Hemostasis is the physiological process that stops bleeding at the site of vascular injury. It involves a complex interaction between blood vessels, platelets, and plasma clotting factors. Hemostasis is broadly divided into:
- Primary hemostasis : Formation of a soft platelet plug
- Secondary hemostasis : Formation of a stable fibrin clot by converting fibrinogen into fibrin
Primary Hemostasis
This phase involves:
- Vasoconstriction : Narrowing of blood vessels to reduce blood flow
- Platelet adhesion : Platelets adhere to exposed subendothelial collagen
- Platelet activation : Platelets change shape and release granule contents
- Platelet aggregation : Platelets stick together forming a soft plug
Secondary Hemostasis
- Involves the coagulation cascade , where fibrinogen is converted to fibrin , stabilizing the platelet plug into a hard, insoluble clot.
- This process requires multiple clotting factors working through three interconnected pathways:
- Intrinsic pathway
- Extrinsic pathway
- Common pathway
Coagulation Pathways and Clotting Factors
| Pathway | Clotting Factors Involved | Notes |
|---|---|---|
| Intrinsic | XII (Hageman), XI (Plasma thromboplastin antecedent), IX (Christmas factor), VIII (Anti-hemophilic factor A) | Activated by damage inside vessel; slower pathway |
| Extrinsic | III (Tissue factor), VII (Stable factor) | Activated by external trauma; faster pathway |
| Common | X (Stuart-Prower), V (Labile factor), II (Prothrombin), I (Fibrinogen), XIII (Fibrin-stabilizing factor) | Final pathway producing fibrin clot |
High yield Points
- Factor IV = Calcium ion (Ca²⁺) : Essential cofactor in all three pathways for enzymatic activation
- Many clotting factors are serine proteases , including Factors II, VII, IX, and X, meaning they act as enzymes to cleave peptide bonds in the cascade.
- Most clotting factors are synthesized in the liver hepatocytes , including:
- I (Fibrinogen), II (Prothrombin), V, VII, IX, X, XI, XII, XIII
- Exceptions:
- Factor VIII (Anti-hemophilic factor A) is produced by endothelial cells
- Factor III (Tissue factor or thromboplastin) is expressed by extrinsic tissues and endothelial cells
| Factor | Name/Function | Source | Notes |
|---|---|---|---|
| I | Fibrinogen | Liver | Precursor to fibrin |
| II | Prothrombin | Liver | Converted to thrombin |
| III | Tissue factor (Thromboplastin) | Endothelial cells/tissues | Initiates extrinsic pathway |
| IV | Calcium ion | Dietary/Plasma | Cofactor in coagulation |
| V | Labile factor | Liver | Cofactor in common pathway |
| VII | Stable factor | Liver | Initiates extrinsic pathway |
| VIII | Anti-hemophilic factor A | Endothelial cells | Deficiency → Hemophilia A |
| IX | Christmas factor | Liver | Deficiency → Hemophilia B |
| X | Stuart-Prower factor | Liver | Activation → Common pathway |
| XI | Plasma thromboplastin antecedent | Liver | Intrinsic pathway |
| XII | Hageman factor | Liver | Initiates intrinsic pathway |
| XIII | Fibrin-stabilizing factor | Liver | Cross-links fibrin to stabilize clot |
