Final answer:
Valves in the heart keep the blood flowing in one direction by opening and closing in one direction only. There are four valves in the heart: the mitral valve, aortic valve, pulmonary valve, and tricuspid valve.
Step-by-step explanation:
The blood is kept from flowing back on itself through the use of valves in the heart. These valves open and close in one direction only, allowing blood to move forward through the heart and preventing backward flow. There are four valves in the heart: the mitral valve, the aortic valve, the pulmonary valve, and the tricuspid valve.