the outer layer of blood vessels is composed of
The tunica adventitia is the outermost layer of the arteries and veins. The word 'adventitia' in tunica adventitia comes from the Latin 'adventicius,' which means 'coming from outside.' This strong outer layer is composed of connective tissue that allows the blood vessel to withstand forces acting on the vessel wall.ll arteries and veins contain three layers. The innermost layer is called the tunica intima. The muscular middle layer is called the tunica media, and the outermost layer is called the tunica adventitia.