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Explore the structures and functions of different blood vessels, including arteries, veins, and capillaries. Learn about the anatomy of blood vessel walls and how they contribute to the circulatory system. Discover the health problems associated with arteries such as aneurysms, arteriosclerosis, and atherosclerosis.
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Chapter 21:Blood Vessels and Circulation Primary sources for figures and content: Marieb, E. N. Human Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004. Martini, F. H. Fundamentals of Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.
6 Classes of Blood Vessels • Arteries: • carry blood away from heart • Branch and decrease in diameter • Arterioles: • Are smallest branches of arteries • Connect to capillaries • Capillaries: • are smallest blood vessels • location of exchange between blood and interstitial fluid
6 Classes of Blood Vessels • Venules: • Smallest veins • collect blood from capillaries • Veins: • return blood to heart • Converge and increase in diameter • Anastomoses: - Bypass connection between vessels
The Largest Blood Vessels • Attach to heart • Pulmonary trunk: • carries blood from right ventricle • to pulmonary circulation • Aorta: • carries blood from left ventricle • to systemic circulation
The Smallest Blood Vessels • Capillaries • Have small diameter and thin walls • Chemicals and gases diffuse across walls
The Anatomy of Blood Vessels • Arteries, veins, and capillaries • Have different functions • Have different structures
Arteries and Veins • Walls have 3 layers: 1. tunica intima 2. tunica media 3. tunica externa
The Tunica Intima/Tunica interna • Is the innermost layer • Includes: • the endothelial cell lining • Endothelium = simple squamous epithelial-like cells connected by tight junctions • With basal lamina of loose connective tissue containing elastic fibers (elastin) • Arteries have internal elastic membrane • extra layer of elastic fibers on the outer edge
Tunica Media • Is the middle layer • Contains smooth muscle cells in loose connective tissue with sheets of elastin • Binds to inner and outer layers • Arteries have external elastic membrane • extra layer of elastic fibers on the outer edge
Tunica Externa/Tunica adventitia • Is outer layer • Contains collagen rich external connective tissue sheath • Infiltrated with nerve fibers and lymphatic vessels • Large vessels contain vasa vasorum • Arteries = more collagen, scattered elastic fiber bands • Veins = extensive fiber networks, bundles of smooth muscle cells
Vasa Vasorum • Small arteries and veins • Found: • in walls of large arteries and veins • Function: • Supply cells of tunica media and tunica externa
Structure of Vessel Walls Figure 21-1
Structure of Blood Vessels Figure 21-2
1. Arteries • Designed to change diameter, elastic and muscular, thick walls • Tunica externa contains collagen • Pressure • Elasticity allows arteries to absorb pressure waves that come with each heartbeat • Contractility • Arteries change diameter • Controlled by sympathetic division of ANS
Vasoconstriction and Vasodilation • Vasoconstriction • The contraction of arterial smooth muscle by the ANS • Vasodilatation • The relaxation of arterial smooth muscle • Enlarging the lumen • Affect: • afterload on heart • peripheral blood pressure • capillary blood flow
Artery Characteristics • From heart to capillaries, arteries change: • from elastic arteries • to muscular arteries • to arterioles
Elastic Arteries • Also called conducting arteries • Diameter up to 2.5cm • Elastin in all three tunics • Elasticity evens out pulse force • Stretch (ventricular systole) and rebound (ventricular diastole) • Not involved in systemic vasoconstriction
Muscular Arteries • Also called distribution arteries • Are medium-sized (most arteries) • Transport blood to organs and tissues • Diameter 10mm – 0.3mm • More smooth muscle and less elastin in tunica media than elastic arteries • Involved in systemic vasoconstriction via sympathetic stimulation
2. Arterioles • Also known as resistance vessels • Connect blood supply to capillary beds • Are small – diameters 300µm – 10µm • All three tunics thin with few elastic fibers • Involved in local vasoconstriction via endocrine or sympathetic stimulation
Health Problems with Arteries 1. Aneurysm: • Pressure of blood exceeds elastic capacity of wall • Causes bulge or weak spot prone to rupture • Caused by chronic high blood pressure or arteriosclerosis
Health Problems with Arteries 2. Arteriosclerosis: • Variety of pathological conditions causing changes in walls that decrease elasticity (“thickenings”) • Focal calcification = smooth muscle degenerates, replaced by calcium salts • Atherosclerosis 3. Atherosclerosis: lipid deposits
Health Problems with Arteries 4. Stroke = cerebrovascular accident (CVA) • Interruption of arterial supply to portion of brain due to embolism or atherosclerosis • Brain tissue dies and function is lost
3. Capillaries • Only vessels with thin enough wall structure to allow complete diffusion • Designed to allow diffusion to/from the tissue • Diameter 8 µm • Consists of tunica intima only • endothelium + basal lamina • Human body contains 25,000 miles of capillaries
Capillary Structure Figure 21-4
Capillary Function • Location of all exchange functions of cardiovascular system • Materials diffuse between blood and interstitial fluid
Capillary Structure • Endothelial tube, inside thin basal lamina • No tunica media • No tunica externa • Diameter is similar to red blood cell
Types of Capillaries • Continuous capillaries • Normal diffusion to all tissues except epithelium and cartilage • Complete endothelium, tight junctions • Functions: • Permit diffusion of: water, small solutes, lipid-soluble materials • Block: blood cells and plasma proteins • e.g., the blood–brain barrier
Types of Capillaries 2. Fenestrated capillaries • High volume fluids or large solute transfer • Pores/fenestrations span endothelium • Permit rapid exchange of water and larger solutes between plasma and interstitial fluid
Fenestrated Capillaries • Are found in: • choroid plexus • endocrine organs • kidneys • intestinal tract
Sinusoids • Areas in: • liver • spleen • bone marrow • endocrine organs • Have gaps between adjacent endothelial cells
Types of CapillariesSinusoids 3. Sinusoids • Cell or large protein exchange • Gaps between endothelial cells • Permit free exchange of water and large plasma proteins between blood and interstitial fluid • Phagocytic cells monitor blood at sinusoids • Found: liver, bone marrow, lymphoid tissues
Capillary Networks Figure 21-5
Capillaries Networks • Organized into Capillary bed or capillary plexus • Connect 1 arteriole and 1 venule • Not enough total blood to fill all capillaries at once • Flow through capillary bed must be controlled based on need via precapillary spincters
Capillary Sphincter • Guards entrance to each capillary • Opens and closes, causing capillary blood to flow in pulses
Vasomotion • Contraction and relaxation cycle of capillary sphincters • Spincter relaxed = flow in capillary bed • Spincter constricted = capillary bed empty, flow through anastomoses • Causes blood flow in capillary beds to constantly change routes
Structure of Blood Vessels Figure 21-2
Veins vs. Arteries • Are larger in diameter • Have thinner walls • Carry lower blood pressure
4. Veins • Collect blood from capillaries in tissues and organs • Return blood to heart • Can serve as blood reservoir • Thin walls but large lumens • Thin tunica media = little smooth muscle or elastin • Tunica externa = elastin and smooth muscle • Tunica intima = valves to prevent back-flow
3 Vein Categories • Venules (5th type of vessel): • very small veins • Average diameter 20 µm • collect blood from capillary beds • Small venules lack tunica media • Medium-sized veins: - Diameter 2-9 mm • Large Veins: - Diameters up to 3 cm
Valves in the Venous System Valves in tunica intima insure one way movement Figure 21-6
Vein Valves • Valves = Folds of tunica intima • Prevent blood from flowing backward • Pressure from heart drives blood flow in arteries, but pressure in veins often too low to oppose gravity • Compression pushes blood toward heart • Skeletal muscle movement required to “squish” blood through veins
Health Problems with Veins • Resistance to flow (gravity, obesity) causes pooling above valves, veins stretch out • Varicose veins • Hemorrhoids
Blood Reservoirs in Venous System • Venous system contains 65-70% total blood volume • Can constrict during hemorrhage to keep volume in capillaries and arteries near normal
6. Anastomoses • Bypass routes between vessels • Bypass the capillary bed • Not present in retina, kidney, or spleen • More common in veins