▶️ Answer/Explanation
Veins are blood vessels that carry blood back toward the heart under relatively low pressure. To ensure that blood flows in the correct direction and does not flow backward due to gravity, veins contain valves. Unlike arteries, which have thick muscular walls and a narrow lumen to handle high pressure, veins have thinner walls and a much wider lumen. The “one-cell-thick wall” mentioned in option A is actually a characteristic of capillaries, which allows for efficient diffusion. Therefore, the presence of valves is the distinguishing structural feature of veins among the choices provided.
✅ Answer: (D)
✅ Answer: (D)
▶️ Answer/Explanation
Veins are blood vessels that carry blood toward the heart under low pressure. Because the pressure is significantly lower than in arteries, veins have relatively thin walls. To facilitate blood flow with minimal resistance, they possess a large lumen (internal diameter). Additionally, because blood in veins often travels against gravity, they contain valves to prevent the backflow of blood, ensuring it moves in one direction. These three characteristics—thin walls, large lumen, and the presence of valves—are correctly identified in row D.
✅ Answer: (D)
