The circulatory system must be self-sealing; otherwise continued blood loss from even the smallest injury would be life threatening. Normally, all but the most catastrophic bleeding is rapidly stopped, by a process known as Coagulation, through several sequential processes. Coagulation involves the formation of a blood clot (thrombus) that prevents further blood loss from damaged tissues, blood vessels or organs. This is a complicated process with a cellular system comprised of cells called platelets that circulate in the blood and serve to form a platelet plug over damaged vessels and a second system based upon the actions of multiple proteins (called clotting factors) that act in concert to produce a Fibrin clot. These two systems work in concert to form a clot; disorders in either system can yield disorders that cause either too much or too little clotting (Ref.1 & 2).
The substances involved in blood coagulation are normally present in the blood stream or tissues, but remain passive until some stimulus convert them to their active form. The protein coagulation system is dynamically entwined with the platelet and vascular system in the formation of the clot and linked with the Fibrinolytic system in dissolution and limitation of clot. The liver synthesizes a series of inactive glycoproteins (the coagulation system) that circulate in the blood and lead to the production of Fibrin. Activation of one of these clotting factors sequentially activate another in a series of reactions, ultimately yielding cross-linked Fibrin. Fibrin formation in cellular material produces clot. The coagulation factors are designated by roman numerals I to XIII and receive an appended “a” to indicate the activated form. The coagulation proteins form a cascade very similar to a chain reaction. When one coagulation factor becomes activated it activates the next factor in a reaction sequence, i.e., the coagulation cascade, which culminates in Fibrin formation. This chain reaction does not occur in the sequential order suggested by I to XIII. The numbers refer to the order in which the factors were discovered, not to the order in which they react (Ref.1, 2 & 3).
Coagulation has often been represented as two somewhat independent pathways that converge to a common pathway, with Thrombin generation as the end point of the reactions. When a break in a blood vessel occurs, substances are exposed that normally are not in direct contact with the blood flow. These substances (primarily Collagen and von Willebrand factor) allow the platelets to adhere to the broken surface. Once a platelet adheres to the surface, it releases chemicals that attract additional platelets to the damaged area, referred to as platelet aggregation. These two processes are the first responses to stop bleeding. The protein based system (the Coagulation Cascade) serves to stabilize the clot that has formed and further seal up the wound. The coagulation pathway is a proteolytic cascade. Each enzyme of the pathway is present in the plasma as a Zymogen (in an inactive form), which on activation undergoes proteolytic cleavage to release the active factor from the precursor molecule. The coagulation cascade functions as a series of positive and negative feedback loops which control the activation process. The ultimate goal of the pathway is to produce Thrombin, which can then convert soluble Fibrinogen into Fibrin, that forms a clot. The process of generation of Thrombin can be divided into three phases: the Intrinsic and Extrinsic pathways which provide alternative routes for the generation of an active clotting factor: FXa (Activated Factor-X), and the final Common pathway which results in Thrombin formation (Ref.2 & 4).
The Intrinsic pathway is activated when blood comes into contact with sub-endothelial connective tissues or with negatively charged surface that are exposed as a result of tissue damage. Quantitatively it is more important of the two pathways, but is slower to cleave Fibrin than the Extrinsic pathway. The Hageman factor/FXII (Factor-XII), FXI (Factor-XI), PKlk (Prekallikrein), and HK (High Molecular Weight Kininogen) are involved in the activation of this pathway. The first step is the binding of FXII to a sub-endothelial surface exposed by an injury. A complex of PK and HK also interacts with the exposed surface in close proximity to the bound FXII , which becomes activated. During activation, the single chain protein of the native Hageman factor is cleaved into two chains (50 and 28 kDa), that remain linked by a disulphide bond. The light chain (28kDa) contains the active site and the molecule is referred to as activated Hageman factor/ FXII a (Activated Factor-XII). There is evidence that the FXII can autoactivate and FXII a can also activate PK, thus making the pathway self-amplifying, once triggered. The Klk (Kallikrein) produced can then cleave FXII , and a further amplification mechanism is triggered. The FXII a remains in close contact with the activating surface, such that it can activate FXI (Factor-XI), the next step in the Intrinsic pathway which, to proceed efficiently, requires Ca2+ (Calcium). Also involved at this stage is HK, which binds to FXI and facilitates the activation process. Activated factors FXIa , FXIIa, and Kallikrein are all serine proteases. In the presence of Ca2+, FXIa activates FIX to FIXa. FIX is a proenzyme that contains vitamin K-dependent Gamma-Carboxyglutamate residues, whose serine protease activity is activated following Ca2+ binding to these Gamma-Carboxyglutamate residues. FXIa converts FX to FXa; to do this it requires: Calcium ions, FVIIIa (Factor-VIIIa), and a negatively charged surface; in vivo, this is the surface of platelets, i.e., PL (Phospholipids) (Ref.4 & 5).
Eventually the Intrinsic pathway activates FX , a process that can also be brought about by the Extrinsic pathway, an alternative route for the activation of the clotting cascade. It provides a very rapid response to tissue injury, generating FX a almost instantaneously, compared to the seconds or even minutes required for the Intrinsic pathway to activate FX . The main function of the Extrinsic pathway is to augment the activity of the Intrinsic pathway. The Extrinsic pathway takes place at the site of a break in a blood vessel that has the platelet aggregate. There are two components unique to the Extrinsic pathway: TF/FIII(Factor-III), and FVII. TF and FVIIa activate FX , forming FXa. TF is present in most human cells bound to the cell membrane. Once activated, TF binds rapidly to FVII which is then activated to form a complex of TF , FVIIa, Calcium, and a Phospholipid, and this complex then rapidly activates FX (Ref.6 & 7).
The Intrinsic and Extrinsic systems converge at FXa to a single Common pathway which is ultimately responsible for the production of Thrombin (Factor-IIa). Thrombin converts Fibrinogen (Factor-I) to Fibrin (Factor-Ia). In addition, Thrombin also acts to enhance the activation of factor: FV, F8 and FXI in a feedback manner. Fibrinogen is a dimer soluble in plasma. Exposure of Fibrinogen to Thrombin results in its rapid proteolysis that results in the release of Fibrinopeptide-A. The loss of small Peptide-A is not sufficient to render the resulting Fibrinn molecule insoluble, a proces that is required for clot formation, but it tends to form complexes with adjacent Fibrin and Fibrinogen molecules. A second peptide, Fibrinopeptide-B, is then cleaved by Thrombin, and the Fibrin monomers formed by this second proteolytic cleavage polymerize spontaneously to form an insoluble gel. The polymerized Fibrin, held together by noncovalent and electrostatic forces, is stabilized by the transamidating enzyme FXIIIa (Factor-XIIIa), produced by the action of Thrombin on FXIII . These insoluble Fibrin aggregates (Clots), together with aggregated platelets (Thrombus), block the damaged blood vessel and prevent further bleeding (Ref.2, 3 & 8).
Precise control of the blood-clotting system is essential for maintenance of the circulation in all higher animals. Deficient function of this system can lead to fatal bleeding following even a minor injury, whereas overactivity of this system can produce unwanted blood clots, resulting in blockages to critical blood vessels, as occurs in such diseases as heart attack and stroke. Deficiency of one or more clotting factors may lead to Hemophilia (deficiency of FVIII and/or FIX), a rare inherited bleeding disorder in which the blood does not clot normally. Deficiency of FX is one of the rarest congenital coagulation disorders, resulting in a variable clinical phenotype. In contrast, a blood clot becomes harmful when it can block an artery or vein to stop the blood flow. A clot in a brain artery can cause a stroke to occur. A thrombus blocking an artery in the heart can cause a heart attack. A thrombus in the leg or pelvic vein is called a DVT (Deep Vein Thrombosis) (Ref.9 & 10).