Heparin acts as a naturally occurring anticoagulant in the body. It is produced by mast cells and basophils and works by enhancing the activity of antithrombin III, which inactivates several enzymes in the coagulation cascade, primarily thrombin and factor Xa. This action helps to prevent the formation of unwanted blood clots, maintaining normal blood flow.

Let's briefly review the other options to understand why they are incorrect in this context:

- Protein C is a natural anticoagulant as well, but it requires activation (activated Protein C) and works by inactivating factors Va and VIIIa. However, it is not as directly known as heparin in its natural anticoagulant role.
- Warfarin is a synthetic anticoagulant, which works by inhibiting vitamin K-dependent clotting factors. It is not produced naturally in the body.
- Aspirin acts as an antiplatelet agent by inhibiting cyclooxygenase and thereby reducing thromboxane A2 synthesis. It is not an anticoagulant and is not produced naturally by the body.

In summary, heparin is the correct answer because it is a naturally occurring anticoagulant that plays a vital role in regulating blood coagulation by enhancing antithrombin III.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Volume 2, Chapter on Hemostasis and Thrombosis, Page 2345