coagulation pathway

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Coagulation Pathway

The coagulation pathway is a series of amplifying enzymatic reactions that deposit an insoluble fibrin clot at sites of vascular injury. The cascade is organized around three phases: initiation (extrinsic pathway), amplification (intrinsic pathway), and a common pathway that generates thrombin and fibrin.

Coagulation cascade - laboratory vs. in vivo (Robbins Pathology, Fig. 4.6)

A: Laboratory model showing intrinsic and extrinsic arms. B: In vivo model where tissue factor is the dominant initiator and thrombin feedback loops (dashed lines) amplify the response. Red = inactive factors; Blue = active factors; Green = cofactors.

Core Principles

Every step of the cascade involves four components assembled on a phospholipid membrane surface (provided by activated platelets):

An enzyme (activated coagulation factor - serine protease)
A substrate (inactive zymogen)
A cofactor (reaction accelerator)
Calcium ions (bind to γ-carboxylated glutamic acid residues on factors II, VII, IX, X)

The γ-carboxylation of these Gla residues is vitamin K-dependent. Warfarin (coumadin) blocks this step, inhibiting factors II, VII, IX, and X. - Robbins & Cotran Pathologic Basis of Disease, p. 122

1. Extrinsic Pathway (Initiation)

Trigger: Vascular injury exposes subendothelial tissue factor (TF / Factor III)

Steps:

TF is exposed on subendothelial cells and activated monocytes after injury
Small amounts of circulating Factor VIIa bind to TF to form the extrinsic tenase complex (TF-VIIa-Ca²⁺) on phospholipid membranes
This complex activates Factor X → Xa (directly)
The same TF-VIIa complex also activates Factor IX → IXa (crossover activation - key bridge to the intrinsic pathway)

Lab test: Prothrombin Time (PT) - assesses factors VII, X, V, II, and fibrinogen

Miller's Anesthesia 10e, p. 6715

2. Intrinsic Pathway (Amplification/Contact Activation)

Trigger (in vitro): Negatively charged surfaces (glass, kaolin) activate Factor XII (Hageman factor)
Trigger (in vivo): Thrombin generated by the extrinsic pathway feedback-activates Factor XI; the TF-VIIa complex also directly activates Factor IX

Steps:

XII → XIIa (contact activation, aided by high-molecular-weight kininogen [HK] and prekallikrein [PK])
XIIa → XI → XIa
XIa → IX → IXa
IXa + VIIIa (cofactor) + Ca²⁺ + phospholipid = Intrinsic tenase complex
Intrinsic tenase: Factor X → Xa

Note: Factor XII deficiency does NOT cause clinical bleeding, confirming that the intrinsic pathway is primarily an amplification system in vivo, not the main initiator. Factor XI deficiency causes only mild bleeding. - Miller's Anesthesia 10e, p. 6716

Lab test: aPTT (Partial Thromboplastin Time) - assesses factors XII, XI, IX, VIII, X, V, II, and fibrinogen

3. Common Pathway

Both pathways converge on Factor Xa.

Prothrombinase complex:

Factor Xa + Factor Va (cofactor) + Ca²⁺ + phospholipid membrane
Converts prothrombin (Factor II) → Thrombin (Factor IIa) - the "surge" in thrombin generation

Thrombin actions (the master regulator):

Action	Mechanism
Fibrin clot formation	Cleaves fibrinopeptides A and B from fibrinogen → fibrin monomers → fibrin polymer
Clot stabilization	Activates Factor XIII → XIIIa (transglutaminase) → covalently cross-links fibrin strands → insoluble clot
Amplification feedback	Activates Factors V, VIII, XI → amplifies its own production
Platelet activation	Activates PAR-1 receptors on platelets → potent platelet aggregation
Pro-inflammatory	PAR activation on endothelium/immune cells → angiogenesis, tissue repair

Robbins & Cotran, p. 123; Miller's Anesthesia 10e, p. 6717

Coagulation cascade - detailed (Harper's Illustrated Biochemistry, Fig. 55-2)

Extrinsic Xase (top left), Intrinsic Xase (top right), and Prothrombinase (center) converge on thrombin. Dashed arrows = thrombin feedback loops. HK = high-molecular-weight kininogen; PK = prekallikrein.

Factor Nomenclature Summary

Factor	Name	Role
I	Fibrinogen	Clot substrate
II	Prothrombin	→ Thrombin (IIa)
III	Tissue factor	Initiator cofactor
V	Labile factor	Cofactor in prothrombinase
VII	Proconvertin	Extrinsic pathway
VIII	Antihemophilic A	Cofactor in intrinsic tenase
IX	Christmas factor	Intrinsic pathway
X	Stuart-Prower factor	Common pathway convergence
XI	PTA	Intrinsic amplification
XII	Hageman factor	Contact activation
XIII	Fibrin-stabilizing factor	Cross-links fibrin

4. Natural Anticoagulant Mechanisms

To prevent runaway clot formation, four major systems counterbalance the cascade:

Antithrombin III (ATIII): Inactivates thrombin and other activated serine proteases. Heparin binds ATIII and dramatically accelerates these reactions. Naturally enhanced by heparan sulfate on endothelial surfaces.
Protein C / Protein S system: Thrombin binds thrombomodulin on intact endothelium → activates Protein C. Activated Protein C + Protein S cofactor destroys Factors Va and VIIIa, cutting off the amplification loop.
Tissue Factor Pathway Inhibitor (TFPI): Directly inhibits Factor Xa; the TFPI-Xa complex then inhibits the TF-VIIa complex. This is why the extrinsic pathway is limited early and the intrinsic amplification becomes necessary.
Fibrinolytic system: Plasminogen activators (t-PA, u-PA) from endothelium convert plasminogen → plasmin, which digests fibrin. Regulated by PAI-1 (plasminogen activator inhibitor-1).

Goldman-Cecil Medicine 22e, p. 1799

Clinical Correlations

Deficiency / Drug	Effect	Test Affected
Factor VIII (Hemophilia A)	Severe bleeding	aPTT ↑, PT normal
Factor IX (Hemophilia B)	Severe bleeding	aPTT ↑, PT normal
Factor VII deficiency	Moderate bleeding	PT ↑, aPTT normal
Warfarin	Inhibits II, VII, IX, X, Prot C & S	PT/INR ↑
Heparin	Potentiates ATIII	aPTT ↑
Factor XII deficiency	No clinical bleeding	aPTT ↑
Vitamin K deficiency	Affects II, VII, IX, X	PT ↑ > aPTT ↑
DIC	Consumption of all factors	PT ↑, aPTT ↑, platelets ↓, fibrinogen ↓

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