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Protamine sulfate functions primarily as an antidote for heparin, a commonly used anticoagulant. When heparin is administered, it binds to antithrombin III, enhancing the body’s ability to inhibit thrombin and other clotting factors, thus preventing blood coagulation. If there is a need to reverse the effects of heparin—such as in the case of bleeding or prior to surgical procedures—protamine sulfate is administered. It works by binding to heparin and forming a stable complex that neutralizes its anticoagulant effects, restoring normal blood coagulation.

The other choices do not accurately describe the role of protamine sulfate: it does not enhance coagulation or activate thrombin; rather, its primary function is to counteract the effects of heparin. Additionally, it is not specifically indicated for preventing hypotension, although administration should be done cautiously since protamine itself can cause hypotension if delivered too rapidly.