Hemostasis II: Coagulation and Fibrinolysis PDF

41 Hemostasis II: Coagulation and Fibrinolysis ILOs By the end of this lecture, students will be able to 1. 2. 3. 4. 5. 6. 7. Describe the process of haemostasis that controls blood loss when vessels are damaged Correlate the role of platelets in the haemostatic mechanism Correlate the role of vitam...

41 Hemostasis II: Coagulation and Fibrinolysis ILOs By the end of this lecture, students will be able to 1. 2. 3. 4. 5. 6. 7. Describe the process of haemostasis that controls blood loss when vessels are damaged Correlate the role of platelets in the haemostatic mechanism Correlate the role of vitamin K in the haemostatic mechanism Summarize the role of calcium in the haemostatic mechanism Describe the fate of the blood clot Interpret how blood normally does not clot in healthy vessels. Describe factors that prevent clot extension Hemostasis; is the stepwise natural series of body reaction and events that occur following vessel injury, that intends to seal off the injured area, arrest the bleeding and repairs the induced damage; Figure 1 The stepwise events of the hemostatic process are: 1. Immediate Constriction Phase of blood vessel, to limit blood flow and decrease hemorrhage. 2. Formation Platelet Plug Phase, by adherence, activation and aggregation of Platelets with formation of a temporary Primary Hemostatic Plug. Refer to hemostasis 1: Platelets 3. Blood Coagulation Phase: by sequential activation of Coagulation Factors in Blood to finally yield a mesh of fibrin that will become polymerized by activated factor XIII to stabilize the platelet aggregates that retract into a stable Secondary Hemostatic Plug [Fibrin Clot]. Figure 1: Process of haemostasis With progress in repair of the injured blood vessels, an opposing process issues that gradually breaks down the fibrin clot once it becomes unnecessary. This represents the Fibrinolytic Phase thus preventing uncontrolled extension of thrombus or its embolization. It is also meant to regain patency of blood vessels during repair. ___________________ 1. IMMEDIATE CONSTRICTION PHASE: It is a brief reflexive contraction regulated by: 2. Local myogenic spasm. 3. Nervous reflex induced by the existing pain of injury. 4. Released vasoconstrictors from platelets [Serotonin & TXA2] and from endothelium [Endothelin ]. 3. BLOOD COAGULATION PHASE: Involves sequential activation reactions of a set of molecules termed Coagulation Factors; (Table 1) [donated roman numbers(I to XIII)] to end by converting soluble fibrinogen into an insoluble fibrin fiber, that will further polymerize to cement retracted platelets into a Fibrin Clot. Most Coagulation Factors belong to the plasma proteins and circulate as inactive proteolytic enzyme precursors. They are initially synthetized: - Mainly by the liver. Vit K catalysis the decarboxylation of (factor II, VII, IX, X) that are termed; Vit K-dependent” Coagulation Factors. - Exceptionally Factor XIII is synthetized by endothelial cells. Factor V and Fibrinogen are taken up (not synthesized ) by platelets The Process of Coagulation starts simultaneous during adherence of platelets (in Primary Hemostasis) but generally finishes after to end by Secondary Hemostasis. It proceeds as three overlapping sequential phases; 1. Initiation 2. Amplification 3. Propagation Phases. During such phases some steps of coagulation factor activation, occur on the procoagulant membranes of different cells being: - Extravascular cells having TF (as fibroblasts); in initiation phase - Activated platelets, during propagation and amplification phases. That is why coagulation is no more viewed as a unidirectional proteolytic cascade but rather as “CELL-BIOLOGICAL MODEL OF COAGULATION” 1. Initiation Phase of Coagulation It starts within seconds by activation of TF present on extra-vascularly surfaces, to activate VII to VIIa. Such Extrinsic Pathway continues to activate IX and X to IXa then Xa respectively forming little activated extrinsic coagulation complexes that proceed to activate small amount of Prothrombin (II) to Thrombin(IIa). (figure 2 ; left panel) This little thrombin presents the THROMBIN SPARK that bind to PAR receptors of platelets to ignite: 1. The Amplification Phase of Coagulation 2. The Full Activation of Platelets. 2. Amplification Phase of Coagulation Proceeds at slower rate and starts by : - the adherence of platelets to the exposed von Willebrand Factor at the site of vascular injury (by thrombin spark) - activation of XII (by negativity of subendothelium). Sequentially XI and VIII become activated in such Intrinsic Pathway to proceed in further activation of IX to IXa that complex with VIIIa. Such intrinsic Tenase Complex ( IXa + VIIIa) activate formation of Prothrombinase Complex (Va/Xa/Ca) that trigger formation of big amounts further of thrombin; the THROMBIN BURST to initiate the propagation phase. The trace levels of thrombin generated during the initiation phase stimulates platelet activation, leading to the surface exposure of phosphatidylserine as well as the release of procoagulant molecules such as factor V from granules. Thrombin also activates Factor V as well as Factor VIII and Factor XI which then further stimulate the activation of Factor X, which binds with factor V on platelet phosphatidylserine. (figure 2 ; right panel). 3. Propagation Phase of Coagulation This presents a Common Pathway that begins by activating big amounts of thrombin. (figure 2 ; lower left panel). Only then it will be capable of transforming fibrinogen that is holding the platelet together by their (GPIIbIIIa) receptors into FIBRIN. Furthermore, thrombin burst in presence of Ca, activate XIII permitting it to cross-link the fibrin into polymers that stabilizes the [Fibrin Clot] meanwhile the platelet within would have become retracted. Figure 2: Coagulation cascade In most phases Ca ions (the only non-protein coagulation factor) share in activation of most phases except the first 2 steps in the intrinsic pathway during the amplification phase. What Prevents Clot Extension Once Formed? 1. Natural Anticoagulants as Antithrombin III, heparin and protein C. ▪ Antithrombin III, a non-Vit K dependent plasma protein formed by liver and inhibits IIa, Xa and other of coagulation factors. ▪ Heparin, a natural anticoagulant contained in basophils and mast cell granules, and on endothelial cells surface. It exerts its action via enhancing antithrombin III activity. ▪ Protein C, a natural anticoagulant produced by the liver to inhibit factors V & Vlll activity. ▪ Protein S, a cofactor activating protein C and increase formation of plasmin. 2. Autocrine - Paracrine Mediators secreted from endothelial cells or platelets. ▪ Prostacyclin which promotes vasodilation and inhibits platelet activation. ▪ Nitric oxide which inhibits platelet adhesion and aggregation in response to thrombin 3. Tissue-Factor Pathway Inhibitor (TFPI), an anticoagulant that inhibits extrinsic Tenase complex Why Doesn’t Blood Clot in Healthy Vessels? Several interplaying factors exist to keep blood in a fluid, un-clotted state in the healthy blood vessels. These factors are intended to prevent unnecessary clot formation, or thrombosis, that may result in occlusion of arteries and blockage of blood supply. These factors include: 1. The clotting factors circulate in an inactive form and are permitted to become activated only upon injury 2. If activation occur the liver is capable of regularly removing activated factors. 3. The speed of flow of blood is optimum to prevent activation of clotting factors. It does not permit their coming in contact with TF which can activate the extrinsic pathway. 4. The healthy endothelial lining of blood vessels is smooth and negatively charged to ensure that it will repel negatively charged surface as (collagen) that can activate the intrinsic pathway. 5. Endothelial cells secret mediators with potential anticoagulant action as NO, PGI2. 6. The circulating platelets is equipped with a glycoprotein coat that create repulsion forces preventing their sticking to healthy endothelium. 7. Plasma contains natural anticoagulants as anti-thrombin lll and heparin that antagonize coagulation. 8. Plasma contains a tightly regulated fibrinolytic system in which tissue plasminogen activator (t-PA) can convert plasminogen into plasmin to digest and lyse the fibrin clots preventing their propagation. What is the Fate of a Blood Clot? After beginning of repair, the blood clot is either invaded by fibroblasts and form connective tissue or undergoes lysis. CLINICAL RELEVANCE If thrombi tend to develop or emboli are prone to dislodge drugs that inhibit coagulation can be used to prevent the insult. They can also be used as a treatment to halt the progression of thrombosis in acute insults. Such ANTICOAGULANT AGENTS can block steps in coagulation by: 1. Indirect inhibitors of thrombin formation by activating anti-thrombin III 2. Direct thrombin inhibitors by binding directly to thrombin 3. Factor X inhibitors 4. Vit K inhibitors to block the coagulative profile of Vit K dependent -factors. _______________ FIBRINOLYSIS It is the process of dissolution of an already formed blood clot or a thrombus. It is achieved by the action of a powerful activator called tissue plasminogen activator (t-PA), that is slowly released from injured tissue and endothelium within days of the injury. It will activate the convergence of an inactive plasma protein proenzyme; plasminogen (profibrinolysin) into an active proteolytic enzyme; plasmin (fibrinolysin). Because the latter is a non-specific protease it will digest: - fibrin threads causing complete dissolution of the Fibrin Clot - other circulating proteins as prothrombin, fibrinogen, factors; V and VIII. By this way many small blood vessels in which blood flow has been blocked by the clot are reopened. CLINICAL RELEVANCE If thrombi or emboli already exist as in myocardial infarction, pulmonary embolism and stroke, the only therapeutic method then to re-open the vessels is to lyse the existing block by use of FIBRINOLYTIC or THROMBOLYTIC AGENTS. These are divided as: 5. Fibrin-Specific Plasmin Activators; Acts on plasminogen only bound to clot surface. 6. Fibrin-Non-Specific Plasmin Activators: Acts also on plasminogen in the circulation