Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5^TH 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

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

• 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

Sinusoids

• 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