converts fibrinogen (Factor I) into insoluble fibrin strands.
o    Role: These fibrin strands form a mesh that stabilizes the platelet plug, resulting in the formation of a stable clot.
•    Clot Retraction and Repair
o    Platelets contract to pull the edges of the wound together (clot retraction), which helps to reduce the size of the wound and promotes tissue healing.
o    Role: The clot also helps to stop further blood loss, while the body’s natural healing processes work to repair the damaged vessel.
Conclusion
Blood coagulation is a vital process that ensures the body can effectively respond to injuries and prevent excessive bleeding. The various steps involved in blood coagulation is essential for maintaining hemostasis and preventing complications related to clotting disorders.