2.6 Intravascular Anticoagulants

• What do intravascular anticoagulants prevent?

  Blood clotting in the normal vascular system
• What is the most important factor preventing clotting in the vascular system?

  Smoothness of the endothelial cell surface
• What role does glycocalyx play in clotting prevention?

  It repels clotting factors and platelets
• What is thrombomodulin's function in the endothelial membrane?

  It binds thrombin to slow the clotting process
• How does the thrombomodulin-thrombin complex affect protein C?

  It activates protein C, acting as an anticoagulant
• What happens when the endothelial wall is damaged?

  Smoothness and glycocalyx-thrombomodulin layer are lost
• What activates Factor XII and platelets upon endothelial damage?

  Loss of smoothness and glycocalyx-thrombomodulin layer
• What enhances the activation of Factor XII and platelets?

  Contact with subendothelial collagen
• What are the two main anticoagulants that remove thrombin from the blood?

  Fibrin fibers and antithrombin III
• How much thrombin becomes adsorbed to fibrin fibers during clotting?

  About 85 to 90 percent
• What does the adsorption of thrombin to fibrin fibers prevent?

  Excessive spread of the clot
• What happens to thrombin that does not adsorb to fibrin fibers?

  It combines with antithrombin III
• How long does it take for antithrombin III to inactivate thrombin?

  During the next 12 to 20 minutes
• What is heparin's role as an anticoagulant?

  It enhances the effectiveness of antithrombin III
• Why does heparin have significant anticoagulant effects only under special conditions?

  Its concentration in the blood is normally low
• How much does heparin increase the effectiveness of antithrombin III?

  By a hundredfold to a thousandfold
• What does the heparin-antithrombin III complex remove besides thrombin?

  Activated Factors XII, XI, X, and IX
• Where is heparin produced in the body?

  By basophilic mast cells in connective tissue
• Why are large quantities of heparin needed in the lungs and liver?

  To prevent further growth of embolic clots
• What are the key factors preventing blood clotting in the normal vascular system?

  • Smoothness of endothelial cell surface
  • Glycocalyx layer repelling clotting factors
  • Thrombomodulin binding thrombin
• What are the mechanisms of action for heparin as an anticoagulant?

  • Combines with antithrombin III
  • Increases effectiveness of antithrombin III
  • Removes thrombin and other activated factors
• What is the euglobulin called that is involved in clot lysis?

  Plasminogen
• What does plasminogen become when activated?

  Plasmin
• What type of enzyme is plasmin?

  Proteolytic enzyme
• How does plasmin compare to trypsin?

  It resembles trypsin in function
• What does plasmin digest?

  Fibrin fibers and protein coagulants
• Name two protein coagulants that plasmin digests.

  Fibrinogen and Factor V
• What happens when plasmin is formed?

  It causes lysis of a clot
• What can result from the destruction of clotting factors by plasmin?

  Hypocoagulability of the blood
• What is trapped in a clot when it is formed?

  A large amount of plasminogen
• What activates plasminogen to form plasmin?

  Tissue plasminogen activator (t-PA)
• When is t-PA released?

  After a clot has formed
• What does t-PA convert plasminogen into?

  Plasmin
• What happens to the blood clot after t-PA acts on plasminogen?

  It is removed from the bloodstream
• What is one important function of the plasmin system?

  To remove minute clots from vessels
• What could happen if minute clots are not cleared from vessels?

  They could occlude the vessels
• How does the role of plasmin contribute to hemostasis?

  It balances clot formation and removal
• If a patient has excessive clotting, what might be a potential issue with plasmin activity?

  Insufficient activation of plasminogen