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The division of coagulation in two pathways is arbitrary, originating from laboratory tests in which clotting times were measured either after the clotting was initiated by glass, the intrinsic pathway; or clotting was initiated by thromboplastin (a mix of tissue factor and phospholipids), the extrinsic pathway. [31]
The two chains formed after the cleavage at Arg 320, termed the A and B chains, are linked by a disulfide bond in active thrombin. In the alternate pathway for thrombin activation, prothrombin is first cleaved at Arg 320, producing a catalytically active intermediate called meizothrombin. [16]
Therefore, although the coagulation cascade can be triggered in vitro through the intrinsic pathway only, in vivo coagulation is triggered by the extrinsic pathway. However, the model better describing how coagulation works is the so-called cell-based model, a more integrated picture of the whole process, in which phospholipid surfaces, such as ...
The prothrombin time (PT) – along with its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) – is an assay for evaluating the extrinsic pathway and common pathway of coagulation. This blood test is also called protime INR and PT/INR.
Together with factor VIIa, tissue factor forms the extrinsic pathway of coagulation. This is opposed to the intrinsic (amplification) pathway, which involves both activated factor IX and factor VIII. Both pathways lead to the activation of factor X (the common pathway), which combines with activated factor V in the presence of calcium and ...
Blood coagulation pathways in vivo showing the central role played by thrombin. Factor Xa is the activated form of the coagulation factor X, also known as thrombokinase. Factor X is an enzyme, a serine endopeptidase, which plays a key role at several stages of the coagulation system. Factor X is synthesized in the liver.
[4] [5] [6] The ability of FXII to bind to negatively charged surfaces and activate coagulation forms the basis of the aPTT test, in which artificial materials act as a surface for contact activation. This test is used to measure the contact activation pathway (intrinsic pathway) and the common pathway of clotting. [7]
Characteristics of the velocity of passage of the intrinsic coagulation pathway; Poor plasma (the most convenient to work with, but no realization of the thrombocyte clotting mechanism) Contact activation pathway; Prothrombin time test (or prothrombin test, INR, PT) – velocity of passage of the extrinsic blood coagulation pathway Poor plasma