| Blood vessels circulate blood through the body. The three major types of blood vessels are the arteries, veins, and capillaries. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries form at the junction between arteries and veins and they are the only blood vessel to come into direct contact with tissue cells. Oxygen, nutrients, and other substances transported in blood through the arteries pass through thin capillary walls into tissue cells. Wastes and other products of cellular metabolism pass from cells back through the capillary walls and into veins. The walls of arteries and veins are composed of three tunics (layers) that surround a central opening called a lumen, through which blood flows. The innermost layer is the tunica intima, composed of endothelial cells that form a slick lining that minimizes friction as blood moves through the lumen. The middle layer is the tunica media. Composed of smooth muscle cells and sheets of elastic tissue, this layer enables the lumen to narrow or widen to regulate blood flow in the body. The tunica adventitia forms a protective outer layer of the blood vessel wall and it also anchors the blood vessel to surrounding structures. |
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