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Unformatted text preview: Chapter 21: The Circulatory System III—Blood Vessels Chapter Overview The knowledge that blood circulates through the body is something that we take for granted but, in fact, the circulation of blood was not demonstrated until the 17 th century when William Harvey provided experimental evidence that blood travels from the heart, to the tissues, and back to the heart. Harvey’s observations were even more remarkable given that there were no microscopes to show that tiny vessels, capillaries, provide the connection between arteries and veins that makes circulation possible. General Anatomy of the Blood Vessels There are three principal categories of blood vessels: arteries, veins, and capillaries. Arteries carry blood away from the heart, while veins return blood to the heart. Capillaries are microscopic vessels that connect the smallest arteries to the smallest veins. The Vessel Wall The walls of arteries and veins are composed of three layers. 1. Tunica interna (tunica intima) The innermost layer is made of simple squamous epithelium called endothelium. The endothelium is selectively permeable and facilitates the passage of material between the blood and tissue fluid. It secretes chemicals that cause smooth muscle to contract or relax resulting in constriction or dilation, and draws leukocytes to injured areas where defensive action is needed. 2. Tunica media The middle layer consists of smooth muscle, collagen, and varying amounts of elastic tissue. The tunica media facilitates vasoconstriction and vasodilation. 3. Tunica externa (tunica adventitia) The outermost layer is composed of loose connective tissue that anchors the vessel and provides passage for nerves, lymphatic vessels, and smaller blood vessels. Arteries Each beat of the heart creates a surge of pressure in the arteries and, therefore, they have a strong, resilient structure that resists high blood pressure. Their walls contain more muscle compared to veins. Three categories of arteries are distinguished by size: 1. Conducting (elastic or large) arteries are the largest. The tunica media has numerous layers of elastic sheets that alternate with thin layers of smooth muscle, collagen, and elastic fibers. The tunica externa is thick. The structure of the conducting arteries allows them to expand and recoil during systole and diastole, which lessens the fluctuations in blood pressure....
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This note was uploaded on 02/09/2012 for the course BIO 102 taught by Professor William during the Spring '11 term at Harvard.
- Spring '11