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Lecture

Chapter 19 blood vessels.doc


Department
Anatomy and Physiology
Course Code
ANP 1105
Professor
Jacqueline Carnegie

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Chapter 19: Blood Vessels
blood vessels- form a closed delivery system that begins and ends at the heart
Structure of the Blood Vessel Walls
-walls of all blood vessels (except the smallest) have 3 distinct layers or tunics (coverings)
that surround a central blood-containing space (the vessel lumen)
1) Tunica Intima
-the innermost tunic
-this tunic has intimate contact with the blood in the lumen
-contains endothelium; simple squamous epithelium that lines the lumen of all vessels
-the 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
endothelium
2) Tunica Media
-the middle tunic
-circularly arranged smooth muscle cells and sheets of elastin
-activity of smooth muscle is regulated by sympathetic vasomotor nerve fibres of the ANS
-vasocontriction or vasodilation are caused by the smooth muscles of the tunica media
vasocontriction- reduction in lumen diameter as the smooth muscle contracts
vasodilation- 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 pressure
-tunica media is the bulkiest layer in the arteries (arteries have the responsibility for
maintaining blood pressure and continuous blood circulation)
3) Tunica Externa
-outermost layer of a blood vessel wall
-composed of loosely woven collagen fibres that protect and reinforce the vessel and
anchor it to surrounding structures
-infiltrated with nerve fibres, lymphatic vessels
-in larger veins, it is infiltrated with a network of elastin fibres
-in larger veins, tunica externa contain vasa vasorum (literally "vessels of vessels"; a
system of tiny blood vessels)
-they 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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-arteries -> arterioles -> capillary beds -> venules -> veins
-arteries carry blood away from the heart; oxygenated except for pulmonary circulation and
umbilical vessels of a fetus
1) Elastic (Conducting) Arteries
-thick-walled arteries near the heart (i.e.: the aorta and its major branches)
-largest in diameter compared to the other arteries (1 - 2.5 cm)
-have large lumens which make them low-resistance pathways
-conduct blood to medium sized arteries and are referred to as conducting arteries
-contain more elastin than any other vessel type
-elastin is present in all 3 tunics and the tunica media contains the most
-elastin in the tunica media make up "holey" / "swiss cheese-like" laminae
(sheets) of elastic connective tissue that is interspersed between the layers of
smooth muscle cells
-contain substantial amounts of smooth muscle but are inactive in vasoconstriction
-functionally, they are simple elastic tubes
-elastic arteries are pressure reservoirs that expand and recoil as blood is ejected from the
heart
-blood flows continuously rather than starting and stopping with the pulsating rhythm of
the heart
-blood flows more intermittently when vessels become hard and unyielding as in
arteriosclerosis
-the elastic arteries have a pressure-smoothing effect
2) Muscular (Distributing) Arteries
-found distal to elastic arteries
-deliver blood to specific body organs and account for most of the named arteries
-internal diameter ranges from 1cm to 0.3 mm
-proportionally, muscular arteries have the thickest tunica media
-tunica media contains relatively more smooth muscle and less elastic tissue than
elastic arteries
-allows for more it to be more active in vasoconstriction and less distensible
-there is an elastic lamina on each face of the tunica media
3) Arterioles
-smallest of the arteries
-have a lumen diameter ranging from 0.3mm to 10microm
-larger arterioles have all 3 tunics
-tunica media is mostly smooth muscle with scattered elastic fibers
-smaller arterioles that lead into the capillary bed are a single layer of smooth muscle cells
that spiral around the endothelial lining
Capillaries
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