Basic Coagulation Pathways

Hemostasis is generally initiated by damage to the vessel wall and disruption of the endothelium, but in venous thrombosis (VT), it may be initiated in the absence of vessel wall damage. Both arterial thrombosis and VT converge on platelets, tissue factor (TF), and thrombin (Figure 1). However, thrombosis in the arterial system occurs somewhat differently than in the venous system. The elements required for initiating VT were described by Virchow as stasis, endothelial injury, and hypercoagulability of the blood. In the arterial circulation, endothelial injury (whether acute or chronic) is key to thrombosis. This is most clearly demonstrated by the typical atherosclerotic plaque. In advanced lesions, the lipid core of the plaque is rich in inflammatory cells, cholesterol crystals, and TF (generated by activated macrophages within the plaque). Plaque ulceration exposes highly thrombogenic lipid to the blood stream, activating coagulation and platelet aggregation and leading to the deposition of clot. A platelet-rich thrombus is observed in arterial thrombus. In contrast, venous stasis and changes in blood composition (leading to hypercoagulability) can incite the formation of thrombus from local procoagulant events, including small endothelial disruptions at venous confluences, saccules, and valve pockets.

The adhesion of platelets to exposed subendothelial collagen is the first step in the formation of an effective hemostatic platelet plug, resulting in platelet activation. This interaction is mediated by von Willebrand factor (vWF), whose platelet receptor is glycoprotein (Gp) Ib. Similarly, fibrinogen forms bridges between platelets by binding to the GpIIb/IIIa receptor, resulting in platelet aggregation. Activation of platelets also leads to the release of the prothrombotic contents of platelet granules containing receptors for coagulation factors Va and VIIIa, as well as fibrinogen, vWF, and adenosine diphosphate (ADP). Platelet activation also leads to the elaboration of arachidonic acid metabolites such as thromboxane A₂, further promoting platelet aggregation (as well as vasoconstriction). Platelet shape changes result in exposure of negatively charged procoagulant phospholipids normally located within caveolae of the platelet membrane. Platelets also release microparticles (MPs) rich in TF and other procoagulants, which accelerate and concentrate the thrombus generation to a site of injury.

One pathway of thrombosis begins with protein disulfide isomerase de-encryption of TF, which activates factor VII (VIIa). The TF–VIIa complex then activates factors IX and X to IXa and Xa in the presence of calcium. Feedback amplification occurs because VIIa, IXa, and Xa are all capable of activating VII to VIIa, especially when bound to TF. Factor Xa is also capable of activating factor V to Va. Factors Xa, Va, and II form on the platelet phospholipid surface in the presence of Ca²⁺ to initiate the prothrombinase complex, which catalyzes the formation of thrombin from prothrombin. Thrombin feedback amplifies the system not only by activating factor V to Va but also by activating factors VIII (normally circulating bound to vWF) to VIIIa and XI to XIa. After activation, factor VIIIa dissociates from vWF and assembles with factors IXa and X on the platelet surface in the presence of Ca²⁺ to form a complex called the Xase complex, which catalyzes the activation of factor X to Xa.

Thrombin (factor II) is central to coagulation by its action of cleavage and release of fibrinopeptide A (FPA) from the α chain of fibrinogen and fibrinopeptide B (FPB) from the β chain of fibrinogen. This causes fibrin monomer polymerization and cross linking, stabilizing the thrombus and the initial platelet plug. Thrombin also activates factor XIII to XIIIa, which catalyzes this cross-linking of fibrin as well as that of other plasma proteins, such as fibronectin and α₂-antitrypsin, resulting in their incorporation into the clot and increasing resistance to thrombolysis. In addition, factor XIIIa activates platelets as well as factors V and VIII, further amplifying thrombin production.

Coagulation can also occur through the intrinsic pathway, with activation of factor XI to XIa, which subsequently converts factor IX to IXa, promoting formation of the Xase complex and ultimately thrombin. Another mechanism by which this occurs in vitro is through the contact activation system, in which factor XII (Hageman factor) is activated to XIIa when complexed to prekallikrein and high-molecular-weight kininogen (HMWK) on a negatively charged surface; factor XIIa then activates factor XI to XIa. Thrombin and factor XIa are also capable of activating factor XI. This pathway is probably not as physiologically important in the venous system as it is in the arterial system.

Natural Anticoagulants Balance Thrombosis

Physiologic anticoagulants balance thrombin formation and localize thrombotic activity to sites of vascular injury. Antithrombin (AT) is a central anticoagulant protein that binds to thrombin and interferes with coagulation by four major mechanisms. First, inhibition of thrombin prevents the removal of FPA and FPB from fibrinogen, limiting fibrin formation. Second, thrombin becomes unavailable for factor V and VIII activation, slowing the coagulation cascade. Third, thrombin-mediated platelet activation and aggregation is inhibited. Fourth, AT has been shown to directly inhibit factors VIIa, IXa, Xa, XIa, and XIIa. Thus, patients with a genetic deficiency of AT are at much higher risk to develop venous thromboembolism (VTE) than the normal population.

Another natural anticoagulant mechanism is activated protein C (APC), which is produced on the surface of intact endothelium when thrombin binds to its receptor, thrombomodulin (TM), and endothelial protein C receptor. The thrombin–TM complex inhibits the actions of thrombin and also activates protein C to APC. APC in the presence of its cofactor protein S inactivates factors Va and VIIIa, thereby reducing Xase and prothrombinase activity.

The third innate anticoagulant is tissue factor pathway inhibitor (TFPI). This protein binds the TF–VIIa complex, inhibiting the activation of factor X to Xa and formation of the prothrombinase complex.

Physiologic Thrombolysis

Thrombus formation is balanced by controlled thrombolysis to prevent pathologic intravascular thrombosis as well as to resolve formed thrombus. The central fibrinolytic enzyme is plasmin, a serine protease generated by the proteolytic cleavage of the proenzyme plasminogen. Its main substrates include fibrin, fibrinogen, and other coagulation factors. Plasmin also interferes with vWF-mediated platelet adhesion by proteolysis of GpIb.

Activation of plasminogen occurs by several mechanisms. In the presence of thrombin, vascular endothelial cells produce and release tissue plasminogen activator (tPA) as well as α₂-antiplasmin, a natural inhibitor of excess fibrin-bound plasmin. As a clot is formed, plasminogen, tPA, and α₂-antiplasmin become incorporated into the fibrin clot. In contrast to free circulating tPA, fibrin-bound tPA is an efficient activator of plasminogen.

A second endogenous activator of plasminogen is through the urokinase-type plasminogen activator (uPA), which is also produced by endothelial cells but has less affinity for fibrin. Although the process is not completely understood, it is hypothesized that plasmin in small amounts (produced through tPA) activates uPA, leading to further plasminogen activation and amplification of fibrinolysis.

The third mechanism of plasminogen activation involves factors of the contact activation system; activated forms of factors XII, kallikrein, and XI can each independently convert plasminogen to plasmin. These activated factors can also catalyze the release of bradykinin from HMWK, which further augments tPA secretion.

To contain and localize thrombolysis, plasminogen activation is inhibited. In plasma, plasminogen activator inhibitor 1 (PAI-1) is the primary inhibitor of the plasminogen activators. It is secreted in an active form from liver and endothelial cells and stabilized by binding to vitronectin (and inhibits thrombin in this form). PAI-1 is stored in the alpha granules of quiescent platelets. PAI-1 levels are elevated by hyperlipidemia, and PAI-1 elevation appears to synergize with factor V Leiden genetic abnormalities.