Final answer:
Blood vessels, except capillaries, are composed of three layers known as tunica intima, tunica media, and tunica externa, with each layer serving specific structural and functional roles.
Step-by-step explanation:
The three layers that arteries and veins are composed of are known as the tunica intima, tunica media, and tunica externa. These layers are integral to the structure and function of the blood vessels.
The tunica intima is the innermost layer and is composed of a smooth lining of endothelial cells that are in contact with red blood cells.
The tunica media, which is the middle layer, contains smooth muscle and connective tissue, particularly thick in arteries, that controls the diameter of the vessels. Lastly, the tunica externa is the outermost layer composed mainly of connective tissue, and it's the thickest layer in veins.