the answer is one–way valves in veins
B. one–way valves in veins
Veins are thin-walled blood vessels as compared to the arteries. The veins are superficially located under the skin in the body. They bring deoxygenated blood to the heart form the body cells. Since the blood pressure in the veins is low, the presence of valves along their length to prevent backflow of blood away from the heart.