<prompt> The blood within the vessels does not clot normally becauseVitamin K antagonists are present in plasmaThrombin has a positive feedback on plasminogenSodium citrate in plasma chelates calcium ionsVascular endothelium is smooth and coated with glycocalyx  <response>: 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: Ganongs Review of Medical Physiology Twenty-Third Edition Page No: 533 