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Chapter 19 blood vessels.doc

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Anatomy and Physiology
Jacqueline Carnegie

Chapter 19 Blood Vessels blood vessels form a closed delivery system that begins and ends at the heart Structure of the Blood Vessel Wallswalls of all blood vessels except the smallest have 3 distinct layers or tunics coverings that surround a central bloodcontaining space the vessel lumen 1 Tunica Intimathe innermost tunic this tunic has intimate contact with the blood in the lumencontains endothelium simple squamous epithelium that lines the lumen of all vesselsthe endothelium is continuous with the endocardial lining of the heart and its flat cells fit closely together to form a slick surface that minimizes friction as blood moves through the lumen for blood vessels larger than 1mm in diameter a subendothelial layer that consists of a basement membrane and loose connective tissue supports the endothelium2 Tunica Mediathe middle tuniccircularly arranged smooth muscle cells and sheets of elastin activity of smooth muscle is regulated by sympathetic vasomotor nerve fibres of the ANSvasocontriction or vasodilation are caused by the smooth muscles of the tunica mediavasocontriction reduction in lumen diameter as the smooth muscle contractsvasodilation increase in lumen diameter as the smooth muscle relaxes tunica media activity is critical in regulating circulatory dynamics since small changes in vessel diameter greatly influence blood flow and blood pressuretunica media is the bulkiest layer in the arteries arteries have the responsibility for maintaining blood pressure and continuous blood circulation 3 Tunica Externaoutermost layer of a blood vessel wallcomposed of loosely woven collagen fibres that protect and reinforce the vessel and anchor it to surrounding structures infiltrated with nerve fibres lymphatic vesselsin larger veins it is infiltrated with a network of elastin fibresin larger veins tunica externa contain vasa vasorum literally vessels of vessels a system of tiny blood vesselsthey nourish the more external tissues of the blood vessel wall the innermost luminal portions of the vessel obtains its nutrients directly from blood in the lumen Arterial System
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