Mechanism of Blood Clotting

Complete study notes covering the physiological phases of hemostasis, the biochemical coagulation cascade, modern cell-based models, fibrinolysis, and clinical laboratory diagnostics.

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3.1 Definition and Overview

Hemostasis is the physiological process that stops bleeding from a damaged blood vessel. It is a rapid, localized, and carefully regulated process that involves three overlapping phases:

1. Vascular spasm (vasoconstriction)
2. Platelet plug formation (primary hemostasis)
3. Coagulation cascade (secondary hemostasis)
4. Clot stabilization and fibrinolysis

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3.2 Vascular Spasm

This is the immediate reflex constriction of a damaged vessel to reduce blood loss. It is most effective in smaller vessels and is mediated by:

• Local myogenic spasm: Direct damage to vascular smooth muscle triggers contraction.
• Endothelin release: Released from damaged endothelial cells.
• Neural reflexes: Triggered by pain receptors.

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3.3 Platelet Plug Formation (Primary Hemostasis)

This process converts a temporary leak into a mechanical plug through three distinct steps:

Step 1: Adhesion

• Vessel injury exposes subendothelial collagen.
• von Willebrand factor (vWF) binds to the exposed collagen.
• Platelet glycoprotein Ib (GPIb) receptors bind to vWF, causing platelets to adhere to the damaged wall.

Step 2: Activation

• Adhesion triggers a platelet shape change (pseudopod formation).
• Release of granule contents:
  — Dense granules: Release ADP, serotonin, and Ca2+.
  — Alpha granules: Release fibrinogen, factor V, vWF, and PDGF.
• Synthesis and release of thromboxane A2 (TXA2), a potent vasoconstrictor and platelet activator.
• Platelet membrane expresses glycoprotein IIb/IIIa (GPIIb/IIIa) receptors.

Step 3: Aggregation

• GPIIb/IIIa receptors bind fibrinogen, linking platelets together.
• This forms a loose platelet plug (white thrombus), which is later reinforced by fibrin.

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3.4 Coagulation Cascade (Secondary Hemostasis)

Secondary hemostasis involves a series of enzymatic reactions involving plasma proteins called clotting factors. Most factors are synthesized in the liver. Factors II, VII, IX, and X are Vitamin K-dependent.

3.4.1 Clotting Factors

*Note: Factor VI was originally assigned to activated factor V but is no longer used in modern nomenclature.

3.4.2 The Three Pathways

A. Extrinsic Pathway (Tissue Factor Pathway)

• Trigger: Tissue injury exposing Tissue Factor (Factor III) to blood.
• Sequence: Factor III + Factor VII activates Factor X.
• Speed: Fast-acting; generates thrombin within seconds.
• Lab Test: Prothrombin Time (PT).

B. Intrinsic Pathway (Contact Activation Pathway)

• Trigger: Blood exposure to negatively charged subendothelial surfaces.
• Sequence: Factor XII → XI → IX (with VIII as cofactor) → activates Factor X.
• Speed: Slower; requires several minutes.
• Lab Test: Activated Partial Thromboplastin Time (aPTT).

C. Common Pathway

• Both pathways converge at Factor X activation.
• Factor Xa + Va (Prothrombinase complex) + Ca2+ + phospholipids converts Prothrombin (II) to Thrombin (IIa).
• Thrombin converts Fibrinogen (I) to Fibrin monomers.
• Fibrin monomers polymerize into a mesh.
• Factor XIIIa cross-links the fibrin to create a stable, permanent clot.

3.4.3 Cell-Based Model (Modern Understanding)

Modern hematology emphasizes that coagulation occurs in three phases on cell surfaces:

1. Initiation: Tissue factor-bearing cells activate small amounts of X and thrombin.
2. Amplification: Thrombin activates platelets and cofactors (V, VIII, XI).
3. Propagation: Large-scale thrombin generation (thrombin burst) on platelet surfaces produces a massive fibrin clot.

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3.5 Clot Retraction and Repair

• Platelets contain actin and myosin filaments (contractile proteins).
• Clot contraction pulls wound edges together.
• This squeezes serum out of the clot (Serum = Plasma without clotting factors).
• Platelet-Derived Growth Factor (PDGF) stimulates fibroblast migration and tissue repair.

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3.6 Fibrinolysis (Clot Removal)

Fibrinolysis prevents excessive clot propagation and removes the clot once healing has occurred.

Mechanism:

1. Plasminogen (inactive) is incorporated into the clot during formation.
2. Tissue plasminogen activator (t-PA) from endothelial cells converts plasminogen to Plasmin.
3. Plasmin digests fibrin into fibrin degradation products (FDPs).
4. D-dimer is a specific FDP from cross-linked fibrin.

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3.7 Natural Anticoagulant Mechanisms

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3.8 Clinical Laboratory Tests of Coagulation