Final answer:
A chemical substance that prevents the clotting of a blood specimen is called an anticoagulant, with examples including plasma anticoagulants, antithrombin, heparin, and potassium oxalate.
Step-by-step explanation:
A chemical substance that prevents clotting of a blood specimen is known as an anticoagulant. These agents include plasma anticoagulants like the protein C system, which inactivates clotting factors in the intrinsic pathway, and TFPI (tissue factor pathway inhibitor), which inhibits conversion of factor VII in the extrinsic pathway. Antithrombin acts in the common pathway by inactivating factor X and opposing the conversion of prothrombin to thrombin. Heparin is another anticoagulant used both naturally by basophils and therapeutically in pharmaceutical form to prevent blood clots in patients. Potassium oxalate, K2C2O4, can also be used as an anticoagulant in blood sample collection by precipitating calcium ions as CaC2O4 H2O, since calcium is essential for blood coagulation processes.