Secondary hemostasis is defined as the formation of fibrin through the coagulation cascade. This involves circulating coagulation factors, which act as enzymes [which require activation] and cofactors (factors V and VIII), calcium and platelets (platelets provide a source of phospholipid [PF3] and a binding surface upon which the coagulation cascade proceeds). Defects in the coagulation cascade manifest as more serious bleeding than primary hemostasis, including bleeding into cavities (chest, joints) and subcutaneous hematomas. Petechial hemorrhages are not seen in disorders of secondary hemostasis. These disorders do share common bleeding symptoms with defects in primary hemostasis, including epistaxis and bleeding after surgery or wounds (the latter are often the first indication of a hemostatic defect), which makes it impossible to distinguish between defects in these 2 pathways based on clinical signs alone.
The coagulation cascade is traditionally separated into 3 pathways: intrinsic, extrinsic and common pathways.
A diagram of the coagulation cascade may be useful for understanding the complex relationships between these various pathways.
Initiation of coagulation: The coagulation cascade is initiated by the extrinsic pathway with the generation/exposure of tissue factor (factor III). Tissue factor is expressed by endothelial cells, subendothelial tissue and monocytes, with expression being upregulated by cytokines (TNFalpha, IL-6). Tissue factor then binds to factor VII and this complex activates factor X. Factor X, in the presence of factor V, calcium and platelet phospholipid ("prothrombinase complex") then activate prothrombin to thrombin. This pathway is rapidly inhibited by a lipoprotein-associated molecule, called tissue factor pathway inhibitor. However, the small amount of thrombin generated by this pathway (before inhibition) activates factor XI of the intrinsic pathway, which amplifies the coagulation cascade.
Amplification of coagulation: The coagulation cascade is amplified by the small amounts of thrombin generated by the extrinsic pathway. This thrombin activates the intrinsic pathway by activation of factors XI and VIII. Activated factor IX, together with activated factor VIII, calcium and phospholipid ("tenase complex"), amplify the activation of factor X, generating large amounts of thrombin. Thombin, in turn, then cleaves fibrinogen to form soluble fibrin monomers, which then spontaneously polymerize to form the soluble fibrin polymer. Thrombin also activates factor XIII, which, together with calcium, serves to crosslink and stabilize the soluble fibrin plymer, forming crosslinked (insoluble) fibrin.
Alternate pathway: A second route of stimulation of the intrinsic pathway (called the alternate pathway) is the direct activation of factor IX by the tissue factor-factor VII complex. However, this is a minor pathway and the major stimulator of the intrinsic pathway is thrombin, through activation of factor XI.
The contact pathway: The contact pathway consists of prekallikrein (Fletcher factor), high-molecular weight kininogen (Williams, Fitzgerald factor) and factor XII (Hageman factor). The contact system has important anticoagulant, profibrinolytic and proinflammatory roles and has minimal influence on the coagulation cascade (by the factor XIIa-mediated activation of factor XI).
Prekallikrein circulates in a 1:1 complex with high-molecular weight kininogen. This complex assembles with factor XII on the surface of cell membranes (the so-called negative surface for activation). Activation of factor XII by this assembly onto the surface of platelets, endothelial cells and granulocytes, converts prekallikrein into kallikrein, a potent enzyme. Kallikrein amplifies the activation of factor XII and converts high-molecular weight kininogen into bradykinin and is, itself, a neutrophil chemotaxin. Bradykinin is an important vascular mediator, causing vasodilation, increased vascular permeability, and vascular smooth muscle growth and proliferation. Thus in an injured vessel, bradykinin serves to stimulate vessel repair. In addition, bradykinin has an antithrombotic role, by inhibiting thrombin-mediated platelet aggregation. The contact system is also a strong activator of fibrinolysis. Kallikrein and factor XIIa can convert plasminogen directly into plasmin and bradykinin is the most potent and specific stimulator of tissue plasminogen activator release from endothelial cells. Thus the contact pathway is more important for activation of fibrinolysis and inflammation and modulation of vascular biology than it is for hemostasis.
The complex interplay between the different processes of hemostasis is illustrated by the participation of platelets in both primary (formation of the platelet plug) and secondary hemostasis (provision of a phospholipid surface necessary for coagulation), by the role of factor XII in the stimulation of fibrinolysis (rather than initiation of coagulation), and by the importance of thrombin as an activator of platelets, amplifier of the coagulation cascade, and inhibitor of fibrinolysis.
Thrombin plays a pivotal role in hemostasis, one that cannot be over-emphasized, by functioning both as a thrombotic and antithrombotic molecule. As a thrombotic molecule, thrombin promotes coagulation in several ways, the most obvious being the generation of fibrin from fibrinogen. Fibrin generation relies upon large amounts of thrombin, which requires the complex interaction between the intrinsic and extrinsic pathways of coagulation (as described above). In addition, thrombin is a platelet agonist, promoting platelet aggregation, and also activates factors V and VIII, which are necessary cofactors for the "prothrombinase" and "tenase" complexes, respectively. Thrombin also activates factor XIII, which is essential for the crosslinking of the fibrin polymer to produce a stable, 3-dimensional fibrin lattice. Another mechanism by which thrombin acts as a thrombotic molecule is by inhibition of fibrinolysis by generation of a thrombin-activatable fibrinolytic inhibitor (TAFI). As an antithrombotic molecule, thrombin, conversely, acts as its own inhibitor. Thrombin binding to thrombomodulin on endothelial cells activates anticoagulant proteins C and S. These proteins inhibit activation of factors V and VIII, which limits thrombin generation (see Inhibitors for more information on TAFI, and proteins C and S).