As in most systems in the body, there is a balance between the mechanisms that facilitate and mechanisms that inhibit a specific function.
While the described clotting mechanisms facilitate, there are built-in mech-anisms that inhibit the clotting process and dissolve the clots that do form. Both clotting and anticlotting mechanisms are equally important. A number of mechanisms are involved in the anticlotting process.
Anticlotting mechanisms include removal of clotting factors by the liver and reduction in the supply of clotting factors as they get used. Although some enzymes secreted by platelets potentiate aggregation of platelets, other enzymes in the blood vessel walls inhibit platelet clumping. Antithrombin III is a sub-stance present in the plasma that inhibits the active form of clotting factors IX, X, XI, and XII. The en-dothelial cells and white blood cells secrete a sub-stance called prostacyclin, which inhibits platelet adhesion and release. Mast cells and basophils se-crete an anticoagulant heparin.
In addition, there are many other complex mecha-nisms that inhibit clotting. These fibrinolytic (break-down of fibrin) mechanisms also rely on a cascade of reactions similar to the clotting mechanism illus-trated. Some of the factors involved in preventing clotting have been isolated and are used to treat indi-viduals with myocardial infarction.
Anticoagulants are often given to individuals with a tendency to form a thrombus. These anticoagulants inhibit vitamin K or stimulate the built-in system that prevents clotting inside blood vessels. A well-known drug (also found naturally in the body), heparin facilitates the activity of antithrombin III, retarding thrombus formation. Streptokinase, an enzyme produced by bacteria, is fibrinolytic and often used as an anticoagulant. If blood must be stored outside the body, substances that remove calcium are introduced to prevent clotting. Because calcium is involved in muscle contraction, it is not feasible to prevent clot-ting by removing calcium from the blood when it is inside the body.