بسم الله الرحمن الرحيم

Mechanism of Haemostasis & Coagulation DR. Khaled Khalil

Guyton and Hall “Textbook of Medical Physiology”12thed (pg454-460) At the end of the session the students should be able to: Define hemostasis and enumerate the steps involved in it. Explain the mechanism of blood coagulation Discuss the anticoagulant mechanism of the body Identify the natural and synthetic anticoagulants Guyton and Hall “Textbook of Medical Physiology”12thed (pg454-460)

Haemostasis Means: stoppage of bleeding when a small blood vessel is injured Local vascular spasm Platelet plug formation Blood clot formation Fibrinolysis and fibrosis of the blood clot

A) Local vascular spasm Immediately after the blood vessel is injured by the following mechanisms: Nervous reflexes initiated by pain from traumatized blood vessel. Local myogenic contraction of smooth muscles due to direct trauma, the more the traumatized the vessel the greater the degree of spasm Humoral: through chemical substances released from the site of injury (serotonin,TxA2, adrenaline,….) Vascular spasm reduce blood flow to the injured BV

B) Platelet Plug Formation Upon damage to blood vessel endothelium (which exposes subendothelial collagen) platelets: 1) Platelet adhesion: With the help of von Willebrand factor (VWF) adhere to collagen through platelets membrane glycoprotein (Ib) and initial release reactions (ADP, TxA2, serotonin) 2) Platelet activation: stimulated by ADP and thromboxane A2. Activated platelets swell, develop pseudopodia, sticky and discharge their granules (ADP, serotonin, TxA2, catecholamines, vWF, fibrinogen,…..) 3) Platelet aggregation: Release serotonin and ADP, attract more and more platelets forming a mass of aggregated platelets (fibrinogen & Gp IIb/IIIa) (platelet cohesion). 4) Formation of a platelet plug large enough to close a small rent in the BV wall. Platelet plug initially is loose then become firm by contraction of the platelet contractile proteins and by fibrin threads

2- Platelet Plug Formation Platelets adhesion is potentiated by vWF which is plasma protein

Platelet Plug Formation 1) Aspirin inhibit TxA2 synthesis, so inhibit platelet activation and aggregation 2) Activated platelet expose the platelet membrane phospholipids (PF3) which is essential for blood coagulation. 3) Normally there is thousands of minute rupture in small BV (capillaries), that closed by platelet plug formation (purpura)

C) Blood Colt formation

Coagulation (Clotting) Factors Most of them are plasma proteins (β globulin) formed in the liver Vitamin K-dependent clotting factors are: II, VII, IX, X Most of them are present as proenzymes (inactive) Once activated, it induces a cascade reaction

C) Blood Coagulation The third mechanism for hemostasis. (begin 20 sec - 2 m) (within 3- 6 m blood clot is formed) A series of reactions in which blood is transformed from a liquid to a gel The main three steps of this series of reactions are: 1) Formation of Prothrombin activator complex: through intrinsic and extrinsic pathways 2) Convertion of Prothrombin into thrombin 3) Thrombin catalyzes the convertion of fibrinogen into a fibrin threads

1) Prothrombin activator complex Formation of prothrombin activator occur by 2 pathways: I) Extrinsic pathway: initiated by substance outside the blood (tissue thromboplastin factor III) released due to trauma. 2) Intrinsic pathway: initiated by substances all present in the plasma (factor XII). There is cooperation between the 2 pathways in the process of blood coagulation.

Coagulation Phase 1: Two Pathways to Prothrombin Activator May be initiated by either the intrinsic or extrinsic pathway Triggered by tissue-damaging events Each pathway cascades toward factor X Once factor X has been activated, it complexes with Ca2+ ions, PF3, and factor V to form prothrombin activator

A) Extrinsic pathway Tissue thromboplastin (factor III) released from damaged tissue and vascular wall (consist of lipoprotein & phospholipids) 1) Factor III activate factor VII to VIIa & form a complex with it. 2) Factor III/VIIa complex in the presence of Ca2+ activate factor X into Xa 3) Xa with factor V, PF3 (platelet phospholipid) and Ca2+ form the prothrombin activator enzyme complex 4) Prothrombin activator act on prothrombin converting it thrombin which then act on fibrinogen converting it to fibrin monomer

B) Intrinsic pathway Exposure of the blood to damaged endothelium & rough subendothelial collagen or contact with water wettable surface (glass) initiate the activation of the intrinsic pathway (slow & terminates in 3 – 6 m) occur in the following steps: 1) Contact activation of factor XII into XIIa 2) XIIa activate factor XI into active XIa 3) XIa activate factor IX into active IXa 4) IXa in the presence of factor VIII and Ca2+ and PF3 activate factor X into Xa 5) Xa with factor V, PF3 (platelet phospholipid) and Ca2+ form the prothrombin activator enzyme complex Prothrombin activator act on prothrombin converting it to thrombin which then act on fibrinogen converting it to fibrin monomer

blood Colt formation

Coagulation Phase 3: Common Pathways to the Fibrin Mesh Thrombin catalyzes the conversion of fibrinogen into fibrin monomer that polymerized into fibrin threads. Insoluble fibrin threads entrapping blood cells, platelets and plasma forming loose blood clot Fibrin causes plasma to become a gel-like trap Fibrin in the presence of calcium ions & active factor XIII that: Cross-links fibrin Strengthens and stabilizes the clot

Role of Ca+2 in blood clotting Ca+2 required for acceleration of most of blood clotting reactions Absence of Ca2+ prevent blood clotting by the 2 pathways. Citrate and oxalate salts (Ca2+ precipitating agents) can be used as in vitro anticoagulants.

Clot Retraction and Repair Clot retraction – stabilization of the clot by squeezing serum from the fibrin strands Repair of BV wall Platelet-derived growth factor (PDGF) stimulates rebuilding of blood vessel wall, Fibroblasts form a connective tissue patch Stimulated by vascular endothelial growth factor (VEGF), endothelial cells multiply and restore the endothelial lining

Natural Anti-Clotting Mechanisms; A- Endothelial Factors Healthy smooth endothelium will not provide surface for platelets adhesion (NO & PGI2) Glycocalyx on endothelial surface is repellent for platelets & clotting factors Thrombomodulin; protein bound to endothelial membrane. It has anticoagulant effect via: Binding thrombin Thrombomodulin-thrombin complex activates Protein C which inactivates factors V & VIII

B) Blood factors: (natural anti-clotting) 1) Continuous blood flowing: prevent contact activation of platelet & clotting factors 2) Fibrin formed during clot formation: adsorb 85% of thrombin & localize it action to the site of clot formation prevent the extension of the clot 3) Antithrombin III: a plasma protein secreted by the liver bind to & inactivate thrombin & bind to & inactivate other active clotting factors (9, 10, 11, 12) Its action is markedly enhanced by heparin 4) Heparin: Sulphated polysaccharide (glucosaminoglycan) (secreted by mast cells & basophils) prevent blood clotting through increase the activity of antithrombin III 1000 times & stimulate fibrinolysis

B) Blood factors: (natural anti-clotting) 5) Protein C & protein S: Plasma proteins secreted by the liver & require vit K activated by thrombomodulin-thrombin complex & then inactive clotting factors (V & VIII) in the presence of protein S & activate fibrinolysis by its binding to t-PA inhibitors 6) α1 antitrypsin & α2 macroglobulin: hydrolyze & inactivate thrombin and many other clotting factors 7) Fibrinolytic system: dissolve & breakdown many small thrombi formed in small BV & prevent their extension

Thank You