The primary purpose of heart valves is to prevent blood from flowing backward.

pump blood backward.

exchange carbon dioxide for oxygen.

protect the heart from trauma.

prevent blood from flowing backward

Explanation:

think of the heart as the panama canal. There are four chambers, the atria and the ventricles. It goes from right atrium to right ventricle, and then to the lungs, where it gets oxygen, and then into the left atrium and finally into the left ventricle which pumps the blood through the aorta into the systemic circulation. Valves prevent backflow of the blood, they open and close at difference stages of the heart cycle.