• Coagulation is the final and most definitive process of hemostasis and allows for the creation of a highly stable, long-lived clot. The entire process of coagulation is directed toward creating fibrin, a highly fibrous protein that essentially forms a mesh, entrapping blood cells and platelets, creating an unyielding gel-like substance that can prevent blood loss from large tears in the vasculature. A highly regulated cascade of events is required for formation of fibrin and here we discuss the basic architecture and components of this pathway. A schematic understanding of coagulation is critical for any student as disorders of coagulation are a frequently encountered issue in nearly all areas of medicine.
The Coagulation Factors
  • Factor Basics
    • The factors of the coagulation cascade are enzymes that are synthesized with an inhibitory domain that normally blocks their enzymatic activity, allowing them to float through the blood without initiating a clot. When coagulation is triggered, the inhibitory domains of the factors at the top of the cascade are cleaved, activating their enzymatic activity. These factors in turn cleave the inhibitory domains of their immediate downstream coagulation factor, and so on, generating a rapidly expanding cascade of factor activation that ultimately leads to generation of fibrin.
  • Synthesis
    • Nearly all of the coagulation factors (except von Willibrand Factor) are synthesized by the liver. The hepatic synthesis of four coagulation factors, Factor II (Thrombin), VII, IX, and X, require the presence of Vitamin K. These facts help explain why liver disease and Vitamin K deficiency yield defects in blood clotting. Additionally, the enzymatic activity of several clotting factors requires the presence of ionized calcium and explains why chelation of calcium by citrate in blood draw tubes prevents blood from clotting.
Basic Architecture
  • Intrinsic and Extrinsic Pathways
    • The identification of coagulation factors and their inter-relationship is indeed a triumph of early biochemistry. These studies largely used in vitro systems to initiate clotting in test tubes. Two basic pathways were identified, one which was initiated by contact of blood with glass itself, and thus termed the "Intrinsic Pathway", and the other which required the extrinsic addition of additional factors, and thus termed the "Extrinsic Pathway". Further studies have shown significant cross-talk between these two pathways when clots are actually initiated in the vasculature; however, this basic division of coagulation into two pathways has remained in use.
  • Final Common Pathway
    • However the coagulation cascade is organized, the final key step is generation of fibrin which is created by cleavage of the precursor fibrinogen, a soluble protein that is present at high concentration within plasma. The enzyme which catalyzes this reaction is "Thrombin" (Factor II) which is itself the active form of the precursor prothrombin. Prothrombin is cleaved to thrombin by activated Factor X and activation of Factor X can occur either by the intrinsic or extrinsic pathways. Therefore, activation of Factor X can be thought of as the point at which the extrinsic and intrinsic pathways of coagulation ultimately converge.
  • Coagulation Inhibitors and Fibrinolysis
    • Given the potentially disastrous consequences of inappropriate clotting, it is unsurprising that a variety of inhibitory factors exist which serve to prevent inappropriate coagulation and extensive spreading of an initiated clot. Additionally, fibrinolytic mechanisms exist to breakdown clots once they have formed in order to recanalize blocked vasculature.
Intrinsic Pathway
  • Initiation
    • The intrinsic pathway was so named because it could be initiated simply by contact of whole blood with the glass wall of the test tube. Physiologically, the intrinsic pathway appears to be directly initiated by exposure of collagen in the vascular wall following trauma. This exposure of collagen appears to directly activate Factor XII, also known as Hageman Factor, which then sets off the subsequent cascade.
  • Cascade
    • The intrinsic pathway then proceeds as follows: Factor XII -> Factor XI -> Factor IX -> Factor X. It should be pointed out that activation of Factor X by Factor IX is enhanced by Factor VIII and the presence of phospholipid complexes provided by activated platelets (See Platelet Plugging).
Extrinsic Pathway
  • Initiation
    • The extrinsic pathway is triggered by the presence of Tissue Factor a lipoprotein complex that is released from injured vasculature. Because this pathway requires the "extrinsic" introduction of Tissue Factor for activation, even in a test tube, the pathway received the "Extrinsic" namesake.
  • Cascade
    • Tissue Factor in combination with Factor VII activates Factor X which in turn converts prothrombin to activated thrombin.
Final Common Pathway
  • Activation of Factor X
    • The Intrinsic and Extrinsic arms of coagulation converge on activation of Factor X. In the case of the extrinsic arm, this is done by a complex of Tissue Factor and Factor VII, while in the intrinsic arm this is done by a complex of Factor IX and Factor VIII. Once activated, Factor X converts Prothrombin to Thrombin. However, it is important to note that the activity of Factor X is enhanced by two major entities: Factor V and activated platelets
    • First, activated Factor V is a major accelerator of conversion of prothrombin to thrombin by Factor X. Interestingly, Factor V is activated by thrombin itself. This feature allows for significant auto-acceleration of thrombin generation as follows: Initially, Factor V is inactive and so the capacity of Factor X to generate thrombin is limited; however, as thrombin accumulates it generates activated Factor V which in turn enhances the capacity of Factor X to generate large amounts of thrombin.
    • Secondly, the phospholipid complex generated by platelet plugging significantly accelerates generation of thrombin by Factor X. It does so by providing a scaffold for these factors to organize and thus increasing their localized concentrations. This not only increases the rate of coagulation but in turn limits extension of clotting into surrounding healthy tissue.
  • Generation of Fibrin by Thrombin
    • Activated thrombin ultimately converts fibrinogen to fibrin. Generation of fibrin is the ultimate output of the coagulation cascade as fibrin acts as a fibrous mesh that entraps platelets and other blood elements to plug the vascular defect.
Coagulation Inhibition
  • Overview
    • Activation of the coagulation cascade also initiates a number of mechanisms which inhibit several key steps within the cascade. These inhibitory factors help ensure that clots remain relatively localized and do not spread into adjacent healthy tissue. Interestingly, patients with inherited defects in many of these inhibitory proteins are highly hypercoagulable and display chronic recurrent thromboses.
  • Protein C and S
    • Protein C and S are two hepatically-synthesized, Vitamin K-dependent proteins that inactivate Factor VIII and Factor V. Inherited mutations of either Protein C or S can lead to hypercoagulability. Also of note, some patients possess a single amino acid mutation in Factor V, known as Factor V Leiden Mutation, which renders the Factor V protein impervious to inactivation by Protein C and S. Factor V Leiden represents one of the most common inherited causes of hypercoagulability.
  • Antithrombins and Heparin
    • Antithrombins are soluble proteins that bind to and inactivate thrombin as well as components of the intrinsic pathway. Mutation of the most important antithrombin, Antithrombin III, is a potential cause of heritable hypercoagulability. The activity of Antithrombin III is massively enhanced when bound by a soluble, negatively charged polysaccharide known as heparin. Although the physiological role of heparin is likely negligible, this molecule is a powerful anticoagulant when used in pharmacological doses.
  • Once acute bleeding has been stopped by the coagulation cascade, mechanisms are slowly engaged to break down the clot in order to re-establish blood flow. The primary mechanism by which occurs is by cleaving of fibrin. This cleavage results in release of fibrin split products, known as D-dimers, which can be detected in the blood as indicators of recent clotting.
  • The most important fibrinolytic enzyme is plasmin which is derived from a soluble precursor protein, plasminogen, that becomes trapped within clots during the clotting process. This trapped plasminogen is converted to plasmin by the enzyme Tissue Plasminogen Activator (tPA), a protein synthesized by the endothelium several days following clot formation. This physiological process allows breakdown of clots a few days after acute bleeding is stopped. Pharmacological administration of tPA is now used as a last resort to break down catastrophic clots that are blocking critical arteries as occurs in strokes and myocardial infarctions.