Haemostasis and thrombosis:

1. Haemostasis and thrombosis: Thrombosis is the formation of a clotted mass of blood within the cardiovascular system

2. Continued……. • It involves the interaction of blood vessel wall, formed elements of blood and coagulation system • Thrombus may be distinguished from a blood clot which maybe formed within cardiovascular system after death or in a test tube. It involves only blood clotting system

3. Continued…. • Formation of blood clot is life saving when a vessel ruptures or is cut, but it is also life threatening because it may obstruct blood flow and cause ischaemic injury or thromboembolism

4. Haemostasis • It is the arrest of bleeding by vasoconstriction and clot formation. • Clot formation (thrombosis) involves three factors: • Vascular wall • Platelets • Blood clotting system

5. 1-Endothelium • Normal endothelium is thromboresistant but when injured it promotes thrombosis • Endothelium secretes antithrombotic and prothrombotic factors • Normally, there is balance between two types of factors but a variety of stimuli may favor antithrombosis (excessive bleeding) or prothrombosis (thrombosis)

6. A. Antithrombotic factors • These inhibit thrombosis and include: i-Antiplatelet effect: Intact endothelium prevents platelets from coming in contact with thrombogenic subendothelial extracellular matrix (ECM). • Non activated platelets do not adhere to the endothelium • Activated platelets are inhibited from adhering to uninjured endothelium by prostacyclin (PGI2) and nitric oxide

7. ii- Anticoagulant properties • Heparin- like molecules allow antithrombin III to inactivate thrombin factor xa and other coagulation factors. • Thrombomodulin, a surface protein binds to thrombin and converts to anticoagulant. • It activates protein C which inhibits clotting by proteolysis.

8. iii- Fibrinolytic properties • Endothelial cells synthesize tissue plasminogen activator (t-PA) which promotes fibrinolytic activity and dissolves intravascular thrombi

9. B- Prothrombotic properties • Endothelial cells produce a number of factors which promote thrombosis. • These include: i- von Willibrand factor (vWF) leads to adhesion of platelets to endothelial collagen ii- Tissue factor (thromboplastin) activates extrinsic clotting pathway especially ixa and xa iii- Inhibitors of plasminogen activator (PAI2)-decrease fibrinolysis

10. 2- Platelets • Play a central role in thrombosis • After vascular injury, platelets come in contact with components of extracellular matrix (ECM) including collagen, proteoglycans,fibronectin and other glycoproteins. • On contact with ECM, platelets undergo following changes (become activated) • A) platelet adhesion and shape change, mediated by vWF • B) secretion (release reaction): secretion of contents of two granules occurs after adhesion i.e. • Alpha granules- fibrinogen, fibronectin, platelet factor 4 transforming growth factor (TGF- alpha) etc • Dense bodies release ADP, ATP, ionized Ca+, histamine, 5-HTP (serotonin) and epinephrine (adrenaline)- play an important role in intrinsic pathway of blood clotting and formation of thrombin

11. 3- Platelet aggregation • Platelets secrete thromboxane A2 (TXA2) which along with ADP stimulates platelet aggregation.

12. Causes of thrombosis • The causes of thrombosis are divided into three categories- called “Virchow’s Triad” • Endothelial injury: factors causing endothelial injury in the heart and arteries include : Mechanical injuries like contusions, lacerations, ruptures and crushing injuries. Parasites- e.g. Strongylus vulgaris and Dirofilaria immitis Bacteria- Erysipelas, Streptococcus, Arcanobacterium etc Viral infections and degenerations like arteriosclerosis Tumors

13. Alterations in normal blood flow • Rate of blood flow is extremely important in thrombosis • Stasis and turbulence change the normal axial to slowing moving plasmatic or laminar stream • This allows platelets and other cells to come in contact with the endothelium and predispose to thrombosis.

14. 3- Hypercoagulability • Increased thrombosis is associated with several conditions like trauma (fractures), burns, cancer, and suppurative diseases like pneumonia. Fibrinogen, prothrombin,and other factors maybe increased. • Production of prothrombin is decreased in hepatic diseases. • Deficiency of bile results in Vit. K deficiency and prolongs clotting time and decreases fibrin in thrombi.

15. Pathogenesis of thrombosis • Endothelial injury exposes subendothelial extracellular matrix ( ECM) including collagen and releases tissue factor. • Platelets adhere to the collagen and become activated and release secretory granules. • ADP released from activated platelets causes aggregation of platelets. • Phospholipid complexes provides binding sites for calcium and coagulation factors in the intrinsic clotting pathway. • ADP and thromboxane A2 (TXA2) enlarge platelet aggregates- called primary haemostatic plug which is reversible.

16. Contd…. • Thrombin binds to the platelet surface receptors and along with ADP and TXA2 causes further aggregation of platelet. • The platelet undergo contraction and form secondary haemostatic plug which is irreversible. • Thrombin converts fibrinogen to fibrin which binds the platelets and stabilizes the plug. • Thrombi formed in the rapid blood flow e.g. in the heart and major arteries consists mostly of platelets with little or no fibrin- called white or pale thrombi. • Thrombi formed in the slow moving blood streams as in veins contain fibrin in which erythrocytes and leukocytes are trapped and these are called red thrombi.

17. Classification of thrombi: • Thrombi are classified according to location in the cardiovascular system, size, colour and composition etc. some of the important types of thrombi are; • 1.Valvular (vegetative,verrucous) are formed on the cardiac valves. • 2. Mural –formed on the wall of heart chambers. • 3. Occluding thrombi are attached to the entire circumference of the vessels. • 4. Saddle thrombus- formed on the bifurcation of a blood vessel. • 5. Canalized thrombi- in which new blood channels have been formed. These are lined with endothelium and allow partial restoration of blood flew. • 6. Septic thrombi contain bacteria. Aseptic thrombi do not contain bacteria or parasite.

18. Fate of thrombi: • Propagation: may continue to enlarge and cause obstruction of the vessel. • Emboli formation : Thrombi may break and give rise to emboli which lodges in smaller vessels. • Abscessation: Abscesses appear when pyogenic bacteria are present. • Dissolution: Plasminogen- plasmin system may be activated and thrombus may be completely digested by fibrinolytic activity. • Organization and re-canalizations: Fibroblasts and angioblasts may spread from surrounding vascular wall and cause fibrosis and vascularization of the thrombus. • Calcification: Old thrombi, particularly in the veins undergo dystrophic calcification.

19. Significance: • Negligible effects- when there is abundant collateral circulation and anastomosis • Beneficial effects: Primary haemostatic plug assists in control of hemorrhage and it is life saving. • Harmful effects: • Thrombosis in end arteries results in infarction as in coronary arteries. • Portion of thrombus may break and form emboli and cause infarction. • Thrombi may cause local congestion and edema. • Thrombus weakens the vessel wall and may cause aneurysm, which may rupture and cause fatal hemorrhage.

20. Differentiation of postmortem clot from venous thrombi: • Post mortem clots are of two types • 1. Red or current jelly clots: in these components of blood are evenly distributed. These occur when there is rapid clotting. • 2. Yellow or chicken fat clots: in them components of blood are not evenly distributed. Ventral part of clot is red and is composed of rbcs’. The upper part is yellow and composed of plasma proteins.These occur when clotting time is increased and there is increased erythrocytes sedimentation rate (ESR).

21. Thrombus Granular, dry and rough White or pale in colour Attached to the vessel wall Stratified in structure Composed mainly of platelets Postmortem clot A rubbery, gelatinous coagulum Intense red or yellow in colour Not attached to the vessel wall Uniform in structure Composed mainly of fibrin. Compression of thrombus and post mortem clot

22. 6. Vascular endothelium below the thrombus is damaged and rough. 7. Forms in a flowing stream of blood. 8. Forms in the living animal 9. May be partially organized. 10.Caused by endothelial injury. 6. Vascular endothelium below the thrombus is undamaged, smooth and glistening. 7. Forms in a stagnant column of blood. 8. Forms in the dead animal 9. No indication of organization. 10. Initiated by thromboplastin ( tissue factor) . Contd….