The blood within the vessels does not clot normally because:

D i.e. Vascular endothelium is smooth & coated with glycocalyxBlood clotting is prevented by endothelium d/t their smoothness and its glycocalyx- thrombomodulin layerQ. Thrombomodulin, a thrombin binding protein, is expressed by all endothelial cells except those in the cerebral microcirculationQ.Vascular endothelium is smooth and is coated with a layer of glycocalyx (mucopolysaccharide)Q which prevents activation of intrinsic system and repels clotting factor & platelets, thereby preventing clotting of blood with in vessels.Antithrombin (AT) III is the most impoant anticoagulant in blood which prevents clotting of blood in normal vascular systemQ. Heparin is powerful anticoagulant but its concentration in blood is low, so by itself it has little or no anticoagulant propey in normal vascular system. However, heparins binding with ATIII increases effectiveness of AT III upto 1000 times. Fibrin (not fibrinogen) also has antithrombin action during clot formation.