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Physiology

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Blood

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Which of the following is not true? (AIPG 2011)



A Factor X is common to both extrinsic and intrinsic pathway.

B Calcium is essential for coagulation pathway.

C Negatively charged surface proteins are required for activation of extrinsic pathway.

D Intrinsic pathway can occur in vitro.

Ans. C Negatively charged surface proteins are required for activation of extrinsic pathway.

I. In the clotting pathway, the conversion of fibrinogen to fibrin is catalyzed by thrombin. Thrombin is a serine protease that is formed from its circulating precursor, prothrombin, by the action of activated factor X.

II. Factor X can be activated by either of two systems, known as intrinsic and extrinsic.

III. The initial reaction in the intrinsic system is conversion of inactive factor XII to active factor XII (XIIa). This activation, which is catalyzed by high-molecular-weight kininogen and kallikrein, can be brought about in vitro by exposing the blood to glass (negative charge example in vitro during capillary tube method), or in vivo by collagen fibers underlying the endothelium.

IV. Active factor XII then activates factor XI, and active factor XI activates factor IX. Activated factor IX forms a complex with active factor VIII, which is activated when it is separated from von Willebrand factor. The complex of IXa and VIIIa activate factor X. Phospholipids from aggregated platelets (PL) and Ca2+ are necessary for full activation of factor X.

V. The extrinsic system is triggered by the release of tissue thromboplastin, a protein–phospholipid mixture that activates factor VII. Tissue thromboplastin and factor VII activate factors IX and X. In the presence of PL, Ca2+ and factor V, activated factor X catalyzes the conversion of prothrombin to thrombin.

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