The blood in the vessels normally does not clot because of the following:
Vitamin K antagonists are present in plasma
Thrombin has a positive feedback on plasminogen
Sodium citrate in plasma chelates calcium ions
Vascular endothelium is smooth and coated with glycocalyx
Blood in the vessels do not clot because of two reasons : 1. Endothelial membrane is coated by highly charged polysaccharide called glycocalyx. Due to its high density glycocalyx contribute to the non thrombogenic propeies of surface of intact epithelium. 2. The luminal surface of the endothelium is smooth.