The blood in the vessels normally does not clot because:
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. Ref: Grays Anatomy, 38th Edition, Page 1456 ; Robbin's Illustrated Pathology, 7th Edition, Page 126 ; Illustrated Human and Social Biology By Brian S. Beckett, Page 72 ; Encyclopedia of Molecular Pharmacology, Volume 1 By Stefan Offermanns, Page 503 ; Cerebral Vasospasm By Robe Loch Macdonald, Page 78
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