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Although all the blood vessels of the body are actually one continuous system, anatomists generally divide them into three functional categories: Arteries, veins, and capillaries.
Arteries and veins are both composed of three major layers, or tunics. The outer layer is the tunica adventitia, which is a fibrous protective layer with a lot of collagen. The tunica media, or middle layer, is composed of elastic connective tissue and smooth muscle. The innermost layer, or tunica intima, is primarily elastic tissue with a lining of endothelial cells.
The major difference between arteries and veins is the thickness of the layers. In arteries, all the layers are thicker and more robust than in veins, with the exception of the endothelial lining. Arteries are subjected to the full pressure of the heart, so they need to be more elastic to maintain shape, and they need more muscle to help control blood flow to various areas. Veins are under much less pressure so they tend to be less elastic, but veins have valves, which are extra flaps of endothelium, to help keep blood from flowing backwards in response to gravity in between heart beats.
Capillaries are composed of only a single layer of tunica intima. This makes them very thin, which allows gasses and other materials to be exchanged between the blood and body cells easily. Because of their simple structure, capillaries are both prone to damage and easy to repair. Their tiny diameter allows them to have a large surface area in contact with body cells.
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