Blood is normally separated from the activators of haemostasis by the endothelial cell. Injury to the vessel wall exposes collagen and sets in motion a series of events leading to haemostasis.
Haemostasis is a complex process depending on interactions between the vessel wall, platelets and coagulation factors.
An immediate reflex vasoconstriction of the injured vessel and adjacent vessels results in a transient reduction of blood flow to the affected area. Damage to the endothelium of the vessel results in activation of platelets and coagulation; release of serotonin and thromboxane A2 (TXA2) from activated platelets contributes to the vasoconstriction.
Platelet adhesion to collagen is dependent on platelet membrane receptors, glycoprotein Ia (GPIa), which binds directly to collagen, and glycoprotein Ib (GPIb), which binds to von Willebrand factor (VIII:vWF) in the plasma and VIII :vWF in turn adheres to collagen. Following adhesion, platelets undergo a shape change from a disc to a sphere, spread along the subendothelium and release the contents of their cytoplasmic granules, i.e. the dense bodies (containing ADP and serotonin) and the ex-granules (containing platelet-derived growth factor, platelet factor 4, l3-thromboglobulin, fibrinogen, VIII:vWF and other factors).
The release of ADP leads to exposure of a fibrinogen receptor, the glycoprotein IIb-IIIa complex (GPIIb-IIIa), on surfaces of adherent platelets; fibrinogen binds platelets into activated aggregates (platelet aggregation) and further platelet release occurs. A self-perpetuating cycle of events is set up leading to formation of a platelet plug at the site of the injury.
Further platelet membrane receptors are exposed during aggregation, providing a surface for the interaction of coagulation factors; this platelet activity is referred to as platelet factor 3 (PF-3). The presence of thrombin encourages fusion of platelets, and fibrin formation reinforces the stability of the platelet plug. Central to normal platelet function is platelet prostaglandin synthesis, which is induced by platelet activation and leads to the formation of TXA2 in platelets. TXA2 is a powerful vasoconstrictor and also lowers cyclic AMP levels and initiates the platelet release reaction.
Prostacyclin (PGI2) is synthesized in vascular endothelial cells and opposes the actions of TXA2. It produces vasodilatation and increases the level of cyclic AMP, preventing platelet aggregration on the normal vessel wall as well as limiting the extent of the initial platelet plug after injury.
Coagulation and fibrinolysis
Coagulation involves a series of enzymatic reactions leading to the conversion of soluble plasma fibrinogen to fibrin clot (Fig. 6.26). The coagulation factors are either enzyme precursors (factors XII, XI, X, IX and thrombin) or cofactors (V and VIII), except for fibrinogen, which is the subunit of fibrin. The enzymes apart from factor XIII are serine pro teases and hydrolyse peptide bonds.
EXTRINSIC PATHWAY. Coagulation is initiated by tissue factor, which is expressed on the surface of perivascular tissue cells, coming into contact with plasma after an injury. The complex of activated factor VII and tissue factor activates factor X but its main role in vivo is to activate factor IX in the intrinsic pathway.
INTRINSIC PATHWAY. Factor XII was thought to be activated by ‘contact’ with the injured surface and then to initiate a series of reactions beginning with activation of factor XI and leading to activation of factor X. The upper part of the intrinsic pathway includes kallikrein and high molecular weight kininogen (HMWK) but recent evidence suggests that this part of the intrinsic pathway is not important for in vivo haemostasis. It is now thought that factor IX is activated by a complex of tissue factor and factor VII. Activated factor IX together with factor VIII and calcium ions activate factor X. Factor XI is activated in vivo by thrombin and only maIces an important contribution after major trauma.
Factor VIII is a complex protein consisting of a small molecule with coagulant activity (VIII:C) and a larger part, von Willebrand factor (VIII :vWF) , which is associated with platelet adhesion. VIII:C is a single chain protein with a molecular weight of about 350000. VIII :vWF a glycoprotein with a molecular weight of about 200000. It readily forms multimers in the circulation with molecular weights of up to 20 X 106. The high molecular weight multimeric forms of VIII :vWF are the most effective in producing platelet adhesion.
COMMON PA THWA Y. Activated factor X eventually leads to the conversion of prothrombin to thrombin. Thromhydrolyses bin the peptide bonds of fibrinogen, releasing fabrinopeptides A and B, and allowing polymerization between fibrinogen molecules to form fibrin. At the same time thrombin, in the presence of calcium ions, activates factor XIII, which stabilizes the fibrin clot by cross-linking adjacent fibrin molecules. The presence of thrombin helps in the activation of factors XI, V, VIII and XIII.
LIMITATION OF COAGULATION. Coagulation is limited to the site of injury by removal of activated coagulation factors by rapid blood flow at the periphery of the damage area, by plasma inhibitors of activated coagulation factors and by fibrinolysis.
Antithrombin III (AT-III) is the most potent inhibitor of coagulation; it inactivates the serine proteases by foreming stable complexes with them and its action is greatly protentiated by heparin. Active protein C is generated from its vitamin K-dependent precursor by the action of thrombin; thrombin activation of protein C is enhanced when thrombin is bound to thrombomodulin, which is an endothelial cell receptor (Fig. 6.27). Active protein C destroys factor V and factor VIII reducing further thrombin generation. Protein S is a cofactor for protein C by allowing binding of activated protein C to the platelet surface. Other natural inhibitors of coagulation are 0’2- macroglobulin, O’J-antitrypsin, O’J-antiplasmin and heparin cofactor II.
FIBRINOLYSIS, which helps to restore vessel patency, also occurs in response to vascular damage. In this system (Fig. 6.28), an inactive plasma protein, plasminogen, is converted to plasmin by plasminogen activators derived from the plasma or blood cells (intrinsic activation) or the tissues (extrinsic activation).
Plasmin is a serine protease which breaks down fibrinogen and fibrin into fragments X, Y, D and E, collectively known as fibrin (and fibrinogen) degradation products (FDPs). Degradation of cross-linked fibrin also yields D-dimer and D-dimer-E fragments. Plasmin is also capable of breaking down coagulation factors such as factors V and VIII.
The fibrinolytic system is activated by the presence of fibrin. Plasminogen is specifically adsorbed to fibrin and fibrinogen by lysine-binding sites. However, little plasminogen activation occurs in the absence of fibrin, as fibrin also has a specific binding site for plasminogen activators, whereas fibrinogen.
The most important plasminogen activator is tissuetype plasminogen
activator (t-PA); vascular endothelium is the major source of t-PA in plasma. Its release is stimulated by thrombin. Another plasminogen activator is urokinase synthesized in the kidney and released into the urogenital tract. Intrinsic plasminogen activators such as factor XlI and kallikrein are of minor physiological importance.
T- PA is inactivated by plasminogen activator inhibitor 1 . Activated protein C inactivates PAl – I and therefore induces fibrinolysis. Inactivators of plasmin (such as a2-antiplasmin) are also present in the and contribute to the regulation of fibrinolysis.