⚡Plasma Medicine Unit 8 – Blood coagulation and hemostasis

Key Concepts and Definitions

• Hemostasis involves a complex interplay of blood vessels, platelets, and coagulation factors to stop bleeding and maintain blood flow
• Coagulation is the process by which blood forms clots to prevent blood loss from damaged blood vessels
• Fibrinolysis breaks down fibrin clots once the damaged vessel has been repaired to restore normal blood flow
• Thrombosis occurs when a clot forms inappropriately inside a blood vessel, obstructing blood flow (venous thrombosis, arterial thrombosis)
• Embolism happens when a clot or other material (fat, air, tumor) travels through the bloodstream and lodges in a distant blood vessel, blocking blood flow (pulmonary embolism)
• Disseminated intravascular coagulation (DIC) is a systemic activation of the coagulation cascade, leading to widespread clotting and consumption of clotting factors
• Anticoagulants are substances that prevent or delay blood clotting, either naturally occurring (antithrombin) or administered therapeutically (heparin, warfarin)

Blood Components and Their Roles

• Platelets, also known as thrombocytes, are small, disc-shaped cell fragments that play a crucial role in hemostasis by adhering to damaged blood vessels and forming a platelet plug
• Red blood cells (erythrocytes) transport oxygen to tissues and help maintain blood viscosity, which is essential for proper blood flow and hemostasis
• White blood cells (leukocytes) are involved in the immune response and can release cytokines that influence coagulation and inflammation
• Plasma is the liquid component of blood, containing water, electrolytes, proteins (albumin, globulins, fibrinogen), and clotting factors
  • Fibrinogen is a soluble plasma protein that is converted into insoluble fibrin during the coagulation process, forming the backbone of the clot
• Endothelial cells line the inner surface of blood vessels and play a key role in regulating hemostasis by producing anticoagulant and procoagulant factors, as well as maintaining a non-thrombogenic surface

Stages of Hemostasis

• Vascular spasm is the immediate constriction of the damaged blood vessel to reduce blood flow and limit blood loss
• Platelet adhesion occurs when platelets bind to exposed collagen and other subendothelial proteins at the site of injury via specific receptors (glycoprotein Ib/IX/V complex, integrin α2β1)
• Platelet activation follows adhesion, causing platelets to change shape, release granule contents (ADP, serotonin, thromboxane A2), and express surface receptors (glycoprotein IIb/IIIa) that promote platelet aggregation
• Platelet aggregation involves the cross-linking of activated platelets via fibrinogen and other adhesive proteins to form a stable platelet plug
• Coagulation cascade is a series of enzymatic reactions that generate thrombin, which converts fibrinogen to fibrin, stabilizing the platelet plug and forming a firm clot
  • The cascade is divided into the extrinsic pathway (tissue factor-dependent) and the intrinsic pathway (contact activation-dependent), which converge on the common pathway
• Fibrinolysis is the process of clot dissolution, mediated by the enzyme plasmin, which cleaves fibrin into soluble fragments (D-dimer) and restores blood flow once the vessel has healed

Coagulation Cascade

• The extrinsic pathway is initiated by tissue factor (TF), a membrane-bound protein expressed by subendothelial cells and activated monocytes/macrophages upon injury
  • TF binds and activates factor VII (FVIIa), forming the TF-FVIIa complex, which activates factors IX and X
• The intrinsic pathway is initiated by the contact activation system, involving high molecular weight kininogen (HMWK), prekallikrein, and factor XII (FXII)
  • FXII is activated upon contact with negatively charged surfaces, leading to the activation of FXI, which in turn activates FIX
• The common pathway begins with the activation of factor X (FXa) by either the extrinsic (TF-FVIIa) or intrinsic (FIXa-FVIIIa) pathways
  • FXa forms the prothrombinase complex with factor Va, calcium, and phospholipids, which efficiently converts prothrombin (FII) to thrombin (FIIa)
• Thrombin is a key enzyme in the coagulation cascade, with multiple functions:
  • Converting fibrinogen to fibrin monomers, which polymerize to form the clot
  • Activating platelets and factors V, VIII, XI, and XIII, amplifying the coagulation response
  • Activating thrombin-activatable fibrinolysis inhibitor (TAFI), which helps stabilize the clot by inhibiting fibrinolysis
• Factor XIII, activated by thrombin, cross-links fibrin monomers to form a stable, insoluble fibrin clot

Clotting Factors and Inhibitors

• Clotting factors are designated by Roman numerals (I-XIII) and are mostly synthesized in the liver
  • They circulate as inactive zymogens and are activated through proteolytic cleavage during the coagulation cascade
• Vitamin K-dependent factors include factors II (prothrombin), VII, IX, and X, as well as the anticoagulant proteins C and S
  • Vitamin K is essential for the post-translational modification (γ-carboxylation) of these factors, which is necessary for their calcium-binding and biological activity
• Tissue factor pathway inhibitor (TFPI) is a key regulator of the extrinsic pathway, inhibiting the TF-FVIIa complex and early FXa generation
• Antithrombin is the primary physiological inhibitor of thrombin and other coagulation factors (Xa, IXa, XIa, XIIa)
  • Its activity is greatly enhanced by heparin and heparan sulfate proteoglycans on the endothelial surface
• Protein C, when activated by thrombin-thrombomodulin complex, inactivates factors Va and VIIIa, limiting the generation of thrombin
  • Protein S serves as a cofactor for activated protein C (APC)
• Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor of tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA), regulating fibrinolysis

Disorders of Hemostasis

• Hemophilia A is an X-linked recessive disorder caused by a deficiency in factor VIII, leading to impaired intrinsic pathway function and prolonged bleeding
• Hemophilia B, also known as Christmas disease, is an X-linked recessive disorder caused by a deficiency in factor IX, with similar clinical manifestations to hemophilia A
• Von Willebrand disease (VWD) is the most common inherited bleeding disorder, caused by a deficiency or dysfunction of von Willebrand factor (VWF), which is essential for platelet adhesion and stabilization of factor VIII
• Thrombotic disorders can be caused by inherited or acquired factors that disrupt the balance between procoagulant and anticoagulant mechanisms
  • Examples include factor V Leiden (resistance to APC), prothrombin gene mutation (G20210A), antithrombin deficiency, and antiphospholipid syndrome
• Disseminated intravascular coagulation (DIC) is a life-threatening condition characterized by systemic activation of the coagulation cascade, leading to widespread microvascular thrombosis and consumption of clotting factors and platelets
  • It can be triggered by severe sepsis, trauma, malignancy, or obstetric complications (amniotic fluid embolism, placental abruption)
• Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are rare but serious disorders characterized by microangiopathic hemolytic anemia, thrombocytopenia, and organ dysfunction due to microvascular thrombosis
  • TTP is caused by a deficiency of ADAMTS13, a protease that cleaves ultra-large VWF multimers, while HUS is typically associated with Shiga toxin-producing Escherichia coli (STEC) infection or complement dysregulation (atypical HUS)

Diagnostic Tests and Measurements

• Prothrombin time (PT) assesses the extrinsic and common pathways of the coagulation cascade
  • It is prolonged in deficiencies of factors VII, X, V, II, and fibrinogen, as well as in vitamin K deficiency or antagonism (warfarin therapy)
  • The international normalized ratio (INR) is a standardized measure of PT, used to monitor warfarin therapy
• Activated partial thromboplastin time (aPTT) evaluates the intrinsic and common pathways of the coagulation cascade
  • It is prolonged in deficiencies of factors XII, XI, IX, VIII, X, V, II, and fibrinogen, as well as in heparin therapy or lupus anticoagulant presence
• Thrombin time (TT) assesses the conversion of fibrinogen to fibrin by thrombin
  • It is prolonged in hypofibrinogenemia, dysfibrinogenemia, or the presence of heparin or fibrin degradation products (FDPs)
• Fibrinogen level measures the concentration of fibrinogen in plasma, which can be decreased in DIC, liver disease, or inherited disorders (afibrinogenemia, hypofibrinogenemia)
• D-dimer is a specific degradation product of cross-linked fibrin, and its elevation indicates recent or ongoing thrombosis or fibrinolysis
  • It is used to rule out deep vein thrombosis (DVT) and pulmonary embolism (PE) in low-risk patients, and to monitor DIC and thrombolytic therapy
• Platelet count and function tests (aggregometry, flow cytometry) are used to diagnose and monitor platelet disorders, such as thrombocytopenia, Bernard-Soulier syndrome, or Glanzmann thrombasthenia
• Genetic tests can identify inherited disorders of hemostasis, such as factor V Leiden, prothrombin gene mutation, or ADAMTS13 deficiency in TTP

Clinical Applications in Plasma Medicine

• Plasma-derived factor concentrates are used to treat hemophilia A (factor VIII), hemophilia B (factor IX), and other rare factor deficiencies (factor VII, XI, XIII)
  • Recombinant factor concentrates are also available for hemophilia A and B, offering improved safety and supply
• Desmopressin (DDAVP) is a synthetic analog of vasopressin that increases VWF and factor VIII levels, and is used to treat mild hemophilia A and type 1 VWD
• Fresh frozen plasma (FFP) contains all coagulation factors and is used to treat multiple coagulation factor deficiencies (DIC, liver disease, massive transfusion) or to reverse warfarin in emergency situations
• Cryoprecipitate is a concentrated source of fibrinogen, factor VIII, VWF, and factor XIII, and is used to treat hypofibrinogenemia, dysfibrinogenemia, or inherited deficiencies of these factors
• Platelet transfusions are indicated in patients with thrombocytopenia or platelet function disorders who are actively bleeding or require invasive procedures
• Anticoagulants, such as heparin, low molecular weight heparin (LMWH), and direct oral anticoagulants (DOACs), are used to prevent and treat thrombotic disorders, including DVT, PE, and atrial fibrillation
  • Vitamin K antagonists (warfarin) are still widely used for long-term anticoagulation in patients with mechanical heart valves or antiphospholipid syndrome
• Thrombolytic agents, such as tissue plasminogen activator (tPA), are used to dissolve acute thrombi in conditions such as acute ischemic stroke, massive PE, or acute myocardial infarction (AMI)
• Antifibrinolytic agents, such as tranexamic acid and ε-aminocaproic acid, inhibit plasmin and are used to control bleeding in conditions with enhanced fibrinolysis, such as trauma, postpartum hemorrhage, or dental procedures in patients with bleeding disorders
• Plasma exchange (plasmapheresis) is a therapeutic modality that removes patient plasma and replaces it with donor plasma or albumin
  • It is used to treat TTP (removes anti-ADAMTS13 antibodies and replaces ADAMTS13), atypical HUS (removes mutated complement factors), and other autoimmune or inflammatory disorders with circulating pathogenic factors