Normal Hemostasis - Coagulation/Fibrinolysis Flashcards Preview

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Flashcards in Normal Hemostasis - Coagulation/Fibrinolysis Deck (12):

What is the definition of the hemostasis, primary hemostasis, and secondary hemostasis?

  • Hemostasis: Prevention of hemorrhage following vascular injury
  • Primary Hemostasis: Vasoconstriction and platelet plug formation
  • Secondary Hemostasis: Fibrin formation


What are the major components involved in the clotting pathway?

  • Vessel wall: endothelium, collagen
  • Platelets & their glycoprotein receptors; neutrophils, monocytes
  • Clotting factors (fibrinogen, prothrombin, calcium, Factors V through XIII (but there is no VI)
  • Fibrinolytic system


Describe the activation of coagulation.

  • Vessel injury provokes vasoconstriction (neural)
  • Endothelium releases endothelin, a vasoconstrictor, as well as P-selectin and von Willebrand factor from Weibel-Palade bodies (organelles in endothelial cells)
  • von Willebrand factor is necessary for platelet adherence to the subendothelial connective tissue in high-shear vessels such as arteries and arterioles
    • It binds to collagen and platelet glycoprotein 1b, and is degraded by ADAMTS13, a protease also released by the endothelium
  • Collagen activates FXII and also binds glycoprotein VI, activating platelets
  • P-selectin exposed by activated platelets binds neutrophil PSGL-1, activates neutrophils and promotes rolling of neutrophils and platelets on activated endothelium
    • Also, activated neutrophils release neutrophil extracellular traps (NETS) that activate platelets and trap microorganisms
  • Blebs form on leukocytes and are shed as microparticles bearing tissue factor, a glycoprotein
    • These dock and fuse with activated platelets
    • In addition, glycoprotein Ib-V-IX on platelet surface binds vWF/FVIII, FXI, & thrombin
    • Glycoprotein IIb/IIIa binds prothrombin and fibrinogen, and FV and fibrinogen are released from platelet α-granules  
  • Thus, all clotting factors are localized on platelet surface & can interact


Describe the coagulation cascade.

  • At each step of the cascade, an enzyme cleaves (by proteolysis) the next factor, activating it
  • The cleaved factor (a zymogen) becomes an enzyme (or cofactor for an enzyme) and works on the next step, activating more enzyme: a biological amplification process
  • The glass test-tube activates factor XII, initiating clotting by the intrinsic pathway
  • However, in vivo, clotting is initiated by tissue factor, which forms a complex with FVII
  • The tissue factor-FVIIa complex activates FIX as well as FX, so that both intrinsic & extrinsic pathways are in play
  • The integrity of the pathways is tested by the prothrombin time (PT) & activated partial thromboplastin time (aPTT)


What is the principle of using the prothrombin time (PT) and the activated partial thromboplastin time (aPTT) as a diagnostic test?

  • Prothrombin Time (PT)
    • Addition of a high concentration of tissue factor results in formation of TF-VIIa complex which activates FX
    • FXa with FV & calcium, converts prothrombin to thrombin, & thrombin converts fibrinogen to fibrin (insoluble clot)
    • Note that the clotting time becomes truly prolonged when the concentration of one or more clotting factors (FV, VII, X, prothrombin) is less than 30% of normal
  • Activated Partial Thromboplastin Time (aPTT)
    • Addition of a silicate activates FXII; provision of phospholipid & calcium accelerates the clotting time
    • As with the PT, the aPTT becomes clearly prolonged when the concentration of any of the clotting factors, except FVII, is less than 30% of normal


Deficiencies in which clotting factors with change the PT and aPTT? Which ones cause bleeding?

  • The classic cascade is helpful in determining which factors are deficient, based on whether the PT is prolonged (factors II, V, VII, X), the aPTT is prolonged (all but FVII), or both are prolonged (factors II, V, X, and fibrinogen)
    • FXII, FXI, PreKallikrein, & High Molecular Weight Kininogen were discovered because patients deficient in these factors had a prolonged aPTT
    • However, only FXI deficiency is associated with a mild/moderate bleeding disorder
  • FXII, Prekallikrein,& HMWKininogen involved in complement cascade and bradykinin formation
  • FXI binds to activated platelets, is activated by thrombin, and catalyzes the conversion of FIX to FIXa (but tissue factor-FVIIa complex also does this)
    • Factor XI participates in thrombus formation; deficiency might protect against thrombosis


What are the three phases of coagulation and what are the factors involved in each?

  • The 3 phases of coagulation are initiation, amplification, and propagation
    • Tissue factor and FVII are involved in the initiation phase
    • Factors V, VIII, and XI in the amplification phase
    • Factor X and prothrombin in the propagation phase
  • All the reactions occur on tissue factor bearing cells or microvesicles and activated platelets


What is the role of vitamin K in the clottin gcascade?

  • Vitamin K is the coenzyme for the carboxylase that attaches carboxyl groups to glutamic acid residues of clotting factors II,VII, IX, X and inhibitors proteins C, S, and Z
  • These negatively charged groups enable the clotting or inhibitor proteins to form Ca2+ bridges with negatively charged membrane phospholipids, an essential step for their activity
  • During the carboxylase reaction, vitamin K is oxidized but returned to its reduced form by the liver enzyme, Vitamin K Oxide Reductase Complex 1 or VKORC1


What is the role of thrombin in the clotting cascade?

  • Thrombin binds to and cleaves Protease Activated Receptor 1 (PAR1) on the platelet surface
  • PAR1 promotes platelet degranulation, changes in phospholipid composition, and activation of glycoprotein IIb/IIIa
  • Thrombin releases FVIII from von Willebrand factor and activates it, FV, FXI, & FXIII
  • In the absence of FVIII or FIX (hemophilia), FVIIa, FXa and thrombin are rapidly inactivated by tissue factor pathway inhibitor (TFPI) & antithrombin, and fibrin formation is greatly attenuated


What is the structure of fibrinogen and what is its role in the clotting cascade?

  • It is a dimer of 3 interwoven peptide chains
  • When 2 small fibrinopeptides are cleaved from the Aα & Bß-chains by thrombin, the dimers aggregate by forming non-covalent bonds between terminal D-domains on one molecule and the central E-domain of a second molecule giving a pattern that looks like bricks in a wall
    • There is also lateral (side-to-side) aggregation
  • Thrombin-activated FXIII then catalyzes bond formation between glutamic acid of one D-domain and lysine of an adjacent D-domain
    • This accounts for the incredible strength of the fibrin strand


What are some of the major coagulation cascade inhibitors?

  • Intact endothelium generates a variety of antithrombotic agents, including nitric oxide, prostacyclin, and ecto-ADPase that inhibit platelet aggregation
    • Tissue factor pathway inhibitor (TFPI) that blocks the FVIIa-tissue factor complex
    • Thrombomodulin that binds thrombin and activates protein C
    • Proteoglycans that activate antithrombin
    • ADAMTS13 is present on the endothelial cell membrane and cleaves the highest molecular weight multimers of von Willebrand factor as they are released from the endothelial Weibel-Palade bodies
  • Antithrombin: a serine protease inhibitor (serpin) that blocks thrombin and activated factors X, IX, XI, XII, and the tissue factor-VIIa complex
  • Activated protein C, with free protein S (PS), inactivates FVa and FVIIIa, preventing continuing thrombosis
    • Two-thirds of circulating  PS is bound to the complement-4B-binding protein (C4BP), one-third is free.  PS levels <5% of normal associated with thrombosis
  • Protein Z binds to Z protease inhibitor and the complex inhibits factor Xa
    • It does not appear to play an important role in thrombosis
  • Phospholipid-binding proteins include annexin V and beta2-glycoprotein-1 (β2GP1), bind to phosphatidyl serine, preventing binding of the coagulation complexes
    • Antibodies to β2GP1 associated with thrombosis in patients with lupus anticoagulant syndrome


Describe the process of fibrinolytic system.

  • Consists of plasminogen, which forms plasmin, the enzyme that lyses fibrin thrombi
  • Tissue plasminogen activator (t-PA) and urokinase convert plasminogen to plasmin, but this process is most effective when plasminogen is bound to lysine-binding sites on fibrin
  • Plasminogen activator inhibitor-1 (PAI-1), lipoprotein(a), and thrombin activatable fibrinolysis inhibitor (TAFI) either bind to or degrade these lysine binding sites and thereby inhibit fibrinolysis
  • Epsilon aminocaproic acid and tranexamic acid are pharmacologic lysine-mimetics
  • Antiplasmin directly inhibits plasmin