What are the three parts of blood vessels and their functions?

Answer 1

There are three layers in arteries and veins. The inner layer is the tunica intima, and it is very thin. The middle layer is the tunica media, and it's the muscular layer. For that reason, it's much thicker in arteries than in veins. The outer layer is the tunica adventitia, and is mostly connective tissue with some nerves.

In contrast, capillaries don't have all three layers. They need to be thin to allow oxygen and other substances to pass to tissues.

The tunica intima provides a layer that allows substances to move through. The muscular layer allows the vessel to change its diameter. The tunica adventita provides protection and connection.