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Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5TH Edition, , Benjamine Cummings Publisher, 2001

 Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.ter

Chapter 20

The Cardiovascular System: Blood Vessels


Blood Vessels

•      Blood is carried in a closed system of vessels that begins and ends at the heart

•      The three major types of vessels are arteries, capillaries, and veins

•      Arteries carry blood away from the heart, veins carry blood toward the heart

•      Capillaries contact tissue cells and directly serve cellular needs

Continuous Capillary Structure

Fenestrated Capillary Structure

Discontinuous Sinusoidal Capillary Structure

Generalized Structure of Blood Vessels

•      Arteries and veins are composed of  three tunics – tunica interna, tunica media, and tunica externa

•      Capillaries are composed of endothelium with sparse basal lamina

•      Lumen – central blood-containing space surrounded by tunics


•      Tunica interna (tunica intima)

•    Endothelial layer that lines the lumen of all vessels

•    In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present

•      Tunica media

•    Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system

•    Controls vasoconstriction/vasodilation of vessels

•      Tunica externa (tunica adventitia)

•    Collagen fibers that protect and reinforce vessels

•    Larger vessels contain vasa vasorum

Elastic (Conducting) Arteries

•      Thick-walled arteries near the heart; the aorta and its major branches

•    Large lumen allow low-resistance conduction of blood

•    Contain elastin in all three tunics

•    Withstand and smooth out large blood pressure fluctuations

•    Allow blood to flow fairly continuously through the body

Muscular Arteries and Arterioles

•      Muscular arteries – distal to elastic arteries; deliver blood to body organs

•    Have thick tunica media with more smooth muscle and less elastic tissue

•    Active in vasoconstriction

•      Arterioles – smallest arteries; lead to capillary beds

•    Control flow into capillary beds via vasodilation and constriction


•      Capillaries are the smallest blood vessels

•    Walls consisting of a thin tunica interna, one cell thick

•    Allow only a single RBC to pass at a time

•    Pericytes on the outer surface stabilize their walls

•      There are three structural types of capillaries: continuous, fenestrated, and sinusoids

Continuous Capillaries

•      Continuous capillaries are abundant in the skin and muscles, and have:

•    Endothelial cells that provide an uninterrupted lining

•    Adjacent cells that are held together with tight junctions

•    Intercellular clefts of unjoined membranes that allow the passage of fluids

•      Continuous capillaries of the brain:

•    Have tight junctions completely around the endothelium

•    Constitute the blood-brain barrier

Fenestrated Capillaries

•      Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

•      Characterized by:

•    An endothelium riddled with pores (fenestrations)

•    Greater permeability to solutes and fluids than other capillaries


•      Highly modified, leaky, fenestrated capillaries with large lumens

•      Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

•      Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

•      Blood flows sluggishly, allowing for modification in various ways

Capillary Beds

•      A microcirculation of interwoven networks of capillaries, consisting of:

•    Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a  postcapillary venule

•    True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed