Haemostasis Study Documents

Haemostasis • • • Maintain blood in fluid state, while circulating in the vasculature Arrest bleeding at site of injury or blood loss by formation of haemostatic plug Ensure eventual removal of plug when healing is complete Components of haemostasis • Blood vessels • Platelets • Coagulation factors and their inhibitors • Fibrinolytic system and its inhibitors Haemostatic plug • Primary haemostasis o Platelet adhesion o Platelet activation and granule release o Platelet aggregation • Secondary haemostasis o Coagulation Blood vessels • General structure o Intima (endothelium and subendothelium) o Media o Adventitia • Endothelial cell function o Luminal surface is covered by the glycocalyx, a proteoglycan coat, which can activate ATIII ( an inhibitor of coagulation enzymes) o Express coagulation proteins (thrombomodulin, endothelial protein C receptor), plasminogen activator, protein S, vWF, ADPase o Metabolizes vasoactive peptides o Produces angiotensin II (local vasoconstrictor), prostacyclin, endothelin, thromboxane o Produces NO (vasodilator) • Subendothelium o Collagen, elastic tissues, proteoglycans, glycoproteins eg fibonectin and vWF (exposed when vessel wall is damaged) • Vasoconstriction o o Vessel wall with muscular coat contract following injury to arrest blood loss. Also occur in microcirculation where smooth muscle is absent Platelets • Small fragments of cytoplasm derived from megakaryocytes • Do not contain nucleus • 1.5-3.5 μm • Bound by lipid bilayer membrane (consists of phospholipids, cholesterol, glycolipids and glycoproteins) • Beneath the outer membrane are microtubules which maintain the shape of the platelets and depolymerize during aggregation • Cytoplasmia: 3 types of granules o δ granules o α granules o Lysosomal granules • Contractile system- dense tubular system, cicumferential microfilaments, actin, myosin and calmodulin; to maintain a disc shape; to change shape • Canalicular membrane system that communicates with the exterior • Function in primary haemostasis: o Adhesion o Activation with shape change and granule release o Aggregation Platelet adhesion • Initiated by loss of endothelial cells • Exposure of collagen and vWF • vWF and other adhesive proteins deposit in damaged vessel and bind to collagen • GPIb binds to vWF • GPIa-IIa and GPVI bind to collagen and other platelet receptors • GPIIb/IIIa binds fibrinogen and vWF Platelet activation • Activated GPIb receptors: G protein activation • Platelet shape change (disc to tiny sphere with numerous projecting pseudopods), spreading, secretion, and clot retraction • Platelet granule release: ADP, ATP, serotonin, calcium, vWF, FV, fibrinogen • Agonist react with specific receptors → further activation and platelet aggregation • Activated GpIIb/IIIa • Arachidonic acid metabolism o Arachidonic acid released by PLA2 from membrane phospholipids o Cyclo-oxygenase metabolise AA to form cyclic- endoperoxides, most of which converted to TXA2 • TXA2 o Local vasoconstrictor o Mobilize Ca2+ and stimulates secondary granule release Platelet aggregation • Fibrinogen bind simultaneously to GpIIb-IIIa receptors on 2 different platelets → receptor cross-linking Platelet procoagulant properties • Platelet membrane changes its configuration-Flip-flop, bringing the negatively charged phosphatidyl-serine and – inositol to the surface (prerequisite for the formation of complexes of coagulation factors) • Membrane blebbing leading to microparticle formation which are negatively charged and rich in factor Va • Microparticles bind factors VIII, Va, and Xa, allowing them to form both the tenase and prothrombinase complexes on their surface • Surface expression of P-selectin → recruitment on leukocytes Secondary haemostasis • Blood coagulation cascade • Generation of thrombin • Fibrinogen to fibrin monomer • Cross linking of fibrin monomer to form fibrin • Fibrin enmeshes the platelet aggregates and stabilizes the clot Blood coagulation • Except for fibrinogen, all coagulation factors are enzyme precursors or co-factors • Series of proteolytic reaction • All enzymes except FXIII are serine proteases • Local concentration of clotting factors at the site of injury • Surface mediated reaction on exposed collagen, platelet phospholipid and tissue factor Initiation • Takes place on tissue factor (TF) bearing binding cells exposed after vascular injury e.g. fibroblast • FVIIa/TF complex (extrinsic tenase complex) and Ca2+ activate both FX & FIX • FXa/Va (prothrombinase) and FII produces small amounts of Thrombin (IIa) Amplification • Thrombin, a potent platelet activator via specific receptors • Thrombin activates factors V, VIII and XI on the activated Platelet Surface • Consolidates the production of FXa • Sets the stage for the ‘procoagulant complex assembly’ on the activated Platelet Surface Propagation • ‘Intrinsic Tenase’ (FVIIIa/FIXa FXa) is assembled on the activated platelet surface • Generates a ‘thrombin burst’ → hemostatic fibrin clot at the site of tissue injury