3.2 Hemostasis, Thrombosis & Coagulopathies

Primary vs Secondary Hemostasis

Hemostasis is the coordinated response that stops bleeding while limiting thrombosis to the injury site. Primary hemostasis forms a platelet plug. Secondary hemostasis generates a fibrin mesh that stabilizes that plug. Endothelial antithrombotic surfaces (prostacyclin, nitric oxide, thrombomodulin, heparan sulfate–antithrombin, tPA) normally restrain activation; injury exposes subendothelial collagen and tissue factor and tips the balance toward clot formation.

Bleeding phenotype is a diagnostic clue. Disorders of primary hemostasis (thrombocytopenia, platelet function defects, von Willebrand disease) produce mucocutaneous bleeding: petechiae, purpura, epistaxis, gingival bleeding, menorrhagia, immediate post-cut oozing. Disorders of secondary hemostasis (factor deficiencies such as hemophilia) produce delayed deep bleeding: hemarthroses, muscle hematomas, retroperitoneal bleeds. Mixed disorders (DIC, severe liver disease) can show both patterns.

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Platelet Adhesion and Aggregation Pathways

After vascular injury, circulating von Willebrand factor (vWF) bridges exposed collagen to platelet glycoprotein GPIb-IX-V, tethering platelets at high shear (adhesion). Collagen also engages platelet GPVI and integrins, activating platelets. Activated platelets release dense-granule ADP and alpha-granule contents (including more vWF and fibrinogen) and synthesize thromboxane A2 (TXA2) via cyclooxygenase-1. ADP acts on P2Y12 receptors; TXA2 amplifies activation. Conformational activation of GPIIb/IIIa (integrin αIIbβ3) allows fibrinogen (and vWF) to cross-link adjacent platelets—aggregation.

Bernard-Soulier syndrome: giant platelets, thrombocytopenia, defective GPIb-mediated adhesion. Glanzmann thrombasthenia: normal count, defective aggregation from GPIIb/IIIa failure. Both are primary-hemostasis disorders with mucocutaneous bleeding and usually normal PT/PTT.

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Coagulation Cascade: Intrinsic, Extrinsic, and Common Pathways

The cascade is a series of zymogen-to-serine-protease activations on phospholipid surfaces with calcium. For CBSE mechanism mapping, classic pathway teaching still organizes PT/PTT interpretation even though in vivo initiation is tissue-factor driven with amplification loops.

Extrinsic pathway: Tissue factor (TF) exposed on injured cells binds factor VII; the TF–VIIa complex activates factor X (and IX). Prothrombin time (PT) / INR primarily screens this limb (VII is the shortest-lived vitamin K–dependent factor, so PT prolongs early in warfarin effect or vitamin K deficiency).

Intrinsic pathway: Contact activation involves XII, XI, IX, and VIII (VIIIa is the cofactor for IXa). Partial thromboplastin time (PTT) screens this limb. Factors VIII and IX deficiencies (hemophilia A and B) prolong PTT with normal PT. Factor XII deficiency prolongs PTT but does not cause clinical bleeding—an important discordance.

Common pathway: Factor Xa with cofactor Va converts prothrombin (II) to thrombin (IIa). Thrombin cleaves fibrinogen (I) to fibrin monomers, activates factor XIII (cross-links fibrin), and feeds back to activate V, VIII, and XI—amplification. Severe common-pathway defects prolong both PT and PTT.

Thrombin also has cellular roles: platelet activation via PARs and, when bound to thrombomodulin, activation of protein C, which with protein S proteolytically inactivates Va and VIIIa—natural anticoagulation.

Vitamin K–Dependent Factors

Vitamin K is a cofactor for γ-carboxylation of glutamate residues on factors II, VII, IX, X and anticoagulant proteins C and S. Without γ-carboxylation, these proteins cannot bind calcium/phospholipid surfaces effectively. Dietary deficiency, fat malabsorption, broad-spectrum antibiotics reducing gut vitamin K supply, and warfarin (inhibits vitamin K epoxide reductase, VKORC1) deplete functional carboxylated factors. Because protein C has a short half-life, early warfarin can create a transient hypercoagulable window—especially relevant in protein C deficiency and warfarin-induced skin necrosis concepts.

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Hemophilia vs von Willebrand Disease

Hemophilia A is factor VIII deficiency (X-linked); hemophilia B is factor IX deficiency (X-linked). Both impair intrinsic tenase activity → isolated PTT prolongation, normal PT, normal platelet count, normal bleeding time/PFA in classic pure factor deficiency. Clinical hallmark: hemarthroses and deep hematomas after minor trauma. Severity tracks residual factor activity.

von Willebrand disease (vWD) is the most common inherited bleeding disorder. vWF mediates platelet adhesion and stabilizes circulating factor VIII. Deficiency or dysfunction yields mucocutaneous bleeding; factor VIII may fall secondarily, mildly prolonging PTT in some patients. Platelet count is usually normal (except type 2B, which can have thrombocytopenia). Desmopressin releases endothelial vWF stores in many type 1 patients—a mechanism contrast with hemophilia, where factor replacement (or bypassing agents) is required for significant deficiency.

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DIC and HIT: Conceptual Mechanisms

Disseminated intravascular coagulation (DIC) is systemic activation of coagulation, usually driven by massive tissue factor exposure (sepsis, trauma, obstetric catastrophes, acute promyelocytic leukemia). Widespread thrombin generation consumes platelets and factors, deposits fibrin microthrombi (MAHA with schistocytes), and secondary fibrinolysis raises D-dimer/FDP while fibrinogen falls. Labs: thrombocytopenia, prolonged PT/PTT, low fibrinogen, high D-dimer, schistocytes. The paradox is simultaneous thrombosis and bleeding from consumptive coagulopathy.

Heparin-induced thrombocytopenia (HIT) is an immune adverse effect, not simple marrow suppression. Heparin binds platelet factor 4 (PF4); IgG antibodies against the heparin–PF4 complex engage platelet Fc receptors, causing platelet activation, thrombocytopenia, and a prothrombotic state (arterial or venous clots). Conceptually: falling platelets plus new thrombosis in a heparin-exposed patient. This is the opposite of the bleeding risk intuition that usually accompanies low platelets.

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Antithrombotic Drug Mechanisms (Basic Science Level)

CBSE emphasizes molecular targets rather than clinical dosing nomograms.

Unfractionated heparin binds antithrombin (AT) and accelerates AT-mediated inactivation of thrombin (IIa) and factor Xa (and other serine proteases). A unique pentasaccharide sequence is required for AT activation; longer chains are needed to bridge AT to thrombin, whereas anti-Xa activity needs less chain length—hence low-molecular-weight heparins are more anti-Xa relative to anti-IIa. Heparin’s effect is monitored conceptually with PTT (unfractionated). Protamine can neutralize heparin by charge interaction.

Warfarin inhibits VKORC1, blocking regeneration of reduced vitamin K needed for γ-carboxylation of II, VII, IX, X, C, and S. Onset depends on factor half-lives; factor VII falls first (PT rises early). Genetic variation in VKORC1 and CYP2C9 modulates sensitivity—high-yield pharmacology genetics.

Direct oral anticoagulants (DOACs): Direct factor Xa inhibitors (for example, rivaroxaban, apixaban) bind Xa and block prothrombinase activity. Direct thrombin inhibitors (for example, dabigatran) bind thrombin’s active site, preventing fibrinogen cleavage and thrombin-mediated feedback. They act independently of antithrombin, unlike heparinoids.

Antiplatelet agents (context for primary hemostasis pharmacology): Aspirin irreversibly acetylates COX-1, reducing TXA2 synthesis for the platelet’s lifespan. P2Y12 antagonists block ADP amplification. GPIIb/IIIa antagonists block the final common aggregation step.

Thrombolytics (alteplase and related agents) convert plasminogen to plasmin, which proteolyzes fibrin (and, with systemic activity, fibrinogen). The therapeutic goal is clot lysis; the mechanism-based toxicity is hemorrhage from nonselective fibrinogen/fibrin degradation.

Thrombosis Principles (Brief)

Virchow’s triad—stasis, endothelial injury, hypercoagulability—frames pathologic clotting. Inherited thrombophilias (factor V Leiden resistant to activated protein C, antithrombin deficiency, protein C/S deficiency, prothrombin G20210A) and acquired states (antiphospholipid syndrome, malignancy, HIT) appear as mechanism vignettes. Arterial thrombi are more platelet-rich (white thrombi) at high shear; venous thrombi are fibrin- and red-cell–rich (red thrombi) under stasis—linking back to why antiplatelet therapy is emphasized for arterial disease and anticoagulation for venous thromboembolism in clinical reasoning.

Integrate bleeding cases as: phenotype (mucosal vs deep) → platelet count/function and vWF → PT vs PTT map → fibrinogen/D-dimer if consumption suspected → drug exposure (heparin, warfarin, DOACs, antiplatelets). That sequence mirrors how CBSE items encode hemostasis mechanisms.