Final answer:
The infoldings that form valves in many veins are part of the tunica intima. These valves prevent backflow and ensure blood flows towards the heart, with structural support from the tunica media and tunica externa.
Step-by-step explanation:
Many veins have infoldings of the tunica intima that form valves which are crucial in directing blood flow towards the heart. These valves are especially significant in the veins of the lower limbs, helping to prevent backflow of blood and ensure its one-directional movement despite the challenges posed by gravity. The vein walls consist of three layers: the tunica intima, the tunica media, and the tunica externa. While arteries have thick walls to handle high pressure, veins have thinner walls and larger lumens, appearing somewhat flattened as they operate under lower pressure.
The valves in veins are made from sections of thickened endothelium, supported by connective tissue, extending into the lumen to maintain proper blood circulation. The tunica externa, also known as the tunica adventitia in some texts, is the outermost layer composed mostly of connective tissue, providing additional support and structure to the vein. This interaction of structural features assists with the vital function of returning deoxygenated blood back to the heart for reoxygenation.