To understand how various anticoagulants work, it is important to understand the body’s normal reaction to a cut or injury that involves blood loss. After a cut, a chain of events begins that will rapidly seal the wound and reduce the loss of blood. This chain includes:

• Vasoconstriction. The blood vessels narrow, restricting the amount of blood lost.
  
  
• Platelet aggregation. Platelets, also known as thrombocytes, play a major role in blood coagulation, clotting and hemostasis (stoppage of bleeding). When a small blood vessel is injured, platelets begin to stick to each other and to the sides of the injured tissue and form a plug, which becomes covered with strands of thread-like fibrin. Soon the blood clot (plug) shrinks in place and blocks any further loss of blood. Platelets also release chemicals that begin the healing process. Without this important clotting function of the platelets, excessive and potentially life-threatening amounts of blood would be lost after simple cuts or scrapes. Antiplatelets (e.g., clopidogrel) are medications that inhibit blood clotting by reducing the platelets’ stickiness.
  
  
• Blood coagulation (or plasma factors). Plasma is the liquid part in which the blood cells are suspended. Blood coagulation occurs due to a series of reactions in plasma. Scientists have identified more than 13 distinct chemical reactions that result in clot formation. Anticoagulant medications affect these processes in one of two ways:
  
  
• Vitamin K antagonists. For several of the plasma reactions, vitamin K serves as a catalyst, which speeds the reactions. Some medications can inhibit vitamin K’s ability to activate these clotting factors and therefore prevent or lessen blood coagulation.
  
  
• Fibrin and thrombin inhibitors. Some medications can inhibit the production of these chemicals in the blood and therefore prevent or lessen coagulation of the blood.