Platelets. Very small blood cells with no nucleus containing an enzyme, called thrombokinase, which is essential to blood clotting. Even minor injuries will cause the thrombocytes to disintegrate, release the enzyme, and thus trigger blood clotting. If the number of thrombocytes in the blood drops to below 30,000 per liter, there is a major risk of bleeding. The thrombocytes are produced in the red bone marrow.