The intrinsic pathway
The extrinsic pathway
The blood clotting system or coagulation pathway, like the complement system, is a proteolytic cascade. Each enzyme of the pathway is present in the plasma as a zymogen (inactive form), which on activation undergoes proteolytic cleavage to release the active factor from the precursor molecule. The coagulation pathway 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, which forms a clot. The generation of thrombin can be divided into three phases, the intrinsic and extrinsic pathways, which provide alternative routes for the generation of factor X, and the final common pathway, which results in thrombin formation
The intrinsic pathway
The intrinsic pathway is activated when blood comes into contact with sub-endothelial connective tissues or with negatively charged surfaces that are exposed as a result of tissue damage. Quantitatively it is the most important of the two pathways, but is slower to cleave fibrin than the extrinsic pathway. The Hageman factor (factor XII), factor XI, prekallikrein and high molecular weight kininogen (HMWK) are involved in this pathway of activation. The first step is the binding of factor XII to a sub-endothelial surface exposed by an injury. A complex of prekallikrein and HMWK also interacts with the exposed surface in close proximity to the bound factor XII, which becomes activated. During activation, the single chain protein of the native factor XII is cleaved into two chains (50 and 28 kDa), which remain linked by a disulphide bond. The light chain (28 kDa) contains the active site and the molecule is referred to as activated Hageman factor (factor XIIa). There is evidence that the Hageman factor can autoactivate, thus the pathway is self-amplifying.
Activated factor XII, in turn, activates prekallikrein. The kallikrein produced can then also cleave factor XII, and a further amplification mechanism is triggered. The activated factor XII remains in close contact with the activating surface, such that it can activate factor XI, the next step in the intrinsic pathway, which, to proceed efficiently, requires calcium. Also involved at this stage is HMWK, which binds to factor XI and facilitates the activation process. Activated factors XIa, XIIa and kallikrein are all serine proteases.
The intrinsic pathway ultimately activates factor X, a process which can also be brought about by the extrinsic pathway. Factor X is the first molecule of the common pathway and is activated by a complex of molecules containing activated factor IX, factor VIII, calcium and phospholipid, which is provided by the platelet surface, where this reaction usually takes place. The precise role of factor VIII in this reaction is not clearly understood. Its presence in the complex is essential, as evidenced by the consequences of factor VIII deficiency experienced by haemophiliacs. Factor VIII is modified by thrombin, a reaction that results in greatly enhanced factor VIII activity, promoting the activation of factor X.
The extrinsic pathway
The extrinsic pathway is an alternative route for the activation of the clothing cascade. It provides a very rapid response to tissue injury, generating activated factor X almost instantaneously, compared with the seconds, or even minutes, required for the intrinsic pathway to activate factor X. The main function of the extrinsic pathway is to augment the activity of the intrinsic pathway.
There are two components unique to the extrinsic pathway, tissue factor or factor III, and factor VII. Tissue factor is present in most human cells bound to the cell membrane. Once activated, tissue factor binds rapidly to factor VII, which is then activated to form a complex of tissue factor, activated factor VII, calcium and a phospholipid, and this complex then rapidly activates factor X.
The intrinsic and extrinsic systems converge at factor X to a single common pathway which is responsible for the production of thrombin (factor IIa).
The end result of the clotting pathway is the production of thrombin for the conversion of fibrinogen to fibrin. Fibrinogen is a dimer which is soluble in plasma. Exposure of fibrinogen to thrombin results in rapid proteolysis of fibrinogen and the release of fibrinopeptide A. The loss of small peptide A is not sufficient to render the resulting fibrin molecule insoluble, a process 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 polymerise spontaneously to form an insoluble gel. The polymerised fibrin, held together by non-covalent and electrostatic forces, is stabilised by the transamidating enzyme factor XIIIa, produced by the action of thrombin on factor XIII. These insoluble fibrin aggregates, together with aggregated platelets, block the damaged blood vessel and prevent further bleeding.
Figure 1: The clotting cascade
Once haemostasis is restored and the tissue is repaired, the clot or thrombus must be removed from the injured tissue. This is achieved by the fibrinolytic pathway. The end product of this pathway is the enzyme, plasmin. Plasmin is formed by activation of the proenzyme, plasminogen by either plasma or tissue activators. Tissue plasminogen activators are found in most tissues, except the liver and the placenta, where they are synthesised by endothelial cells and are found concentrated in the walls of blood vessels. Plasminogen activator is also a product of macrophages. The level of tissue activator in the plasma is normally low, but can be increased by exercise and stress.
Two forms of plasminogen are present in the plasma; one has a glutamic acid at the N-terminal of the polypeptide chain, and is called native or glu-plasminogen, and the other has a lysine. The latter form arises as a result of partial degradation of the parent molecule by autocleavage.
Triggering of fibrinolysis occurs when the plasminogen activator, plasminogen, and fibrin are all in close proximity. Both plasminogen and its activator bind avidly to fibrin as the clot forms. This close association prevents inhibition of plasmin activity by inhibitor, and allows proteolysis of the fibrin to proceed after the production of lys-plasminogen. Plasmin inhibitors (antiplasmins) which can control plasmin activity include: alpha-1 antitrypsin, alpha-2 antiplasmin, C1 inhibitor and antithrombin III.
Plasmin attacks fibrin at at least 50 different sites, reducing its size such that it no longer has haemostatic activity. Many fragments are formed during this process, and some retain the capacity to polymerise; thus, some of the early degradation products can compete with fibrinogen for thrombin and act as inhibitors of clot formation. This may prevent the clot being removed before the tissue is repaired.
Normally, the clotting mechanism is balanced by opposing reactions preventing coagulation, e.g. anti-thrombin III which inhibits active factors II,IX, X, XI and XII. Prostacyclin secreted by vascular endothelium inhibits platelet aggregation.
Updates in perioperative coagulation: physiology and management of thromboembolism and haemorrhage.
Bombeli T, Spahn DR.
Br J Anaesth. 2004 Aug;93(2):275-87