Blood Vessels The central opening of a blood vessel, the lumen, is surrounded by a wall consisting of three layers: The tunica intima is the inner layer facing the blood. It is composed of an innermost layer of endothelium (simple squamous epithelium) surrounded by variable amounts of connective tissues. • The tunica media, the middle layer, is composed of smooth muscle with variable amounts of elastic fibers. • The tunica adventitia, the outer layer, is composed of connective tissue. The cardiovascular system consists of three kinds of blood vessels that form a closed system of passageways: • Arteries carry blood away from the heart. The three kinds of arteries are categorized by size and function: • Elastic arteries (conducting arteries) are the largest arteries and include the aorta and other nearby branches. The tunica media of elastic arteries contains a
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