Final answer:
Arteries have thick, round walls that appear wavy, while veins have larger lumens and thinner walls that often appear flattened.
Step-by-step explanation:
Arteries and veins have different appearances due to their structural differences. Arteries have thick, round walls and appear wavy under a microscope. Veins, on the other hand, have larger lumens and thinner walls, often appearing flattened. This is because arteries are part of a high-pressure system, while veins are part of a lower-pressure system.