Normal hemostasis is a consequence of tightly regulated processes that maintain blood in a fluid, clot-free state in normal vessels while inducing the rapid formation of a localized hemostatic plug at the site of vascular injury. The pathologic form of hemostasis is thrombosis; it involves blood clot (thrombus) formation in uninjured vessels or thrombotic occlusion of a vessel after relatively minor injury. Both hemostasis and thrombosis involve three components: the vascular wall, platelets, and the coagulation cascade. We begin our discussion with the process of normal hemostasis and a description of its regulation.
Normal Hemostasis
The sequence of events in hemostasis at a site of vascular injury is shown in Figure 4-6. After initial injury a brief period of arteriolar vasoconstriction occurs mostly as a result of reflex neurogenic mechanisms and is augmented by the local secretion of factors such as endothelin (a potent endothelium-derived vasoconstrictor; . The effect is transient, and bleeding would resume were it not for activation of the platelet and coagulation systems.
Endothelial injury also exposes highly thrombogenic subendothelial extracellular matrix, allowing platelets to adhere and be activated. Activation of platelets results in a dramatic shape change (from small rounded disks to flat plates with markedly increased surface area) and release of secretory granules. Within minutes the secreted products have recruited additional platelets (aggregation) to form a hemostatic plug; this is the process of primary hemostasis
Tissue factor is also exposed at the site of injury. Also known as factor III and thromboplastin, tissue factor is a membrane-bound procoagulant glycoprotein synthesized by endothelium. It acts in conjunction with factor VII (see below) as the major in vivo pathway to activate the coagulation cascade, eventually culminating in thrombin generation. Thrombin cleaves circulating fibrinogen into insoluble fibrin, creating a fibrin meshwork deposition. Thrombin also induces further platelet recruitment and granule release. This secondary hemostasis sequence lasts longer than the initial platelet plug.
Polymerized fibrin and platelet aggregates form a solid permanent plug to prevent any additional hemorrhage. At this stage counter-regulatory mechanisms (e.g., tissue plasminogen activator, t-PA) are set into motion to limit the hemostatic plug to the site of injury .
The following sections discuss these events in greater detail.
Endothelium
Antithrombotic Properties
Under most circumstances, endothelial cells maintain an environment that promotes liquid blood flow by blocking platelet adhesion and aggregation, by inhibiting the coagulation cascade, and by lysing blood clots.
Antiplatelet Effects
An intact endothelium prevents platelets (and plasma coagulation factors) from interacting with the highly thrombogenic subendothelial ECM. Nonactivated platelets do not adhere to the endothelium, a property intrinsic to the plasma membrane of endothelium. Moreover, if platelets are activated (e.g., after focal endothelial injury), they are inhibited from adhering to the surrounding uninjured endothelium by endothelial prostacyclin (PGI2) and nitric oxide . Both mediators are potent vasodilators and inhibitors of platelet aggregation; their synthesis by endothelial cells is stimulated by several factors (e.g., thrombin and cytokines) produced during coagulation. Endothelial cells also elaborate adenosine diphosphatase, which degrades adenosine diphosphate (ADP) and further inhibits platelet aggregation .
Anticoagulant Effects
Anticoagulant effects are mediated by membrane-associated, heparin-like molecules and thrombomodulin. The heparin-like molecules act indirectly; they are cofactors that allow antithrombin III to inactivate thrombin, factor Xa, and several other coagulation factors (see later). Thrombomodulin also acts indirectly; it binds to thrombin, converting it from a procoagulant to an anticoagulant capable of activating the anticoagulant protein C. Activated protein C, in turn, inhibits clotting by proteolytic cleavage of factors Va and VIIIa; it requires protein S, synthesized by endothelial cells, as a cofactor.
Fibrinolytic Properties
Endothelial cells synthesize tissue plasminogen activator (t-PA), promoting fibrinolytic activity to clear fibrin deposits from endothelial surfaces .
Prothrombotic Properties
While endothelial cells exhibit properties that usually limit blood clotting, they can also become prothrombotic, with activities that affect platelets, coagulation proteins, and the fibrinolytic system. Endothelial injury results in platelet adhesion to subendothelial collagen; this occurs through von Willebrand factor (vWF), an essential cofactor for binding platelets to collagen and other surfaces. vWF (both circulating and collagen bound) is synthesized largely by normal endothelium. Loss of endothelium exposes previously deposited vWF and allows circulating vWF to also bind to the basement membrane; in quick order, platelets adhere via their glycoprotein Ib (GpIb) receptors .
Cytokines such as tumor necrosis factor (TNF) or interleukin-1 (IL-1) as well as bacterial endotoxin all induce endothelial cell production of tissue factor; as we will see below, tissue factor activates the extrinsic clotting pathway. By binding activated IXa and Xa , endothelial cells augment the catalytic activities of these coagulation factors. Finally, endothelial cells also secrete plasminogen activator inhibitors (PAIs), which depress fibrinolysis).
PlateletsPlatelets play a critical role in normal hemostasis. When circulating and nonactivated they are membrane-bound smooth disks expressing several glycoprotein receptors of the integrin family and containing two types of granules:
α-Granules express the adhesion molecule P-selectin on their membranes and contain fibrinogen, fibronectin, factors V and VIII, platelet factor 4 (a heparin-binding chemokine), platelet-derived growth factor (PDGF), and transforming growth factor α (TGF-α).Dense bodies, or δ granules, contain adenine nucleotides (ADP and ATP), ionized calcium, histamine, serotonin, and epinephrine.
After vascular injury, platelets encounter ECM constituents (of which collagen is the most important) and additional proteins (vWF being critical) that are normally not exposed when the endothelial layer is intact. Upon contact with these proteins, platelets undergo three reactions: (1) adhesion and shape change, (2) secretion (release reaction), and (3) aggregation .