Blood Clotting Student Copy - Haemostasis - PDF

Dr Makinde Vincent OLUBIYI HAEMOSTASIS Dr Makinde V. OLUBIYI Dr Makinde Vincent OLUBIYI Introduction Haemostasis is the process by which blood loss from the body is prevented when a blood vessel is cut or ru...

Dr Makinde Vincent OLUBIYI HAEMOSTASIS Dr Makinde V. OLUBIYI Dr Makinde Vincent OLUBIYI Introduction Haemostasis is the process by which blood loss from the body is prevented when a blood vessel is cut or ruptured. Following injury to a blood vessel, the sequence of events in haemostasis is as follows: 1. Vascular spasm 2. Formation of platelet plug 3. Formation of blood clot and clot retraction 4. Growth of fibrous tissue into the clot to form a permanent seal at the point of vessel damage 5. Removal of excess fibrous tissue (fibrinolysis) 6. Repair of blood vessel endothelium. Dr Makinde Vincent OLUBIYI Vascular spasm This is a process in which the injured vessel contracts to narrow or obliterate its own lumen so as to reduce or stop blood loss altogether. This contraction is believed to be due in part to the response of the smooth muscle of the blood vessel to direct stimulation caused by the force causing the cut or rupture of the vessel and in part to nervous reflexes initiated by pain impulses generated from the site of the injury in the vessel or from nearby tissues. This spasm lasts about 20 to 30 minutes, during which time, the next two processes - formation of platelet plug and formation of blood clot - would have occurred thereby providing a more effective seal for the damaged vessel. Dr Makinde Vincent OLUBIYI Formation of Platelet Plug There are about 200,000 to 400,000 platelets per mm of blood. These platelets move freely along the lumen of blood vessels because of the smooth endothelial lining of the blood vessels. However, when there is a cut in blood vessels, the endothelial lining is breached and platelets now come into contact with collagen fibres that are normally below the smooth vascular endothelium. Contact of platelets with collagen or damaged endothelium itself causes the platelets to become sticky, leading to aggregation of the platelets. The platelets release ADP and enzymes that cause formation of thromboxane A which cause further aggregation of platelets, leading to the formation of a platelet plug that further seals off the site of cut in the blood vessel. Dr Makinde Vincent OLUBIYI Formation of blood clot and clot retraction This involves coagulation of the blood that has come-out of the blood vessel at the site of injury. Clot retraction: after the fibrin mesh forms, the clot begins to contract or retract, pulling the wound edges together and reducing the size of the injured area, which aids in tissue repair. Growth of fibrous tissue into the Clot The clot is soon invaded by fibroblasts leading to formation of dense fibrous tissue. The latter completes the final sealing of the cut in the blood vessel. Dr Makinde Vincent OLUBIYI Fibrinolysis Once the vessel is healed, fibrinolysis removes the clot to restore normal blood flow. Plasminogen, trapped in the clot, is activated to plasmin, which dissolves fibrin and breaks down the clot. Repair of blood vessel endothelium The damaged endothelium is replaced by the formation of a new lining, which is the final stage of haemostasis Dr Makinde Vincent OLUBIYI Dr Makinde Vincent OLUBIYI Blood clotting process Dr Makinde Vincent OLUBIYI MECHANISM OF BLOOD COAGULATION Blood coagulation or clotting is triggered by: trauma to the vascular wall and adjacent tissues trauma to the blood contact of the blood with damaged endothelial cells or with collagen and other tissue elements outside the blood vessel. Contact of blood with a foreign surface e.g. glass container used to collect blood The clotting process involves a series of complex mechanisms viz: 1. Formation of thromboplastin (or prothrombin activator) 2. Conversion of Prothrombin (a plasma protein) to thrombin by thromboplastin in the presence of calcium ions 3. Conversion of fibrinogen (a plasma protein) to fibrin in the presence of thrombin. Dr Makinde Vincent OLUBIYI Clotting process (Guyton and Hall, 2016) Dr Makinde Vincent OLUBIYI Formation of thromboplastin (or prothrombin activator) : initiation of coagulation Prothrombin activator is formed in two ways, although, in reality, the two ways interact constantly with each other: 1) extrinsic pathway 2) intrinsic pathway Both extrinsic and the intrinsic pathways involves different plasma proteins called blood-clotting factors. Most of these clotting factors are inactive forms of proteolytic enzymes. When converted to the active forms, their enzymatic actions cause the successive, cascading reactions of the clotting process. Dr Makinde Vincent OLUBIYI Clotting factors Factor I (Fibrinogen) Factor IX (Christmas factor or Factor II (Prothrombin) Plasma thromboplastin component) Factor III (Tissue thromboplastin or Tissue factor) Factor X (Stuart-Power factor) Factor IV (Calcium ions) Factor XI (Plasma thromboplastin antecedent) Factor V (Labile factor or Proaccelerin) Factor XII (Hageman factor) Factor VII (Stable factor) Factor XIII (Fibrin-stabilizing factor) Factor VIII (Antihaemophilic factor) Dr Makinde Vincent OLUBIYI Extrinsic pathway The extrinsic pathway is triggered by trauma to the vascular wall or surrounding extravascular tissues that come in contact with the blood The steps involved in the extrinsic pathway are 1. Release of tissue factor (Factor III) by traumatized tissue: This factor is composed especially of phospholipids from the membranes of the tissue plus a lipoprotein complex that functions mainly as a proteolytic enzyme. 2. Activation of Factor X: The lipoprotein complex of tissue factor further complexes with blood coagulation Factor VII and, in the presence of calcium ions, acts enzymatically on Factor X to form activated Factor X (Xa). Dr Makinde Vincent OLUBIYI Extrinsic pathway (cont´d) 3. The activated Factor X combines with tissue phospholipids that are part of tissue factors or with additional phospholipids released from platelets, as well as with Factor V, to form the complex called prothrombin activator. Within a few seconds, in the presence of calcium ions prothrombin is split to form thrombin and then the clotting process proceeds Dr Makinde Vincent OLUBIYI Intrinsic pathway The intrinsic pathway is triggered by trauma to the blood or exposure of the blood to collagen from a traumatized blood vessel wall. The series of cascading reactions involved in the intrinsic pathway are: 1. Contact of blood with collagen or a foreign surface converts Factor XII to Activated factor XII, a proteolytic enzyme. Simultaneously, the blood trauma also damages the platelets because of adherence to either collagen or a wettable surface (or by damage in other ways). This results in platelet aggregation leading to the release of platelet phospholipid or platelet factor. 2. Factor XI is acted upon by activated Factor XII to form Activated Factor XI. Dr Makinde Vincent OLUBIYI Intrinsic pathway (cont´d) 3. Factor IX is acted upon by activated Factor Xl and converted to Activated Factor IX. 4. Factor X is acted upon by activated Factor IX, Factor VIII and platelet phospholipids and converted to Activated Factor X. 5. Activated Factor X reacts with Factor V and platelet phospholipids (PP) to form Thromboplastin (prothrombin activator). Dr Makinde Vincent OLUBIYI Conversion of Prothrombin to Thrombin Prothrombin is formed in the liver and vitamin K is required in its formation. Prothrombin is converted by prothrombin activator (formed in the extrinsic and intrinsic pathways) in the presence of calcium ions to thrombin. Conversion of Fibrinogen to Fibrin Fibrinogen is also formed in the liver. Thrombin acts on fibrinogen and converts it to fibrin monomer. The fibrin monomers join one another to form long fibrin threads. These fibrin threads form a mesh-work that traps blood cells, platelets and plasma. The initial fibrin thread can be easily broken but it is acted upon by fibrin stabilizing factor (factor XIII). Factor XIII is first activated thrombin and then factor XIIIa act on the fibrin threads to strengthen them so that they can no longer be easily broken Dr Makinde Vincent OLUBIYI Clot retraction This is the final step in the clotting process. The clot formed is usually soft and jelly-like. The fibrin threads in the clot contract a few minutes after the clot is formed and squeeze out most of the fluid (serum) so that what is left is a firm clot. Platelets are important in clot retraction and low platelets count can lead to failure of clot retraction. Interaction of the myosin and actin in the platelets in the blood clot is important for clot retraction. Finally, excess fibrin that may be occluding part of the blood vessel lumen is removed (fibrinolysis) and the damaged endothelium is replaced by new one. Dr Makinde Vincent OLUBIYI Intrinsic and extrinsic pathway Dr Makinde Vincent OLUBIYI Clotting factors Trapped blood cells Dr Makinde Vincent OLUBIYI Dr Makinde Vincent OLUBIYI Haemostais video C:\Users\Asus\Videos\Videos for Lecture slides slides\Blood and body fluids\Coagulation Cascade Animation - Physiology of Hemostasis.mp4 Dr Makinde Vincent OLUBIYI Haemostasis video https://www.youtube.com/watch?v=ud4vHInqjb4 The Clotting cascade https://www.youtube.com/watch?v=cy3a__OOa2M Coagulation cascade animation https://www.youtube.com/watch?v=9QVTHDM90io Hemostasis, Coagulation and Fibrinolysis Dr Makinde Vincent OLUBIYI Why does not blood clot in normal vascular system Blood does not normally clot in the cardiovascular system in a normal person. This is due to the presence of factors that prevent intravascular clotting. These factors are: 1) The smoothness of the endothelium which prevents contact activation of the intrinsic clotting system. 2) A layer of glycocalyx adsorbed to the inner surface of the endothelium, which repels the clotting factors and platelets, thereby preventing activation of the clotting process. 3) Presence of thrombomodulin; this is a protein that is bound with the vascular endothelial lining, which binds any thrombin that is formed intravascularly to form a thrombomodulin - thrombin complex. Dr Makinde Vincent OLUBIYI Apart from making sure that thrombin is not available to take part in the clotting process, the complex also activates a plasma protein C which acts as anticoagulant by inactivating activated Factors V and VIII. 4) Antithrombin action of fibrin and Antithrombin III: during the process of clotting most of the thrombin formed becomes adsorbed to the fibrin fibres. This helps prevent the spread of thrombin into the remaining blood thereby preventing excessive spread of the clot to other parts of the body. The thrombin that does not adsorb to the fibrin fibres combines with antithrombin III, which blocks the effect of the thrombin on the fibrinogen and then also inactivates the thrombin itself Dr Makinde Vincent OLUBIYI 5) Heparin: by itself, heparin has little or no anticoagulant properties, but when it combines with anti thrombin III, it greatly increases it effectiveness in removing thrombin. Heparin is produced majorly by mast cells and basophil leucocytes. These cells continually secrete small quantities of heparin that diffuse into the circulatory system. Mast cells are abundant around the lung tissues and and, to a lesser extent around the liver tissues. Dr Makinde Vincent OLUBIYI Clinical correllates 1) Vitamin K deficiency: Vitamin K is important for the formation of clotting factors in the liver e.g. prothrombin, factor VI, factor IX, factor X. Vitamin K deficiency leads to excessive bleeding Severe liver disease can also lead to clotting defect and severe bleeding. 2) Haemophilia: This is a bleeding disorder that largely resuts from deficiency of cloting factor VIII It occurs in males, while females are just carriers. Factor VIII is made up of two components; a large component and a smaller component. The deficiency of the smaller component causes classic haemophilia, The deficiency of the large component causes a bleeding disorder called Von Willebrand's disease. Dr Makinde Vincent OLUBIYI 3) Thrombocytopenia (very low platelet count): When the blood platelet level falls to 50,000 per microlitre or less, there is a tendency to bleed. The bleeding is usually from small venules or capillaries rather than from larger vessels as in haemophilia. Thrombocytopenia is associated with poor clot retraction and this may in fact be the first suggestion of its existence in an individual. Transfusion with fresh whole blood that contains large amounts of platelets can be life saving in patients with thrombocytopenia. 4) Thrombi and Emboli: An abnormal clot that develops in a blood vessel is called a thrombus. Once a clot has developed, continued flow of blood past the clot is likely to break it away from its attachment and cause the clot to flow with the blood; such freely flowing clots are known as emboli. Dr Makinde Vincent OLUBIYI Anticoagulants The ones most useful clinically are heparin and the coumarins (e.g warfarin) They act by blocking important stages of clotting process or inhibiting some clotting factors e.g conversion of prothrombin to thrombin as well as conversion of fibrinogen to fibrin Substances that bind ionic calcium binding: e.g. sodium citrate, sodium oxalate, sodium fluoride and ethylene diamine tetra-acetic acid (EDTA) This is because prothrombin can only be converted to thrombin by thromboplastin if calcium ions are present. Dr Makinde Vincent OLUBIYI Mechanism of Assessment 1) Bleeding time : is a test that measures how long it takes for a small cut to stop bleeding. It is used to assess platelet function, which is the ability of platelets to stick together and form clots. Normal bleeding time is 1-6 minutes 2) Clotting time: is the time it takes for blood to form a clot. Normal clotting time is 6-10 minutes 3) Prothrombin time: it is inversely related tp the prothrombin concentration Excess calcium ions and tissue extract is mixed with blood causing clotting occurs rapidly. The time between the addition of the procoagulants and the occurrence of clotting is the prothrombin time. The normal value is 12 - 20 seconds.