Blood Functions and Coagulation Process

Secondary Hemostasis

• Secondary hemostasis, also known as coagulation, reinforces the unstable platelet plug with fibrin.
• It involves a series of enzyme-mediated reactions culminating in the formation of thrombin.
• Thrombin converts the soluble protein fibrinogen to insoluble fibrin, stabilizing the platelet plug.
• Fibrin formation is tightly controlled, limiting it to the site of injury, preventing widespread clotting.

Primary vs. Secondary Hemostasis

• Primary hemostasis involves blood vessels and platelets in forming an initial platelet plug in response to vascular injury.
• Secondary hemostasis (coagulation) reinforces the platelet plug with fibrin.

Hemostatic Process

• The process maintains blood fluidity under normal conditions.
• It forms clots when needed.
• It dissolves clots after healing.
• It maintains vascular integrity.

Coagulation Mechanism

• The coagulation cascade involves the activation of inactive coagulation factor precursors (zymogens) to their active enzyme forms.
• Each zymogen first acts as a substrate for the preceding enzyme and then as an enzyme for the subsequent zymogen.

Coagulation Cascade

• The cascade has two initial pathways:
  • Contact activation pathway (intrinsic pathway): Relies on factors present in plasma.
  • Tissue factor pathway (extrinsic pathway): Requires factors in plasma and a tissue factor activator.
• Both pathways converge on the common pathway to form a fibrin clot.

Coagulation Cascade Complexes

• Extrinsic tenase: Consists of Tissue Factor, Factor VIIa, phospholipids, and calcium. It activates Factor X.
• Intrinsic tenase: Consists of Factor IXa, Factor VIIIa, phospholipids, and calcium. It activates Factor X.
• Prothrombinase: Consists of Factor Xa, Factor Va, phospholipids, and calcium. It converts prothrombin to thrombin.

Factor IX Activation

• Factor IXa, in the presence of calcium, activates Factor X, in the intrinsic pathway.
• Factor VIIa/Tissue Factor also activates Factor IX, bypassing the contact activation pathway.

Factor VIII

• Factor VIII is synthesized in the liver and circulates as a complex with von Willebrand factor (VWF).
• The complex protects Factor VIII from degradation, extending its circulatory time.
• Thrombin activates Factor VIII, releasing it from VWF to serve as a cofactor for Factor IXa.

The Common Pathway

• The intrinsic and extrinsic pathways converge on the common pathway, activating Factor X.
• The prothrombinase complex activates prothrombin to thrombin.
• Thrombin cleaves fibrinogen to fibrin.
• Factor XIIIa cross-links fibrin to form an insoluble clot.

Fibrinogen

• Fibrinogen is a plasma protein found in alpha granules of platelets.
• It consists of three polypeptide chains (alpha, beta, gamma) linked by disulfide bonds.
• The chains fold to form a tri-nodular structure with central (E) and outer (D) nodules.

Factor XIII

• Factor XIIIa stabilizes the fibrin clot by cross-linking fibrin monomers via transglutaminase activity.
• It also cross-links fibrinolytic inhibitors (like PAI-1 and TAFI) to the fibrin clot, preventing premature clot degradation.

Thrombin Release

• Prothrombin is a vitamin K-dependent factor that binds to the negatively charged phospholipids of activated platelets.
• Thrombin is formed by the presence of prothrombinase complex. It does not bind to negatively charged phospholipids.

Role of Thrombin

• Thrombin cleaves fibrinogen peptides to form fibrin monomers.
• Thrombin stimulates platelet activation and aggregation.
• Thrombin activates clotting factors (Factor V, VIII, XI) generating more thrombin in a cascade.
• Thrombin activates Factor XIII resulting in cross-linking of fibrin monomers to form a stable fibrin clot.
• Thrombin stimulates endothelium to release tPA to initiate fibrinolysis.

Other Functions of Thrombin

• Thrombin acts as a chemo-attractant for neutrophils, stimulating inflammation.
• Thrombin is mitogenic for fibroblasts.
• Thrombin also has anti-thrombotic functions, promoting wound healing by activating protein C and fibrinolysis.

Tissue Factor

• Tissue factor (TF) is a transmembrane lipoprotein.
• TF is expressed predominantly on non-vascular cells.
• TF is a receptor and cofactor for Factor VII and VIIa.
• TF and activated platelets attract calcium to form procoagulant complexes, thus localizing the reactions at the site of injury.

Extrinsic Pathway

• Initiated by tissue factor (TF) exposed to blood.
• TF binds to activated Factor VII (VIIa) forming the extrinsic tenase complex.
• The complex activates Factor X to Xa, initiating the common pathway.

Intrinsic Pathway

• Initiated by contact with negatively charged surfaces (in vitro).
• Activation leads to formation of primary complex of factors XII, XI, PK, and HK, converting PK to Kallikrein, and activating factor XII.
• Converts factor XI to XIa, activates FIX, and by activating factor IX and VIII together with calcium and phospholipids the intrinsic pathway will activate factor X.