Vascular Bed

It is very important that blood not clot within the vascular system. In the baseline state, vascular endothelial cells provide a nonthrombogenic interface with the circulating blood. Endothelial cells do not normally express molecules that support platelet adhesion or promote activation and activity of the coagulation proteins. In addition, the antithrombotic features of the endothelial surface go beyond simply being “inert” with respect to coagulation. The endothelium also expresses molecules that actively downregulate the coagulation reactions on its surface: principally thrombomodulin to localize activated protein C to the endothelial surface, and heparan sulfates to localize antithrombin (AT) to the endothelial surface. A further discussion of these mechanisms is presented in the section on thrombosis. These properties are crucial in preventing coagulation from being initiated at inappropriate sites within the vasculature and preventing appropriately initiated hemostatic reactions from spreading within the vascular tree.

Extravascular Tissues

When an injury disrupts a blood vessel, it allows blood to contact extravascular cells and matrix. Extracellular matrix proteins, such as collagen, fibronectin, thrombospondin, and laminin, interact with adhesive receptors on blood platelets and support formation of the initial platelet plug at the site of injury. Perivascular tissues also express significant levels of tissue factor (TF).^1,2 Exposure of TF to blood initiates the process of thrombin generation on the surfaces of adherent platelets and ultimately leads to stabilization of the initial platelet plug in a fibrin clot (i.e., secondary hemostasis). Different tissues express different complements of matrix components and procoagulants. The tissue environment plays a role in determining the intensity of the procoagulant response to an injury.

Platelets

Membrane receptors for collagen and other subendothelial and extravascular matrix proteins are present on the platelet membrane and mediate binding of unactivated platelets at sites of injury.^3–5 Platelet binding is also mediated by von Willebrand factor (vWF) bridging between collagen and the platelet receptor glycoprotein (GP) Ib. These receptor binding events also transmit an activation signal to the platelets. Full platelet activation also requires stimulation by thrombin, however, which is produced as the coagulation reactions are initiated. The platelet surface receptor for fibrinogen, GPIIb/IIIa, rapidly changes conformation from an inactive to an active form on platelet activation.⁶ This change in conformation allows platelet aggregates to be stabilized by binding to fibrinogen even before conversion to fibrin begins. Platelet activation also initiates the synthesis of prostaglandins and thromboxanes—compounds that modulate platelet activation and promote vasoconstriction.⁷

Platelet adhesion and activation at a site of injury, in concert with local vasoconstriction, provides initial hemostasis for small caliber vessels. When hemostasis is achieved by these mechanisms, the subsequent stabilization of the platelet plug in a fibrin meshwork can proceed more effectively than if bleeding continues. Initial hemostasis may be established even if a deficiency of plasma coagulation proteins is present. The platelet plug is insufficient, however, to provide long-term hemostasis, and delayed rebleeding occurs if it is not reinforced by a stable fibrin clot during secondary hemostasis.

Coagulation Proteins

Adequate levels and function of each of a series of procoagulant proteins are required for hemostasis. The coagulation proteins can be organized into several groups based on their structural features.

The vitamin K–dependent factors include factors II (prothrombin), VII, IX, and X. These factors each have a structural domain in which several glutamic acid residues are post-translationally modified to γ-carboxyglutamic acid (Gla) residues by a vitamin K–dependent carboxylase.⁸ The vitamin K cofactor is oxidized from a quinone to an epoxide in the process. A vitamin K epoxide reductase cycles the vitamin K back to the quinone form to allow carboxylation of additional glutamic acid residues. The negatively charged Gla residues bind calcium ions. These binding interactions hold the Gla-containing proteins in their active conformation. The calcium-bound form of the Gla-domain is responsible for mediating binding of the coagulation factors to phospholipid membranes. Lipids with negatively charged head groups, particularly phosphatidylserine, are required for binding and activity of the Gla-containing factors.

The carboxylation process is inhibited by the anticoagulant warfarin, which competes with vitamin K for binding to the reductase.⁹ Inhibition by warfarin results in the production of undercarboxylated forms of the vitamin K–dependent proteins, which are nonfunctional.

The vitamin K–dependent procoagulants are zymogens (inactive precursors) of serine proteases. Each is activated by cleavage of at least one peptide bond. The activated form is indicated by the letter “a.” Factors VIIa, IXa, and Xa each require calcium ions, a suitable cell (phospholipid) membrane surface, and a protein cofactor for their activity in hemostasis.

Factor IIa (thrombin) is a little different from the activated forms of the other vitamin K–dependent factors. Its Gla domain is released from the protease domain during activation. It no longer binds directly to phospholipid membranes. It also does not require a cofactor to cleave fibrinogen and initiate fibrin assembly, or to activate platelet receptors. Factor IIa that escapes the vicinity of a hemostatic plug can bind, however, to a cofactor on endothelial cell surfaces, thrombomodulin.¹⁰ After binding to thrombomodulin, factor IIa can no longer activate platelets or cleave fibrinogen. Instead, it triggers an antithrombotic pathway by activating protein C on the endothelial surface.

Proteins C and S are also vitamin K–dependent factors. They do not act as procoagulants, but rather as antithrombotics on endothelial surfaces.¹¹ Protein C is the zymogen of a protease, whereas protein S has no enzymatic activity, but serves as a cofactor for activated protein C. The activated protein C/protein S complex cleaves and inactivates factor Va and factor VIIIa, preventing propagation of thrombin generation on normal healthy endothelium.

Factors V and VIII are large structurally related glycoproteins that act as cofactors. They have no enzymatic activity of their own, but when activated by proteolytic cleavage, they dramatically enhance the proteolytic activity of factors Xa and IXa.

Factor VIII circulates in a noncovalent complex with vWF, which prolongs its half-life in the circulation. The vWF/factor VIII complex binds to the platelet surface via GPIb as vWF mediates adhesion of platelets to collagen under high shear conditions. Cleavage and activation of factor VIII releases it from vWF so that it can assemble into a complex with factor IXa on the platelet surface, where it activates factor X.

Factor V circulates in the plasma, and it is packaged in the alpha granules of platelets. It is released on platelet activation in a partially activated form. Plasma and platelet-derived factor V can be fully activated by cleavage by factor Xa or IIa. Factor Va then assembles into a complex with factor Xa on the platelet surface, where it activates prothrombin to factor IIa.

TF is also a cofactor, but is structurally unrelated to any of the other coagulation factors. Instead, it is related to one class of cytokine receptors.¹² This lineage emphasizes the close evolutionary and physiologic links between the coagulation system and the other components of the host response to injury. Rather than circulating in the plasma as do the other coagulation factors, TF is a transmembrane protein.¹³ TF serves as the cellular receptor and cofactor for factor VIIa. It is primarily expressed on cells outside the vascular space under normal conditions, although monocytes and endothelial cells can express TF in response to inflammatory cytokines. The factor VIIa/TF complex can activate factor IX and factor X, and is the major initiator of hemostatic coagulation.¹³

Another group of related proteins are the contact factors—factors XI and XII, prekallikrein, and high-molecular-weight kininogen. These proteins share the feature of binding to charged surfaces. The only one of this group that is needed for normal hemostasis is factor XI.¹⁴ The other contact factors may play a role, however, in thrombosis in some settings. Factor XI is a zymogen that can be activated to a protease by factor XIIa, but is likely activated primarily by thrombin during the hemostatic process. Factor XIa activates factor IX.

Fibrinogen provides the key structural component of the hemostatic clot. Two small peptides, fibrinopeptides A and B, are cleaved from fibrinogen by thrombin, and the resulting fibrin monomer polymerizes into a network of fibers. The fibrin polymer is stabilized further when it is cross-linked by activated factor XIII. Factor XIIIa is a transglutaminase that is activated by thrombin coincident with fibrin formation.¹⁵

Thrombin plays a key role in activating procoagulant and anticoagulant factors and triggering formation of fibrin. In addition, thrombin has cytokine-like activities that bridge the transition between hemostasis, inflammatory/immune responses, and wound healing. Thrombin is truly a multifunctional molecule that affects the host response to injury at many levels.

Even before the structure and function of the various factors had been defined, their interactions had been studied during plasma clotting. In the 1960s, two groups proposed a “waterfall” or “cascade” model of the interactions of the coagulation factors leading to thrombin generation. These schemes were composed of a sequential series of steps in which activation of one clotting factor led to the activation of another, finally leading to a burst of thrombin generation.^16,17 At that time, each clotting factor was thought to exist as a proenzyme that was activated by proteolysis. The existence of cofactors without enzymatic activity was not recognized until later. The original models were subsequently modified as information about the coagulation factors accumulated and eventually evolved into the Y-shaped scheme shown in Figure 9-1. The “cascade” model shows distinct intrinsic and extrinsic pathways that are initiated by factor XIIa and the factor VIIa/TF complex. The pathways converge on a “common” pathway at the level of the factor Xa/factor Va (prothrombinase) complex.

Figure 9-1 The extrinsic and intrinsic pathways in the modern cascade model of coagulation. These two pathways are conceived as each leading to formation of the factor Xa/Va complex, which generates thrombin (IIa). Lipid/Ca indicates that the reaction requires a phospholipid surface and calcium ions. These pathways are assayed clinically using the prothrombin time (PT) and activated partial thromboplastin time (aPTT). HK, high-molecular-weight kininogen; PK, prekallikrein.

This scheme was not proposed as a literal model of the hemostatic process in vivo; rather, it was derived from studies of plasma clotting in a test tube and was intended to represent the biochemical interactions of the procoagulant factors. The coagulation “cascade” does reflect well the process of plasma clotting, as in the PT and aPTT tests. The lack of any other clear and predictive concept of hemostasis has meant, however, that until more recently most physicians have also viewed the “cascade” as a model of physiology, and the PT and aPTT as reflecting the risk of clinical bleeding.

The limitations of the coagulation cascade as a model of the hemostatic process in vivo are highlighted by certain clinical observations. Patients deficient in the initial components of the intrinsic pathway—factor XII, high-molecular-weight kininogen, or prekallikrein—have a greatly prolonged aPTT, but no bleeding tendency. Patients deficient in factor XI also have a prolonged aPTT, but usually have a mild to moderate bleeding tendency. Other components of the intrinsic pathway have a crucial role in hemostasis because patients deficient in factor VIII or factor IX have a serious bleeding tendency even though the extrinsic pathway is intact. Similarly, patients deficient in factor VII also have a serious bleeding tendency even though the intrinsic pathway is intact. Although the cascade model accurately reflects the protein interactions that lead to plasma clotting, and is an essential guide to interpretation of PT and aPTT results, it is not an adequate model of hemostasis in vivo.

The numbering of the coagulation factors does not follow their order in the cascade. The coagulation factors were numbered roughly in the order in which they were discovered. Because many workers had described the same molecules under different names, designating them with roman numerals seemed the fairest way to reconcile the nomenclature confusion.¹⁸

Step 1: Initiation of Coagulation on Tissue Factor–Bearing Cells

The process of thrombin generation is initiated when TF-bearing cells are exposed to blood at a site of injury. TF is a transmembrane protein that acts as a receptor and cofactor for factor VII. When bound to TF, zymogen factor VII is rapidly converted to factor VIIa through mechanisms not yet completely understood, but that may involve factor Xa or noncoagulation proteases. The resulting factor VIIa/TF complex catalyzes activation of factor X and activation of factor IX. The factors Xa and IXa formed on TF-bearing cells have very distinct and separate functions in initiating blood coagulation.²⁰ The factor Xa formed on TF-bearing cells interacts with its cofactor, factor Va, to form prothrombinase complexes and generate small amounts of thrombin on the TF cells (Fig. 9-2). The small amounts of factor Va required for prothrombinase assembly on TF-bearing cells are activated by factor Xa,²¹ activated by noncoagulation proteases produced by the cells,²² or released from platelets that adhere nearby. The activity of the factor Xa formed by the factor VIIa/TF complex is largely restricted to the TF-bearing cell because factor Xa that dissociates from the cell surface is rapidly inhibited by tissue factor pathway inhibitor (TFPI) or AT in the fluid phase.

Figure 9-2 The initiation step in a cell-based model of hemostasis. Initiation occurs on the tissue factor (TF)–bearing cell as activated factor X combines with its cofactor, factor Va, to activate small amounts of thrombin.

Related posts:

Evolution of the Coronary Care Unit: Past, Present, and Future Vascular Complications after Percutaneous Coronary Intervention Diagnosis and Treatment of Complications of Coronary and Valvular Interventions Pharmacologic Interactions in the CICU Acute Aortic Syndromes: Diagnosis and Management Pacemaker and Implantable Cardioverter-Defibrillator Emergencies

Stay updated, free articles. Join our Telegram channel

Join