Middle Layer, Tunica Media
The middle layer surrounding the Tunica intima is called the Tunica media. In arteries, this layer is considerably thicker compared to veins. While veins primarily consist of collagen fibers and have a limited number of elastic fibers, arteries are predominantly composed of elastic fibers. Additionally, veins contain vasa vasorum, which are small blood vessels, and nervi vasorum, which are nerves. In contrast, arteries do not possess these structures.
Outer Layer: Tunica Externa
The outer layer of blood vessels, also referred to as Tunica adventitia, serves the purpose of anchoring the blood vessels to the surrounding tissues. While this layer is relatively thin in arteries, it is the thickest layer in veins. At this level, both arteries and veins have the nervi vasorum and vasa vasorum, although there are notable differences between the two. Veins contain a higher concentration of smooth muscle fibers, whereas arteries possess few, if any, smooth muscle fibers.