Directions : In the following question more than one of the answers given may be correct. Select the correct answer and mark it according to the code:
Blood does not clot inside the vessels because (1)constant speedy flow of blood prevents accumulation of fibrin threads, if at all formed (2) absence of heparin, an anticoagulant having an tithrombin activity, in blood (3) smoothness of the endothelial lining prevents rupture by platelets and formation of thromboplastin from them (4) presence of monomolecular layer of negatively charged protein adsorbed to the inner surface of endothelium, which attracts the clotting factorsA) 1, 2 and 3 are correct.
B) 1 and 2 are correct.
C) 2 and 4 are correct.
D) 1 and 3 are correct.
Correct Answer: D
Solution :
Probably the most important factors for preventing clotting in the normal vascular system are (i) the smoothness of the endothelial cell surface, which prevents contact activation of the intrinsic clotting system (ii) a layer of glycocalyx on the endothelium, which repels clotting factors and paltelets, thereby preventing activation of clotting (iii) a protein bound with the endothelial membrane, thrombomodul in, which binds thrombin.
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