SI2101 Haematology Lecture 4: Haemostasis PDF

SI2101 Haematology Lecture 4: Haemostasis Dr. Louise Horrigan Physiology [email protected] University ofGalway.ie Learning To learn: The role of the endothelium in Obj...

SI2101 Haematology Lecture 4: Haemostasis Dr. Louise Horrigan Physiology [email protected] University ofGalway.ie Learning To learn: The role of the endothelium in Objectives of haemostasis Today’s Lecture The 4 main stages of blood clotting and clot resolution Detail of the formation of the fibrin mesh Some mechanisms to limit blood clotting Some disorders of blood clotting  Haemostasis involves a delicate balance to prevent blood loss and maintain blood in the fluid state Prevention of haemorrhage and prevention of thrombosis Role of endothelium in maintaining blood flow Endothelium Smooth muscle Blood and collagen fibres  Prostacyclin Intact endothelium Vasodilation Nitric oxide Damaged endothelium Vasoconstriction Endothelin-1 Endothelium 4 Stages of Blood Clotting 1. Vasoconstriction 2. Formation of platelet plug 3. Formation of fibrin network (coagulation) 4. Clot retraction and dissolution University ofGalway.ie 1. Vasoconstriction Injury to a blood vessel causes contraction of the smooth muscle of the vessel 1. Myogenic contraction Reflex contraction due to stress or stretch on the blood vessel 2. Change in the balance of endothelial vasodilators and vasoconstrictors Endothelin-1 released from endothelial cells, which inhibits release of nitric oxide and prostacyclin This can reduce blood flow for many minutes University ofGalway.ie 2. Formation of platelet plug Activated Platelets Involves platelet 1. Adhesion 2. Activation 3. Aggregation Initiated by damage to endothelium and exposure of platelets to subendothelial collagen Platelet adhesion to collagen Platelets can bind to exposed collagen directly and also via von Willebrand factor (vWF) Von Willebrand factor is a glycoprotein produced by endothelial cells and platelets It forms a bridge between platelets and collagen University ofGalway.ie Platelet activation Prostacyclin and NO from intact endothelium inhibit platelet activation Platelet adhesion triggers activation When in contact with damaged endothelium, platelets swell and extend pseudopodia Activated Platelets –Activated platelets release ADP and serotonin from granules as well as thromboxane A2 Promote vasoconstriction and further activation of platelets (positive feedback) Platelet aggregation Activation of platelets triggers activation of fibrinogen receptors Fibrinogen can act as a bridge to link activated platelets with each other to form the platelet plug University ofGalway.ie  Scanning electron micrographs of resting (A) and activated (B) platelets. Charles J Knight Heart 2003;89:1273-1278 Copyright © BMJ Publishing Group Ltd & British Cardiovascular Society. All rights reserved. 4 Stages of Blood Clotting 1. Vasoconstriction 2. Formation of platelet plug 3. Formation of fibrin network (coagulation) 4. Clot retraction and dissolution University ofGalway.ie 3. Formation of the fibrin network Objective: Formation of a fibrin mesh Mechanism: 1. Formation of prothrombinase (prothrombin activator) By intrinsic and extrinsic pathways 2. Conversion of prothrombin to thrombin 3. Thrombin catalyses the formation of the fibrin mesh from fibrinogen University ofGalway.ie Red blood cells trapped in a fibrin network in a blood clot Image licensed under the Creative Commons Attribution-Share Alike 4.0 International license.  Extrinsic Pathway Intrinsic Pathway Tissue damage exposes Exposure of RBC to collagen Tissue Factor (Factor III) or foreign surface XII III XI VII PF3 IX V VIII X On X platelet surface Prothrombinase (Factor X /Factor V) Cascade of enzyme activation requires activated platelets (Platelet Factor 3), Ca2+ and co-factors Conversion of prothrombin to thrombin Prothrombinase complex consists of activated Factor X and Factor V Within seconds, prothrombinase splits prothrombin to form thrombin After a small amount of thrombin is formed, it activates Factor V in the prothrombinase complex, accelerating the activity of prothrombinase Factors VIII and XI leading to amplification of the intrinsic pathway Factor XIII which stabilises the fibrin network University ofGalway.ie Formation of the fibrin mesh following the production of Factors V & X prothrombinase Prothrombinase Complex Ca2+ PF3 Fibrinogen Thrombin Prothrombin Fibrin Fibrin stabilising factor (Factor XIII) Thrombin exerts a positive feedback effect  Formation of fibrin mesh Thrombin acts on fibrinogen to form a fibrin monomer Many fibrin monomers polymerise into long fibrin fibres Ca 2+ required Fibrin-stabilizing factor (Factor XIII) causes fibrin cross-linking – Present in blood and also released from platelets – Activated by thrombin  Vitamin K is an essential requirement for blood clotting. It acts as a cofactor for the enzymatic modification of some clotting factors, thus enabling them to bind calcium. Calcium is necessary for activation of some clotting factors. 4. Clot retraction and fibrinolysis Clot retraction occurs within minutes or hours Platelets contain actin and myosin Contraction - pulls fibrin strands - pull edges of blood vessel together - squeezes out serum Fibrinolysis is the breakdown of the clot Formation of plasmin – a ‘clot buster’ Proteolytic enzyme that digests fibrin threads and other clotting factors Plasminogen converted to plasmin by tissue plasminogen activator (tPA) University ofGalway.ie  Plasminogen activator eg. tPA Plasminogen Plasmin Fibrin Soluble fibrin fragments Three mechanisms to limit blood clotting 1. Antithrombin III inhibits thrombin and other factors at intact endothelium Accelerated by heparin (expressed by endothelial cells) 2. Protein C pathway Thrombomodulin (on surface of endothelial cells) induces a conformational change in thrombin This changes thrombin’s activity away from activation of proclotting factors and towards activation of Protein C With its cofactor Protein S, activated Protein C inactivates factors V and VIII 3. Tissue factor pathway inhibitor (TFPI) Found attached to endothelium Inhibits Factor X and Factor VII/Factor III complex  Some Clotting Disorders Platelet defect or deficiency (eg. thrombocytopenia) Resulting in multiple bruises and petechiae Von Willebrand’s disease Haemophilia Lack of Factor VIII (85%) or Factor IX (15%) X-linked recessive disease Male sufferers, female carriers Liver disease Vitamin K deficiency Hypercoagulation/thrombotic disorders Protein S deficiency Nakakubo Y, Yamamoto K, Fujita M. Atypical Presenting Symptoms of Acute Onset Acquired Haemophilia with Eosinophilic Fasciitis. Eur J Case Rep Intern Med. 2020;7(9):001722. Published 2020 Jun 15. doi:10.12890/2020_001722 Summary of main points Haemostasis involves a delicate balance between pro-coagulant and anti- coagulant mechanisms Clotting is activated where there is damage to the endothelium Platelets play a crucial role in blood clotting Thrombin is the central regulator of haemostasis, involved in both positive and negative feedback loops in the process Vitamin K and calcium are essential for the activation of clotting factors Deficiency of various clotting factors can cause haemophilia Deficiency of anti-coagulant mechanisms can cause thrombosis University ofGalway.ie