Which vessels commonly contain valves to prevent backflow of blood?

• A. Large veins
• B. Arterioles
• C. Capillaries
• D. Arteries

Answer: (A)

Valves are folds of the inner lining that create one-way doors to keep blood moving toward the heart. This unidirectional flow is crucial in the veins, especially in large veins of the limbs where blood has to travel upward against gravity. The valves work with muscle contractions to form a venous pump, preventing backflow when the muscles relax. Arteries, arterioles, and capillaries don’t have valves because they carry blood under higher pressure in a mostly continuous forward flow, so there’s less need to prevent backflow. Therefore, large veins are the vessels commonly containing valves to stop blood from flowing backward.