Learning Objective

By the end of this article, you should be able to describe the coagulation cascade, distinguish between the extrinsic, intrinsic, and common pathways, explain how clot formation is regulated by anticoagulants and calcium, and recognize the clinical implications of coagulation disorders, including Protein C deficiency, haemophilia, and thrombolysis therapy.

Overview

Coagulation is the physiological process of blood clot formation, an essential component of haemostasis, the body’s response to vascular injury that minimizes bleeding and restores vessel integrity. The process is mediated by a tightly regulated cascade of clotting factors that sequentially activate one another, culminating in the formation of a stable fibrin clot.

Extrinsic Pathway of Coagulation

Triggered by external trauma, causing blood to escape the circulation. Steps:

1. Damage allows Factor VII to leave the bloodstream.
2. Tissue factor (Factor III) is released from damaged cells.
3. Factor VII and Factor III form the TF-VIIa complex.
4. This complex activates Factor X → Factor Xa.
5. Together with Factor Va, Factor Xa converts prothrombin → thrombin.

Clinical Note: The extrinsic pathway provides the initial burst of Factor Xa, while the intrinsic pathway amplifies clot formation.

Intrinsic Pathway of Coagulation

Triggered by internal vessel wall injury. Steps:

Factor XII contacts negatively charged collagen, becoming activated.
Activated factors propagate a cascade involving Factors XI and IX.
Platelets form a primary hemostatic plug and release mediators, including Factor VIII.
Factor IX + Factor VIII complex activates Factor X → Factor Xa, feeding into the common pathway.

Key Point: The intrinsic pathway is slower but amplifies and sustains thrombin generation.

Common Pathway of Coagulation

Convergence of intrinsic and extrinsic pathways.
Factor Xa + Factor Va forms prothrombinase, converting prothrombin → thrombin.
Thrombin converts fibrinogen → fibrin.
Factor XIII stabilizes fibrin, forming a durable clot.

Regulation of Clotting

Protein C & Protein S: Activated by thrombomodulin-thrombin complex, degrade Factors Va and VIIIa, limiting clot growth.
Calcium ions (Ca²⁺): Essential cofactor; low levels inhibit multiple steps.
Antithrombin: A protease inhibitor that degrades thrombin and multiple activated clotting factors; its activity is enhanced by heparin.

Proper regulation prevents excessive clotting and thrombotic disease.

Clinical Relevance

Protein C Deficiency

Can be inherited or acquired (e.g., liver disease, sepsis).
Decreased inhibition of the clotting cascade → predisposition to DVT, PE, stroke.

Fibrinolysis

Clot dissolution restores blood flow.
tPA converts plasminogen → plasmin, which cleaves fibrin.
Endothelial tPA release is slow, providing delayed clot resolution.

Haemophilia

Inherited clotting factor deficiency causing bleeding disorders.
- Type A: Factor VIII deficiency (X-linked)
- Type B: Factor IX deficiency (X-linked)
- Type C: Factor XI deficiency
Severity depends on the degree of factor deficiency.
Therapy: Replacement of the missing factor.

Thrombolysis Therapy

Synthetic tPA is used to dissolve pathological clots in ischemic stroke or myocardial infarction.
Major side effect: increased risk of bleeding elsewhere in the body.