The adventitia is the outermost layer of blood vessels, including arteries and veins. It is composed of connective tissue that provides structural support and protection to the vessel. This layer helps anchor the blood vessel to surrounding tissues, ensuring stability and maintaining proper blood flow. In addition to blood vessels, the adventitia can also be found in other tubular structures in the body, such as trachea and esophagus. Its primary function is to provide strength and flexibility, allowing these structures to withstand various pressures and movements while maintaining their integrity.