Final answer:
Veins are the thin-walled blood vessels that conduct blood toward the heart, have larger lumens, thinner walls than arteries, and contain valves to prevent backflow. Veins are equipped for lower pressure and are located closer to the skin surface, often appearing flattened.
Step-by-step explanation:
The thin-walled blood vessels that are mentioned in the question are veins. Veins are the blood vessels that conduct blood toward the heart, and they have larger lumens and thinner walls compared to arteries. Unlike the elastic arteries near the heart, which contain a high percentage of elastic fibers and have thick walls to withstand high pressure, the veins' elasticity is less significant because the blood pressure within them is considerably lower. They are equipped with valves to prevent backflow and help maintain the unidirectional flow of blood towards the heart.
As for the vessel walls, they consist of three tunics: the tunica intima, the tunica media, and the tunica externa. However, the tunica intima in veins is a thin layer composed of endothelium, and the other layers are not as robust as in arteries. The tunica media has a few muscle cells and elastic fibers, while the tunica externa consists mainly of connective tissue fibers. This construction is why veins can appear flattened, especially since they are closer to the skin surface and are a lower-pressure system compared to the arterial system.