5.3 Hemostasis & Blood Coagulation
Key Takeaways
- Primary hemostasis forms a platelet plug through adhesion (via von Willebrand factor), activation, and aggregation.
- Secondary hemostasis is the coagulation cascade: intrinsic (XII→XI→IX→VIII), extrinsic (VII), and common (X→II→I) pathways produce fibrin.
- Fibrinolysis dissolves clots via plasmin, generated from plasminogen by tissue plasminogen activator (tPA).
- Vitamin K is required for synthesis of factors II, VII, IX, and X (and proteins C and S); warfarin inhibits vitamin K epoxide reductase.
- Disorders include hemophilia A (factor VIII), hemophilia B (factor IX), von Willebrand disease (vWF), and DIC (widespread consumption).
Overview of Hemostasis
Quick Answer: Hemostasis has two phases: primary (platelet plug) and secondary (coagulation cascade forming fibrin). A third process, fibrinolysis, removes the clot once healing begins. The PA-CAT Bulletin of Information, rev. 20240815, includes "Blood and Clotting" in the Physiology blueprint.
Primary Hemostasis: The Platelet Plug
Vascular injury exposes subendothelial collagen and von Willebrand factor (vWF). Platelets adhere via the glycoprotein Ib-IX-V complex binding vWF, and via GPVI binding collagen. Adhesion triggers platelet activation, which causes:
- Shape change (discoid → stellate, via cytoskeletal reorganization).
- Degranulation — α-granules release vWF, fibrinogen, factor V; dense granules release ADP, serotonin, calcium.
- Thromboxane A₂ (TXA₂) synthesis from arachidonic acid via cyclooxygenase (COX) — aspirin irreversibly inhibits COX-1 in platelets.
- GPIIb/IIIa receptor activation — binds fibrinogen, linking platelets into a plug (aggregation).
Defects in primary hemostasis cause mucocutaneous bleeding (petechiae, epistaxis, gum bleeding). Von Willebrand disease is the most common inherited bleeding disorder; Glanzmann thrombasthenia is GPIIb/IIIa deficiency; Bernard-Soulier syndrome is GPIb deficiency.
Secondary Hemostasis: The Coagulation Cascade
The cascade is a chain of serine protease zymogens activated by limited proteolysis, amplifying the signal to generate thrombin (factor IIa), which converts fibrinogen (factor I) to fibrin (factor Ia). Factor XIIIa then cross-links fibrin for stability.
Intrinsic pathway (contact activation; assessed by aPTT):
- XII → XIIa → XI → XIa → IX → IXa (with VIIIa as cofactor) → X → Xa
Extrinsic pathway (tissue factor; assessed by PT):
- Tissue factor (thromboplastin) + VIIa → X → Xa
Common pathway (assessed by both PT and aPTT):
- Xa (with Va cofactor, on phospholipid surface) → prothrombin (II) → thrombin (IIa) → fibrinogen (I) → fibrin (Ia)
Most coagulation factors are synthesized in the liver. Factors II, VII, IX, X (and the anticoagulants protein C and protein S) require vitamin K as a cofactor for γ-carboxylation, which allows them to bind calcium and anchor to phospholipid surfaces. Warfarin inhibits vitamin K epoxide reductase, blocking this recycling; factor VII has the shortest half-life, so PT (INR) rises first.
Laboratory Tests
| Test | Measures | Pathway |
|---|---|---|
| PT / INR | Extrinsic + common | VII, X, V, II, I |
| aPTT | Intrinsic + common | XII, XI, IX, VIII, X, V, II, I |
| Bleeding time | Primary hemostasis | Platelet function |
| Thrombin time | Fibrinogen → fibrin conversion | Fibrinogen quality |
| D-dimer | Cross-linked fibrin degradation | Fibrinolysis / clot presence |
Fibrinolysis
Once the vessel heals, the clot must be removed. Tissue plasminogen activator (tPA) converts plasminogen to plasmin, which degrades fibrin into D-dimer and other fragments. α₂-antiplasmin limits plasmin activity. Therapeutic tPA (alteplase) is used in acute ischemic stroke and myocardial infarction; it must be given within defined windows (e.g., 3–4.5 hours for stroke).
Anticoagulant and Antiplatelet Drugs
- Heparin potentiates antithrombin III, which inactivates thrombin and factor Xa. Low-molecular-weight heparins (enoxaparin) preferentially inhibit Xa.
- Warfarin inhibits vitamin K epoxide reductase (oral, delayed onset).
- Direct oral anticoagulants (DOACs) — dabigatran (direct thrombin inhibitor), rivaroxaban/apixaban (direct Xa inhibitors).
- Aspirin irreversibly inhibits COX-1, blocking TXA₂.
- Clopidogrel inhibits the P2Y₁₂ ADP receptor.
- Abciximab blocks GPIIb/IIIa.
Clinical Bleeding Disorders
| Disorder | Defect | Lab Pattern |
|---|---|---|
| Hemophilia A | Factor VIII deficiency | ↑ aPTT, normal PT |
| Hemophilia B | Factor IX deficiency | ↑ aPTT, normal PT |
| Von Willebrand disease | vWF deficiency (qualitative or quantitative) | ↑ bleeding time, may ↑ aPTT |
| DIC | Widespread activation consuming factors and platelets | ↑ PT, ↑ aPTT, ↓ platelets, ↑ D-dimer |
| Liver disease | Reduced factor synthesis (all except VIII) | ↑ PT, ↑ aPTT |
| Vitamin K deficiency | Reduced II, VII, IX, X | ↑ PT early, ↑ aPTT late |
Hemophilia causes deep tissue and joint bleeding (hemarthrosis), unlike the mucocutaneous pattern of platelet disorders — a classic PA-CAT-style distinction.
Regulation and Inhibitors
The cascade is held in check by natural anticoagulants: antithrombin III (inhibits thrombin, Xa, IXa), protein C and protein S (inactivate factors Va and VIIIa), and tissue factor pathway inhibitor (TFPI). Protein C deficiency or factor V Leiden (resistant to activated protein C) causes thrombophilia — increased risk of venous thromboembolism. This regulation explains why clots form only at injury sites and not throughout the vasculature.
Thrombin's Dual Role: Procoagulant and Anticoagulant Functions via Thrombomodulin
A recurring PA-CAT physiology trap treats thrombin purely as a procoagulant enzyme that converts fibrinogen to fibrin. In reality, thrombin is a bifunctional switch whose downstream effect depends on which receptor and cofactor it encounters. When thrombin binds thrombomodulin on intact endothelial cells, its active site is reoriented so that it no longer cleaves fibrinogen or activates platelets. Instead, the thrombin-thrombomodulin complex activates protein C, which complexes with endothelial protein C receptor and protein S to proteolytically inactivate factors Va and VIIIa on the same phospholipid surface where the common and intrinsic tenase complexes assemble. This is why protein C deficiency and factor V Leiden (a factor V variant resistant to activated protein C cleavage) produce thrombophilia rather than bleeding: the natural brake on cofactor amplification is lost.
The clinical implication students often miss is that warfarin initiation transiently worsens thrombosis risk. Because protein C has a short half-life similar to factor VII, the anticoagulant protein C falls early while procoagulant factors II, IX, and X decline more slowly. This 'procoagulant window' explains why bridging with heparin is required in patients starting warfarin for acute thrombosis, and it is the mechanism behind warfarin-induced skin necrosis in undiagnosed protein C deficiency.
Thrombin Actions Sorted by Target
| Thrombin target | Location | Effect | Net result |
|---|---|---|---|
| Fibrinogen (factor I) | Plasma | Cleaves fibrinopeptides A and B | Procoagulant (fibrin mesh) |
| Factor V, VIII, XI, XIII | Plasma/surface | Cofactor and zymogen activation | Procoagulant (amplification) |
| PAR-1 and PAR-4 on platelets | Platelet membrane | Platelet activation and aggregation | Procoagulant (plug growth) |
| Thrombomodulin | Intact endothelium | Binds thrombin, shifts active site | Anticoagulant (blocks fibrinogen cleavage) |
| Protein C (via thrombomodulin) | Endothelial surface | Generates activated protein C | Anticoagulant (degrades Va and VIIIa) |
This dual-role framework resolves a classic exam question: why does a single enzyme both build and dismantle the cascade? The answer is location, cofactor availability, and receptor context, not enzyme identity alone.
A 6-year-old boy has recurrent hemarthrosis of the knees. Labs show prolonged aPTT with normal PT and normal platelet count. Which factor is most likely deficient?
Warfarin exerts its anticoagulant effect by inhibiting which enzyme, and which factor's decline explains the early rise in PT/INR?
Which laboratory pattern is most characteristic of disseminated intravascular coagulation (DIC)?