National League for Nursing (NLN PAX) Practice Exam

The correct response indicates that veins contain valves to prevent blood from flowing in the wrong direction. This is an essential feature of their structure, as veins carry deoxygenated blood back to the heart under lower pressure compared to arteries. The presence of valves is crucial in this context to ensure that blood does not accumulate in the extremities and instead moves efficiently back to the heart, especially when influenced by gravity, such as in the legs.

In contrast, arteries, which transport oxygenated blood away from the heart, do not have valves because the high-pressure flow from the heart keeps the blood moving in one direction. Capillaries are the smallest blood vessels where nutrient and gas exchange occurs; they do not require valves as blood flow is driven by diffusion rather than directed pressure. Lymph ducts, while they do have a similar function to veins in preventing backflow of lymph fluid, are distinct structures primarily associated with the lymphatic system rather than the circulatory system.