Hemostasis. Learning objectives Dr. Mária Dux. Components: blood vessel wall thrombocytes (platelets) plasma proteins

Transcription

1 Hemostasis Learning objectives Dr. Mária Dux Components: blood vessel wall thrombocytes (platelets) plasma proteins Hemostatic balance! procoagulating activity anticoagulating activity 1

2 Thrombocytes Colony Forming Unit-Megakaryocytes Thrombopoietin (glycoprotein): liver, kidney megakaryocyte thrombopoietin thrombopoietin platelets Thrombopoietin in plasma binds to receptors on the surface of circulating platelets; the remaining, unbound thrombopoietin in plasma is available to promote megakaryocyte proliferation. Thus, when the platelet count drops, increased plasma levels of free thrombopoietin stimulate megakaryocytes. Thrombocytes /µl blood Size: 1-3 µm Lifespan: 5-8 days Morphology: No nucleus Surface factors (glycoproteins - Integrin) Actin-Myosin Granules dense granules (ADP, serotonin, adrenaline) α-granules (proteins - clotting factors) lysosomes (enzymes) Mitochondria Glycogen 2

3 Ultrastructure of platelets Adhesion molecules (Integrins) (Ca 2+ ) actin, myosin α2-antiplasmin lysosomes VWF = von Willebrand factor Hemostasis Primary hemostasis local vasoconstriction plug formation Secondary hemostasis blood coagulation WHITE thrombus RED thrombus + defence against infection tissue repair 3

4 Vasoconstriction slows down the bleeding: 1) Response of vascular smooth muscle to injury. Activation of vasomotor nerves. Vasoconstrictors released by activated thrombocytes. (Thromboxane A 2, serotonin, adrenalin) 2) Mechanical compression: - damaged elastic fibres - blood in the extravascular space Short lasting effect! Formation of thrombocyte plug: 1. Adhesion of thrombocytes to injured blood vessel wall - Hydrodynamic margination - Adhesion of thrombocytes blood vessel wall endothelium Platelets exposure of collagen fibers to blood von Willebrand factor (glycoprotein) damaged endothel (platelets) 4

5 Formation of thrombocyte plug: 1. Adhesion of thrombocytes to injured blood vessel wall 2. Activation of thrombocytes Intracellular signal transduction Changes in the structure of thrombocytes Exocytosis (release of stored substances) resting activated Activation of thrombocytes thromboxane A 2 (TXA 2 ) ADP serotonin adrenalin INTACT ENDOTHELIUM contact with collagen TXA 2 thrombin [Ca 2+ ] adenylyl cyclase prostacyclin camp camp, cgmp Ca-ATPase free Ca structural changes exocytosis exposition of GP IIb/IIIa THROMBOCYTE NO cgmp nitric oxide 5

6 Formation of thrombocyte plug: 1. Adhesion of thrombocytes to injured blood vessel wall 2. Activation of thrombocytes 3. Aggregation of thrombocytes blood vessel wall endothelium Platelets von Willebrand factor Bleeding time (Ivy s method): 3-5 min Acetylsalicylic acid Aspirin causes an irreversible inhibition of cyclooxygenase (COX). Inhibition of platelet aggregation 6

7 Secondary hemostasis Coagulation factors: Enzymes: serine proteases, transglutaminase (Factor XIII) Cofactors (Factor V,VIII) Ca 2+ + Vitamin K Surface: subendothelial collagenmatrix membrane of the thrombocytes (phospholipid) Coagulation factors cascade Factor Factor a 7

8 Vitamin K (phylloquinone ) (in green vegetables) Serves as a cofactor in the γ-carboxylation of glutamic acid residues (post-translational processing) on various proteins concerned with blood clotting. Vitamine K dependent factors - II, VII, IX, X, Protein-C, Protein-S (inhibition!) γ - carboxyglutamic acid binds Ca 2+ Tissue Factor Pathway Inhibitor Membrane protein (blood vessel wall) (activated endothel and makrophages) Extrinsic tissue injury Ca ++ tissue factor Prekallikrein Intrinsic negatively charged surface Kininogen It is not the main pathway of activation BUT: 1. It has a positive feedback on the cascade. 2. It is important in fibrinolysis, tissue repair, complement activation. 3. It can be activated on the surface of artificial heart valves. Formation of fibrin Peptides Fibrin polymer 8

9 Complexes of blood coagulation VIIa-PPL-Ca 2+ complex IXa-VIIIa-PPL-Ca 2+ complex Xa-Va-PPL-Ca 2+ complex Xa IXa X Ca 2+ VIIIa Platelet phospholipid Platelet phospholipid Ca 2+ : calcium PT: prothrombin Thr: thrombin Tissue Factor Pathway Inhibitor Membrane protein (blood vessel wall) (activated endothel and makrophages) Extrinsic tissue injury Ca ++ tissue factor Prekallikrein Intrinsic negatively charged surface Kininogen It is not the main pathway of activation BUT: 1. It has a positive feedback on the cascade. 2. It is important in fibrinolysis, tissue repair, complement activation. 3. It can be activated on the surface of artificial heart valves. Formation of fibrin Peptides Fibrin polymer 9

10 VIIIa Va FIBRIN XIIIa XI XIa PROTEIN C PROTEIN Ca Ca ACTIVATION + tissue repair defence against infection Formation of fibrin weak bonds formed through noncovalent bindings strong covalent bonds factor XIIIa (fibrin-stabilizing factor) - transglutaminase 10

11 RETRACTION (within a few hours): a compact red clot is formed (actin/myosin attached to fibrin fibres), clear yellowish fluid ( serum coagulation factor-free plasma) is squeezed out. serum + coagulation factors = plasma What is Factor III (Thromboplastin)??? The term thromboplastin was originally assigned to the putative activator of factor II (prothrombin). In light of our present knowledge, however, it is the Xa-Va-PPL-Ca 2+ complex that should be designed as thromboplastin. Since the tissue factor was earlier thought to be directly responsible for the conversion of prothrombin to thrombin, the tissue factor was called tissue thromboplastin. Because of the ambiguities, the terms Factor III and Thromboplastin are best avoided. 11

12 Anti-hemostatic mechanisms Inactivation of thrombin: Antithrombin (III)/ heparin produced by the liver serine protease inhibitor heparin acts as cofactor Protein C/Thrombomodulin Thrombomodulin of the undamaged endothel binds thrombin. The thrombomodulin - thrombin complex activates protein-c. Protein-Ca inactivates factors Va and VIIIa. Fibrinolytic system Tissue-type plasminogen activator (endothelium) Fibrin Plasminogen Plasmin Streptokinase Urokinase Factor XIIa short fragmets α 2 -antiplasmin of thrombocytes inhibits the activation of plasminogen. 12

13 Anticoagulants: In vivo: Heparin Vitamin K antagonists Dicumarol, Warfarin Selective Xa inhibitor (inhibiting thrombin formation) In vitro: Heparin Binding of Ca 2+ Na-oxalate Na-citrate EDTA Tests for coagulation disorders Prothrombin time (PT) assesses the extrinsic + common pathways of coagulation within 20 sec Prothrombin index (Quick) % Partial thromboplastin time (PTT) assesses the intrinsic + common pathways of coagulation within 45 sec INR (International normalized ratio) INR = ( PT test PT normal ) ISI ISI (International Sensitivity Index) 13

14 Hemorrhagic disorders 1) Vascular disorders (increased capillary fragility) 2) Platelet disorders spontaneous bleeding on the skin and mucous membranes purpura 3) Coagulation disorders Bleeding occurs following injury and does not stop easily. Bleeding occurs in joints. Genetic deficiency of coagulation factors: VIII Hemophilia A IX Hemophilia B - royal disease Queen Victoria ( ) and her family Blood can change the course of history Lannoy, Hermans 2010 Haemopilia 14