The tunica externa, also known as the adventitia, is the outermost layer of blood vessels, including arteries and veins. It is primarily composed of connective tissue, which provides structural support and elasticity. This layer helps anchor the blood vessels to surrounding tissues and organs. In larger blood vessels, the tunica externa may contain vasa vasorum, which are small blood vessels that supply nutrients to the vessel walls. This layer plays a crucial role in maintaining the integrity and function of the vascular system, ensuring that blood can flow efficiently throughout the body.