Final answer:
The heart and veins have valves that prevent blood from flowing backward.
Step-by-step explanation:
The parts of the body that have valves to prevent blood from flowing backward are the heart and the veins.
In the heart, there are four valves that keep the blood flowing in one direction. The two atrioventricular (AV) valves prevent blood from moving from the ventricles to the atria, while the two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood from flowing back from the arteries into the ventricles.
In the veins, valves stop the blood from flowing backward as it carries blood toward the heart. This is important to ensure that blood flows efficiently and does not accumulate in the wrong areas of the body.