Medical School · Year 1 · Cardiovascular · includes a quiz and discussion video

Lecture 10: Hemostasis and Thrombosis

Unit 1.7: Cardiovascular System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the stages of normal hemostasis
2. Explain the role of platelets in primary hemostasis
3. Describe the coagulation cascade and its regulation
4. Explain the fibrinolytic system
5. Describe the pathophysiology of arterial and venous thrombosis
6. Apply hemostasis concepts to clinical scenarios and anticoagulation

Overview of Hemostasis

Hemostasis is the physiological process that stops bleeding at sites of vascular injury while maintaining blood fluidity throughout the rest of the circulation. This carefully balanced system prevents hemorrhage without causing inappropriate thrombosis. When the balance tips toward excessive clotting, thrombotic disease results; when it tips toward insufficient clotting, bleeding disorders occur.

Hemostasis proceeds through three overlapping stages. Primary hemostasis involves the formation of a platelet plug at the site of vascular injury. This initial response occurs within seconds and is mediated by platelets adhering to exposed subendothelial matrix and aggregating together. Secondary hemostasis refers to the coagulation cascade, which generates fibrin to reinforce and stabilize the platelet plug. This process takes several minutes and involves the sequential activation of coagulation factors. Fibrinolysis is the final stage, in which plasmin gradually dissolves the fibrin clot once tissue repair is complete, restoring vascular patency.

Virchow's triad describes the three broad categories of factors that predispose to pathological thrombosis. Endothelial injury disrupts the normally antithrombotic vascular surface and may occur with atherosclerosis, trauma, or inflammation. Stasis allows coagulation factors to accumulate and prevents their dilution and clearance, explaining why venous thrombosis is common in immobilized patients or those with heart failure. Hypercoagulability refers to inherited or acquired conditions that shift the hemostatic balance toward clotting.

<image>Panel A: Vascular injury depicted as a tear in a blood vessel wall exposing subendothelium, initiating the hemostatic cascade. Panel B: Primary hemostasis occurring within seconds, showing platelets adhering to exposed collagen and aggregating into a platelet plug. Panel C: Secondary hemostasis occurring over minutes, showing the platelet plug reinforced by a fibrin mesh forming a stable clot, followed by fibrinolysis with gradual clot dissolution over hours to days. Panel D: Virchow's triad illustrated as three overlapping circles labeled endothelial injury, stasis, and hypercoagulability, with the intersection labeled thrombosis.</image>

Vascular Response

The immediate response to vascular injury is vasoconstriction, which reduces blood flow to the injured site and limits blood loss. This response occurs through multiple mechanisms acting in parallel.

The myogenic response involves direct contraction of vascular smooth muscle in response to mechanical injury. Neural reflexes mediated by sympathetic innervation cause vasoconstriction within seconds of injury. Humoral factors released by activated platelets and damaged endothelium sustain vasoconstriction; these include endothelin-1, a potent vasoconstrictor released from endothelial cells, and thromboxane A₂, released from activated platelets. Together, these mechanisms produce vasoconstriction lasting seconds to minutes, limiting hemorrhage while other hemostatic processes proceed.

The endothelium plays a dual role in hemostasis, functioning as an antithrombotic surface when intact but as a prothrombotic surface when injured. Intact endothelium prevents thrombosis through multiple mechanisms. Nitric oxide and prostacyclin inhibit platelet activation and cause vasodilation. Thrombomodulin on the endothelial surface activates the protein C anticoagulant pathway. Heparan sulfate proteoglycans enhance antithrombin activity. Tissue factor pathway inhibitor limits coagulation activation.

When endothelium is damaged, the prothrombotic subendothelium is exposed. Tissue factor, the primary trigger of coagulation in vivo, becomes accessible to circulating factor VII. von Willebrand factor bound to subendothelial collagen tethers platelets to the injured surface. The injured endothelium releases plasminogen activator inhibitor-1, suppressing fibrinolysis and stabilizing any clots that form.

<image>Panel A: Intact endothelium labeled as antithrombotic surface, with endothelial cells releasing NO and PGI2 to inhibit platelets. Panel B: Thrombomodulin displayed on the endothelial surface and heparan sulfate chains enhancing antithrombin activity. Panel C: Injured endothelium labeled as prothrombotic surface, with denuded endothelium exposing subendothelial collagen and tissue factor becoming accessible. Panel D: Von Willebrand factor bound to collagen with platelets attaching, and PAI-1 being released from the injured endothelium.</image>

Platelets

Platelets are small, anucleate cell fragments derived from bone marrow megakaryocytes. They circulate for 7-10 days before removal by the spleen and liver. Normal platelet counts range from 150,000 to 400,000 per microliter. Thrombopoietin, produced primarily by the liver, regulates platelet production by stimulating megakaryocyte proliferation and maturation.

Resting platelets have a discoid shape that transforms to a stellate form with numerous pseudopods upon activation. This shape change dramatically increases surface area for interaction with other platelets and coagulation factors. Platelets contain two major types of secretory granules. Alpha granules store fibrinogen, von Willebrand factor, factor V, platelet-derived growth factor, and other proteins released upon activation. Dense granules (also called delta granules) contain small molecules including ADP, ATP, calcium, and serotonin. The open canalicular system, an invagination of the plasma membrane, further increases surface area and provides channels for granule release.

Platelet surface receptors mediate adhesion, activation, and aggregation. Glycoprotein Ib/IX/V is the receptor for von Willebrand factor and is essential for platelet adhesion to injured vessel walls under high shear conditions. Glycoprotein IIb/IIIa (also called integrin αIIbβ3) is the fibrinogen receptor and the final common pathway for platelet aggregation. Glycoprotein VI binds collagen directly and contributes to adhesion and activation. P2Y₁₂ is the receptor for ADP and is the target of clopidogrel and related antiplatelet drugs. Thromboxane receptors and protease-activated receptors (PAR-1 and PAR-4) for thrombin complete the major activation pathways.

<image>Panel A: Megakaryocyte in the bone marrow with large multilobed nucleus and extended cytoplasmic processes releasing platelets into a sinusoid. Panel B: Cross-section of a resting platelet showing alpha granules containing fibrinogen, vWF, factor V, and PDGF, and dense granules containing ADP, ATP, calcium, and serotonin. Panel C: Platelet surface glycoprotein receptors labeled including GPIb/IX/V, GPIIb/IIIa, GPVI, P2Y12, thromboxane receptor, and PAR-1/4, with the open canalicular system shown. Panel D: Shape change from discoid to stellate form upon platelet activation, demonstrating increased surface area.</image>

Primary Hemostasis

Primary hemostasis refers to the formation of the platelet plug and involves three sequential steps: adhesion, activation, and aggregation. This process occurs within seconds of vascular injury and provides the initial hemostatic response.

Adhesion is the attachment of platelets to the injured vessel wall. Under normal conditions, platelets do not adhere to intact endothelium. Vascular injury exposes subendothelial collagen to which von Willebrand factor rapidly binds. Von Willebrand factor then captures circulating platelets by binding to the platelet glycoprotein Ib/IX/V receptor. This vWF-GPIb interaction is particularly important under conditions of high shear stress found in arteries, where rapid blood flow would otherwise sweep platelets past the injury site before slower interactions could occur. Direct binding of platelet glycoprotein VI to collagen contributes to adhesion and initiates activation signaling.

Activation triggers platelets to change shape, release granule contents, and become competent for aggregation. Multiple agonists can activate platelets. ADP released from damaged cells and from platelet dense granules acts on P2Y₁ and P2Y₁₂ receptors. Thromboxane A₂, synthesized from arachidonic acid by activated platelets, provides an amplification loop by activating additional platelets. Thrombin, generated by the coagulation cascade, is the most potent platelet agonist, acting through PAR-1 and PAR-4 receptors. Collagen binding to GPVI initiates intracellular signaling cascades. Upon activation, platelets undergo shape change, forming pseudopods that increase surface area, and release their granule contents, which recruit and activate additional platelets—the amplification phase.

Aggregation is the binding of platelets to each other to form the platelet plug. Activation causes a conformational change in the glycoprotein IIb/IIIa receptor, converting it from a low-affinity to a high-affinity state capable of binding fibrinogen. Fibrinogen is a divalent molecule that bridges adjacent platelets by binding to GPIIb/IIIa receptors on each. This GPIIb/IIIa-fibrinogen interaction is the final common pathway of platelet aggregation, regardless of the activating stimulus. At very high shear, von Willebrand factor can substitute for fibrinogen in bridging platelets.

<image>Panel A: Adhesion step showing injured vessel wall with exposed collagen, von Willebrand factor bound to collagen, and a platelet tethered via GPIb/IX/V receptor binding to vWF, with GPVI binding directly to collagen. Panel B: Activation step showing the adherent platelet changed from discoid to stellate shape with extended pseudopods, releasing ADP and ATP from dense granules and fibrinogen and vWF from alpha granules. Panel C: Thromboxane A2 molecules emanating from the activated platelet and additional platelets being recruited to the site of injury. Panel D: Aggregation step showing multiple activated platelets linked by fibrinogen molecules bridging GPIIb/IIIa receptors on adjacent platelets, forming the platelet plug.</image>

Coagulation Cascade

The coagulation cascade is a series of enzymatic reactions that culminates in the generation of thrombin and the formation of a fibrin clot. This process reinforces and stabilizes the initial platelet plug.

The traditional model divides coagulation into extrinsic and intrinsic pathways that converge on a common pathway. The extrinsic pathway is initiated when tissue factor, exposed on damaged cells or activated monocytes, binds circulating factor VII. The resulting tissue factor-factor VIIa complex activates factor X to Xa. The intrinsic pathway begins with contact activation: factor XII becomes activated upon contact with negatively charged surfaces, then sequentially activates factor XI, which activates factor IX. Factor IXa, together with its cofactor factor VIIIa, forms the tenase complex that activates factor X to Xa. The common pathway begins with factor Xa combining with its cofactor factor Va to form the prothrombinase complex, which converts prothrombin (factor II) to thrombin (factor IIa). Thrombin cleaves fibrinogen to fibrin monomers, which spontaneously polymerize. Factor XIIIa, activated by thrombin, cross-links fibrin polymers to create a stable clot.

The cell-based model of coagulation provides a more physiologically accurate understanding. Initiation occurs on tissue factor-bearing cells, where the TF-VIIa complex generates a small amount of thrombin. Amplification occurs when this initial thrombin activates platelets and factors V, VIII, and XI on the platelet surface. Propagation involves large-scale thrombin generation on the activated platelet surface, where the tenase complex (IXa-VIIIa) and prothrombinase complex (Xa-Va) are assembled and protected from plasma inhibitors.

Coagulation factors fall into several functional groups. The vitamin K-dependent factors (II, VII, IX, X, and proteins C and S) undergo gamma-carboxylation, which enables them to bind calcium and phospholipid surfaces. Contact factors (XII, XI, high-molecular-weight kininogen, and prekallikrein) initiate the intrinsic pathway. Cofactors (V, VIII, and tissue factor) accelerate enzymatic reactions but have no enzymatic activity themselves. Fibrinogen and factor XIII form the structural components of the clot.

<image>Panel A: Extrinsic pathway showing tissue factor and factor VII forming the TF-VIIa complex that activates factor X, with calcium ions required at multiple steps. Panel B: Intrinsic pathway showing factor XII through XIIa, XI, XIa, IX, and IXa, with IXa and VIIIa forming the tenase complex that activates factor X. Panel C: Common pathway showing factor Xa and Va forming the prothrombinase complex, converting prothrombin to thrombin, then thrombin converting fibrinogen to cross-linked fibrin via factor XIIIa. Panel D: Cell-based model of coagulation showing three phases: initiation on TF-bearing cells, amplification with thrombin activating platelets and cofactors, and propagation with burst thrombin generation on the platelet surface.</image>

Coagulation Factors and Laboratory Tests

Thrombin occupies a central position in coagulation, with multiple critical functions. Its primary role is converting fibrinogen to fibrin. Thrombin also amplifies its own generation by activating factors V, VIII, and XI. It is a potent platelet agonist, acting through PAR receptors. Thrombin activates factor XIII, which stabilizes the fibrin clot. Paradoxically, thrombin also initiates its own regulation by activating protein C when bound to thrombomodulin on endothelium.

Standard laboratory tests assess different portions of the coagulation cascade. The prothrombin time (PT) measures the extrinsic and common pathways by adding tissue factor and calcium to citrated plasma and measuring time to clot formation; normal values are 11-15 seconds. The international normalized ratio (INR) standardizes PT results across laboratories and is used to monitor warfarin therapy, with a normal value of 1.0. The activated partial thromboplastin time (aPTT) measures the intrinsic and common pathways by adding a contact activator (such as kaolin), phospholipid, and calcium; normal values are 25-35 seconds. The thrombin time measures the final step of converting fibrinogen to fibrin by adding thrombin to plasma; normal values are 14-19 seconds, and prolongation suggests low fibrinogen, abnormal fibrinogen, or the presence of thrombin inhibitors. Direct fibrinogen levels can be measured; normal values are 200-400 mg/dL.

<image>Panel A: Simplified coagulation cascade with the extrinsic pathway (TF, VII) in blue and the intrinsic pathway (XII, XI, IX, VIII) in green. Panel B: Common pathway (X, V, II, I, XIII) in red, showing converging point of both pathways. Panel C: Overlaid brackets indicating which factors each laboratory test measures: PT covering extrinsic plus common pathways, aPTT covering intrinsic plus common pathways, and thrombin time covering fibrinogen alone. Panel D: Reference table listing normal ranges for each test: PT 11-15 sec, INR 1.0, aPTT 25-35 sec, TT 14-19 sec, fibrinogen 200-400 mg/dL.</image>

Anticoagulant Mechanisms

The body possesses multiple natural anticoagulant systems that confine coagulation to the site of injury and prevent disseminated intravascular clotting.

Antithrombin is the primary plasma anticoagulant, a serine protease inhibitor (serpin) that irreversibly inhibits thrombin, factors IXa, Xa, and XIa. Antithrombin acts slowly on its own but becomes approximately 1000-fold more potent when bound to heparin or heparan sulfate on endothelial surfaces. This acceleration explains the therapeutic efficacy of heparin anticoagulation. Inherited antithrombin deficiency is a significant thrombophilia causing recurrent venous thromboembolism.

The protein C system provides feedback inhibition of coagulation. When thrombin binds to thrombomodulin on the endothelial surface, it undergoes a substrate specificity switch: rather than cleaving fibrinogen, thrombin-thrombomodulin activates protein C. Activated protein C, in partnership with its cofactor protein S, proteolytically inactivates factors Va and VIIIa, shutting down the tenase and prothrombinase complexes. Factor V Leiden is a mutation that renders factor Va resistant to inactivation by activated protein C and is the most common inherited thrombophilia.

Tissue factor pathway inhibitor (TFPI) regulates the initiation of coagulation by inhibiting the tissue factor-factor VIIa complex and factor Xa. TFPI is bound to endothelium and is released into plasma where it provides a threshold that must be exceeded before coagulation can propagate.

The intact endothelium provides a non-thrombogenic surface through multiple mechanisms. Nitric oxide and prostacyclin inhibit platelet activation. Heparan sulfate proteoglycans activate antithrombin. Thrombomodulin activates the protein C pathway. ADPase (CD39) degrades ADP released from activated platelets.

<image>Panel A: Blood vessel with endothelium and flowing blood, showing antithrombin as a serpin molecule inhibiting thrombin, with heparan sulfate chains on endothelium enhancing its activity 1000-fold. Panel B: Protein C pathway showing thrombin bound to thrombomodulin on endothelium activating protein C, then activated protein C with protein S inactivating factors Va and VIIIa. Panel C: Tissue factor pathway inhibitor binding and inhibiting the TF-VIIa complex and factor Xa. Panel D: Endothelial surface antithrombotic features including NO and PGI2 inhibiting platelets, heparan sulfate, thrombomodulin, and CD39 degrading ADP.</image>

Fibrinolysis

Fibrinolysis is the enzymatic dissolution of fibrin clots, essential for restoring vascular patency after tissue repair is complete.

Plasminogen is the inactive precursor that is converted to plasmin, the active fibrinolytic enzyme. Plasminogen binds to fibrin as clots form, localizing the fibrinolytic machinery to the clot. Tissue plasminogen activator (tPA), released from endothelial cells, is the primary activator of plasminogen. Importantly, tPA activity increases dramatically when bound to fibrin, ensuring that plasmin generation occurs primarily within clots rather than in circulating plasma. Urokinase plasminogen activator (uPA) contributes to fibrinolysis, particularly in the extravascular space.

Plasmin cleaves fibrin into soluble degradation products. D-dimer is a specific fragment released when cross-linked fibrin is degraded. Because D-dimer requires prior fibrin formation and cross-linking, elevated plasma D-dimer indicates active fibrinolysis of recent thrombi and is clinically useful for excluding venous thromboembolism in patients with low pretest probability.

The fibrinolytic system is tightly regulated to prevent excessive clot dissolution. Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor of both tPA and uPA. PAI-1 levels increase in obesity, insulin resistance, and inflammation, potentially contributing to the thrombotic risk in these conditions. Alpha₂-antiplasmin rapidly inactivates any plasmin that escapes into the circulation. Thrombin-activatable fibrinolysis inhibitor (TAFI) removes plasminogen binding sites from fibrin, reducing the efficiency of fibrinolysis.

Fibrinolytic therapy exploits this system to dissolve pathological thrombi. Alteplase (recombinant tPA) is used in acute myocardial infarction, ischemic stroke (within the therapeutic window), and massive pulmonary embolism. Tenecteplase, a modified tPA with longer half-life, is used in myocardial infarction. Antifibrinolytic drugs including tranexamic acid and aminocaproic acid inhibit plasminogen activation and are used to control bleeding in trauma, surgery, and menorrhagia.

<image>Panel A: Fibrin clot within a blood vessel with tPA molecules released from endothelial cells binding to the fibrin surface and converting plasminogen to plasmin. Panel B: Plasmin cleaving fibrin strands and releasing D-dimer fragments as specific fibrin degradation products. Panel C: Regulatory mechanisms including PAI-1 inhibiting tPA, alpha-2-antiplasmin inhibiting circulating plasmin, and TAFI removing plasminogen binding sites from fibrin. Panel D: Clinical applications showing alteplase and tenecteplase for thrombolysis and tranexamic acid for bleeding control.</image>

Pathological Thrombosis

Arterial and venous thromboses differ fundamentally in their pathophysiology, composition, and treatment.

Arterial thrombosis occurs in high-flow, high-shear vessels and is typically triggered by atherosclerotic plaque rupture or erosion. The exposed thrombogenic core activates platelets, which form the bulk of the arterial thrombus—hence the description "white clot." Fibrin contributes to clot stability, but platelets predominate. Clinical manifestations include myocardial infarction and ischemic stroke. Because platelets are central to arterial thrombosis, antiplatelet agents are the cornerstone of prevention and treatment.

Venous thrombosis occurs in low-flow, low-shear vessels where stasis allows coagulation factors to accumulate without dilution or clearance. Hypercoagulability and endothelial dysfunction contribute, but stasis is often the dominant factor. Venous thrombi are rich in fibrin and trapped red blood cells—the "red clot." Clinical manifestations include deep venous thrombosis and pulmonary embolism. Because coagulation predominates over platelet activation, anticoagulants rather than antiplatelet agents are the primary therapy.

Inherited thrombophilias increase the risk of venous thromboembolism. Factor V Leiden, a mutation causing resistance to activated protein C, is the most common inherited thrombophilia in Caucasian populations, increasing VTE risk 3-8 fold in heterozygotes. Prothrombin G20210A mutation increases prothrombin levels and confers a 2-4 fold VTE risk. Antithrombin, protein C, and protein S deficiencies are less common but confer higher risk (10-50 fold, 7-10 fold, and 5-10 fold, respectively).

Acquired thrombophilias include malignancy (cancer cells express tissue factor and create a hypercoagulable state), antiphospholipid syndrome (antibodies against phospholipids cause both venous and arterial thrombosis), surgery and trauma (tissue factor release and immobility), pregnancy (increased coagulation factor levels), and estrogen therapy (increased clotting factor synthesis).

<image>Panel A: Arterial thrombosis in a high-flow artery with atherosclerotic plaque rupture exposing a lipid core and tissue factor, triggering platelet aggregation forming a white thrombus rich in platelets. Panel B: Venous thrombosis in a low-flow vein with stagnant blood, showing a red thrombus rich in fibrin mesh entrapping red blood cells. Panel C: Treatment comparison showing antiplatelet drugs for arterial thrombosis and anticoagulants for venous thrombosis. Panel D: Table of inherited thrombophilias with relative risks: Factor V Leiden 3-8x, Prothrombin G20210A 2-4x, AT deficiency 10-50x, Protein C deficiency 7-10x, Protein S deficiency 5-10x.</image>

Antithrombotic Therapy

Antiplatelet agents target primary hemostasis and are used primarily for arterial thrombotic disease. Aspirin irreversibly acetylates cyclooxygenase-1, blocking thromboxane A₂ synthesis for the life of the platelet. Aspirin is the cornerstone of antiplatelet therapy for coronary artery disease and secondary stroke prevention. P2Y₁₂ inhibitors block ADP-mediated platelet activation. Clopidogrel is a prodrug requiring hepatic activation and is widely used. Prasugrel is a more potent P2Y₁₂ inhibitor with more consistent antiplatelet effect. Ticagrelor is a reversible P2Y₁₂ inhibitor with rapid onset and offset. Glycoprotein IIb/IIIa inhibitors (abciximab, eptifibatide, tirofiban) block the final common pathway of platelet aggregation and are used in acute coronary syndromes and percutaneous coronary intervention.

Anticoagulants target the coagulation cascade. Unfractionated heparin binds antithrombin and accelerates its inhibition of thrombin, factor Xa, and other coagulation factors. It requires aPTT monitoring and has unpredictable pharmacokinetics. Low-molecular-weight heparins (enoxaparin, dalteparin) provide more predictable anticoagulation, primarily inhibit factor Xa, and can be given subcutaneously without routine monitoring. Warfarin inhibits vitamin K epoxide reductase, depleting vitamin K-dependent factors (II, VII, IX, X) over several days. Warfarin requires INR monitoring and has numerous drug and food interactions. Direct oral anticoagulants offer more predictable anticoagulation without routine monitoring. Dabigatran is a direct thrombin inhibitor. Rivaroxaban, apixaban, and edoxaban are direct factor Xa inhibitors.

Fibrinolytics are used for emergency dissolution of life-threatening thrombi. Alteplase (recombinant tPA) is used for STEMI when primary PCI is unavailable, massive pulmonary embolism causing hemodynamic instability, and acute ischemic stroke within the therapeutic window. Contraindications include recent surgery, active bleeding, prior intracranial hemorrhage, and uncontrolled hypertension.

Antifibrinolytic agents control bleeding by inhibiting plasminogen activation. Tranexamic acid is used in trauma, surgery, postpartum hemorrhage, and menorrhagia. Aminocaproic acid has similar applications.

<image>Panel A: Antiplatelet drugs at the platelet level showing aspirin blocking COX-1 and TXA2 synthesis, P2Y12 inhibitors blocking the ADP receptor, and GPIIb/IIIa inhibitors blocking the fibrinogen receptor. Panel B: Anticoagulants at the coagulation cascade showing heparins enhancing antithrombin, warfarin reducing vitamin K-dependent factor synthesis, direct thrombin inhibitors blocking thrombin, and direct Xa inhibitors blocking factor Xa. Panel C: Fibrinolytic agents showing alteplase and tenecteplase activating plasminogen for clot dissolution, and tranexamic acid inhibiting plasminogen activation. Panel D: Clinical indications for each drug class color-coded by therapeutic category.</image>

Summary

Hemostasis proceeds through three stages: primary hemostasis (platelet plug formation), secondary hemostasis (fibrin clot formation), and fibrinolysis (clot dissolution). Each stage is tightly regulated to confine clotting to the injury site.

Primary hemostasis involves platelet adhesion to exposed subendothelium via von Willebrand factor and GPIb, activation by multiple agonists including ADP and thromboxane A₂, and aggregation through fibrinogen bridging GPIIb/IIIa receptors on adjacent platelets.

The coagulation cascade comprises the extrinsic pathway (triggered by tissue factor), intrinsic pathway (contact activation), and common pathway, all converging on thrombin generation. Thrombin converts fibrinogen to fibrin and amplifies its own generation through positive feedback.

Natural anticoagulants—antithrombin, the protein C/S system, and tissue factor pathway inhibitor—maintain hemostatic balance. Fibrinolysis mediated by plasmin dissolves clots after tissue repair; D-dimer is a specific marker of fibrin degradation.

Arterial thrombosis is platelet-rich and treated with antiplatelet agents. Venous thrombosis is fibrin-rich and treated with anticoagulants. Inherited and acquired thrombophilias increase venous thromboembolism risk.

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown