Cardiovascular System Types of Blood Vessels 1409

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Blood Vessels • Blood vessels are the channels or conduits through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. One system, the pulmonary vessels, transports blood from the right ventricle to the lungs and back to the left atrium. The other system, the systemic vessels, carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins. • Arteries • Arteries carry blood away from the heart. • The wall of an artery consists of three layers. • The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. • The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. • The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. • • Veins • Veins carry blood toward the heart. • The walls of veins have the same three layers as the arteries. Although all the layers are present, there is less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries, which is related to the fact that blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. • Veins also have valves. Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity. • • Capillaries • Capillaries, the smallest and most numerous of the blood vessels, • The primary function of capillaries is the exchange of materials between the blood and tissue cells. • • #TypesOfBloodVessels • #ArtriesVeinsAndCapillaries

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