ch21saladinIM - Chapter21: ChapterOverview...

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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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