Final answer:
Capillaries do not have valves to prevent backflow of fluids, unlike arteries and veins which are equipped with such structures to ensure unidirectional blood flow.
Step-by-step explanation:
The vessel that does not feature valves to prevent backflow of fluids is the capillaries. Arteries and veins have the necessary structures to prevent backflow, ensuring that blood flows in only one direction. The arteries, which have thicker walls due to their proximity to the heart and higher blood pressure, transport blood away from the heart. In contrast, veins return blood to the heart and have valves that prevent the blood from flowing backward due to gravity or low pressure within veins.
Capillaries, however, are small blood vessels that connect arterioles and venules, and they do not have valves because the blood pressure is lower and the flow is unidirectional due to the pressure from the arterial side.