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Formation of prothrombinase
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Formation of prothrombinase

Prothrombin and Thrombin Prothrombin is a plasma protein, an alpha2-globulin, having a molecular weight of 68,700. It is present in normal plasma in a concentration of about 15 mg/dl. It is an unstable protein that can split easily into smaller compounds, one of which is thrombin, which has a molecular weight of 33,700, almost exactly one half that of prothrombin. Prothrombin is formed continually by the liver, and it is continually being used throughout the body for blood clotting. If the liver fails to produce prothrombin, in a day or so prothrombin concentration in the plasma falls too low to provide normal blood coagulation. Vitamin K is required by the liver for normal formation of prothrombin as well as for formation of a few other clotting factors. Therefore, either lack of vitamin K or the presence of liver disease that prevents normal prothrombin formation can decrease the prothrombin level so low that a bleeding tendency results.

Active prothrombinase is formed with the help of two mechanisms:

extrinsic that is initiated by phospholipids, which are released from damaged cells of vessels or from connective tissue;

intrinsic that is initiated by coagulative factors of blood.

Extrinsic (tissue) mechanism – is fast mechanism, last for 5-20 seconds. Reactions occur on membranes of damaged cells. After damage, cells (mainly lysosomes) excrete active enzymes – tissue thromboplastin (F. IIIa) activates proconvertin (F. VII). F.VIIa with phospholipids of tissues and calcium form complex, which activate prothrombinase (F. X). F. Xa with phospholipids, calcium and F. Va (proaccelerin) form complex, which is tissue prothrombinase.



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