The blood within the vessels does not clot normally because
The endothelium of the blood vessels also plays an active role in preventing the extension of clots. All endothelial cells except those in the cerebral microcirculation produce thrombomodulin, a thrombin-binding protein, on their surfaces. InCirculating blood, thrombin is a procoagulant that activates factors V and VIII, but when it binds to thrombomodulin, it becomes an anticoagulant in that the thrombomodulin-thrombin complex activates protein C. Activated protein C (APC), along with its cofactor protein S, inactivates factors V and VIII and inactivates an inhibitor of tissue plasminogen activator, increasing the formation of plasmin. Ref: Ganong's Review of Medical Physiology Twenty-Third Edition Page No: 533
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
Cardiovascular system