Blood Vessels
• Blood vessels are intricate networks of hollow tubes that transport blood through the entire body.
• Blood vessels (except capillaries) are made up of three layers – tunica externa, tunica media and tunica interna, surrounding the central blood carrying canal (known as lumen).
– The tunica externa or tunica adventitia, strong outer covering of arteries and veins is composed of connective tissue as well as collagen and elastic fibres. These fibres allow the arteries and veins to stretch to prevent over expansion due to the pressure that is exerted on the walls by blood flow.
– The tunica media is the middle layer of the walls of arteries and veins. It is composed of smooth muscle and elastic fibres. This layer is thicker in arteries than in veins.
– The tunica intima is the inner layer of arteries and veins. In arteries this layer is composed of anelastic membrane lining and smooth endothelium that is covered by elastic tissues.
Read more: http://www.hourlybook.com/circulatory-system-blood-vessels/