Final answer:
The gray top tube contains potassium oxalate as an anticoagulant by precipitating calcium, and sodium fluoride to preserve glucose levels. Antithrombin, while a natural anticoagulant in the body, is not contained in the gray top tube.
Step-by-step explanation:
The gray top tube typically contains an anticoagulant called sodium fluoride as well as potassium oxalate. The primary anticoagulant effect is through the potassium oxalate, which acts by precipitating calcium, and sodium fluoride acts as a glycolysis inhibitor, preserving glucose in samples for accurate testing. Calcium is necessary for the coagulation pathway; thus, its removal prevents the blood from clotting.
Antithrombin is another crucial anticoagulant found in the blood plasma that inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin. However, it is not what the gray top tube contains; it is a naturally occurring anticoagulant in the body that operates within the common pathway of the coagulation cascade.