Arteries are brighter, since they transport blood rich in oxygen to the organs of the body. The veins afterwards transport the blood, which is at a low oxygen level and thus darker, to the lungs and the liver.
Arteries are elastic vessels that are very strong, able to carry blood away from the heart under high pressure. They subdivide into thinner tubers that give rise to branched, finer arterioles.
Blood vessels are made up of three distinct layers called tunics. These are:
❑ Tunica interna/intima (internal layer- epithelial lining)
❑ Tunica media (middle lining- smooth muscles+ elastic connective tissue)
❑ Tunica externa/adventitia (outer lining – connective tissue)
Tunica interna is made up of a layer of simple squamos epithelium known as endothelium. It rests on a connective tissue membrane with many elastic, collagenous fibers. Tunica media makes up most of an arterial wall including smooth muscle fibers and a thick elastic connective tissue layer. The tunica adventitia is composed of collagen and elastic fibers.
The thickness and histologic composition of these three layers differs in arteries, capillaries and veins giving them their unique structure which enables them to perform their unique functions.
The tunica intima and tunica media of elastic arteries are generally thicker than those of the other types, and the tunica media contains elastic fibers. Capillaries differ in that they have a very thin tunica adventitia and their endothelial cells are fenestrated to increase their permeability.
Arteries are classified into three types on the basis of their diameter:
Elastic arteries/ Conducting arteries
Muscular arteries/distributing arteries
Small arteries/ Resistance arteries
Blood vessel – arteries
