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This 11 page Class Notes was uploaded by Jess Graff on Tuesday March 1, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 22 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 03/01/16
BMS 508.03 2/24/16 Chapter 20 Blood Vessels and Circulation An Introduction to Blood Vessels and Circulation Blood Vessels o Classified by size and histological organization o Instrumental in overall cardiovascular regulation Classes of Blood Vessels • Arteries • Carry blood away from heart • Arterioles • Smallest branches of arteries • Capillaries • Smallest blood vessels • Location of exchange between blood and interstitial fluid • Venules • Collect blood from capillaries • Veins • Return blood to heart • 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 Structure of Vessel Walls • Walls have 3 layers • Tunica intima • Tunica media • Tunica externa • The Tunica Intima (Inner Layer) • Includes: • The endothelial lining • Connective tissue layer • Internal elastic membrane • In arteries, is a layer of elastic fibers in outer margin of tunica intima • The Tunica Media (Middle Layer) • Contains concentric sheets of smooth muscle in loose connective tissue • Binds to inner and outer layers • External elastic membrane of the tunica media • Separates tunica media from tunica externa • The Tunica Externa (Outer Layer) • Anchors vessel to adjacent tissues in arteries • Contains collagen fibers • Elastic fibers • In veins • Contains elastic fibers • Smooth muscle cells • Vasa vasorum (“vessels of vessels”) • Small arteries and veins • In walls of large arteries and veins • Supply cells of tunica media and tunica externa • Differences between Arteries and Veins • Arteries and veins run side by side • Arteries have thicker walls and higher blood pressure • Collapsed artery has small, round lumen (internal space) • Vein has a large, flat lumen • Vein lining contracts, artery lining does not • Artery lining folds • Arteries more elastic • Veins have valves Structure and Function of Arteries • Arteries • Elasticity allows arteries to absorb pressure waves that come with each heartbeat • Contractility • Arteries change diameter • Controlled by sympathetic division of ANS • Vasoconstriction • The contraction of arterial smooth muscle by the ANS • Vasodilation • The relaxation of arterial smooth muscle • Enlarging the lumen • Vasoconstriction and Vasodilation • Affect: • Afterload on heart • Peripheral blood pressure • Capillary blood flow • Arteries • From heart to capillaries, arteries change • From elastic arteries to muscular arteries and arterioles • Elastic Arteries • Also called conducting arteries • Large vessels (e.g., pulmonary trunk and aorta) • Tunica media has many elastic fibers and few muscle cells • Elasticity evens out pulse force • Muscular Arteries • Also called distribution arteries • Medium sized (most arteries) • Tunica media has many muscle cells • Arterioles • Small • Have little or no tunica externa • Have thin or incomplete tunica media • Artery Diameter • Small muscular arteries and arterioles • Change with sympathetic or endocrine stimulation • Constricted arteries oppose blood flow • Resistance (R) • Resistance vessels – arterioles • Aneurysm • A bulge in an arterial wall • Is caused by weak spot in elastic fibers • Pressure may rupture vessel Structure and Function of Capillaries • Capillaries • Are smallest vessels with thin walls • Microscopic capillary networks permeate all active tissues • Capillary function • Location of all exchange functions of cardiovascular system • Materials diffuse between blood and interstitial fluid • Capillary Structure • Endothelial tube, inside thin basement membrane • No tunica media • No tunica externa • Diameter is similar to red blood cell • Continuous Capillaries • Have complete endothelial lining • Found in all tissues except epithelia and cartilage • Functions of continuous capillaries • Permit diffusion of water, small solutes, and lipid-soluble materials • Block blood cells and plasma proteins • Specialized Continuous Capillaries • In CNS and thymus • Have very restricted permeability • Example: the blood–brain barrier • Fenestrated Capillaries • Have pores in endothelial lining • Permit rapid exchange of water and larger solutes between plasma and interstitial fluid • Found in: • Choroid plexus • Endocrine organs • Kidneys • Intestinal tract • Sinusoids (Sinusoidal Capillaries) • Have gaps between adjacent endothelial cells • Liver • Spleen • Bone marrow • Endocrine organs • Permit free exchange • Of water and large plasma proteins • Between blood and interstitial fluid • Phagocytic cells monitor blood at sinusoids • Capillary Beds (Capillary Plexus) • Connect 1 arteriole and 1 venule • Precapillary sphincter • Guards entrance to each capillary • Opens and closes, causing capillary blood to flow in pulses • Thoroughfare Channels • Direct capillary connections between arterioles and venules • Controlled by smooth muscle segments (metarterioles) • Collaterals • Multiple arteries that contribute to one capillary bed • Allow circulation if one artery is blocked • Arterial anastomosis • Fusion of 2 collateral arteries • Arteriovenous anastomoses • Direct connections between arterioles and venules • Bypass the capillary bed • Angiogenesis • Formation of new blood vessels • Vascular endothelial growth factor (VEGF) • Occurs in the embryo as tissues and organs develop • Occurs in response to factors released by cells that are hypoxic, or oxygen-starved • Most important in cardiac muscle, where it takes place in response to a chronically constricted or occluded vessel • Vasomotion • Contraction and relaxation cycle of capillary sphincters • Causes blood flow in capillary beds to constantly change routes Structure and Function of Veins • Veins • Collect blood from capillaries in tissues and organs • Return blood to heart • Larger in diameter than arteries • Have thinner walls than arteries • Have lower blood pressure • Venules • Very small veins • Collect blood from capillaries • Medium-Sized Veins • Thin tunica media and few smooth muscle cells • Tunica externa with longitudinal bundles of elastic fibers • Large Veins • Have all 3 tunica layers • Thick tunica externa • Thin tunica media • Venous Valves • Folds of tunica intima • Prevent blood from flowing backward • Compression pushes blood toward heart Blood Vessels • The Distribution of Blood • Heart, arteries, and capillaries • 30–35% of blood volume • Venous system • 60–65% • 1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin • Capacitance of a Blood Vessel • The ability to stretch • Relationship between blood volume and blood pressure • Veins (capacitance vessels) stretch more than arteries • Venous Response to Blood Loss • Vasomotor center stimulates sympathetic nerves • Systemic veins constrict (venoconstriction) • Veins in liver, skin, and lungs redistribute venous reserve Pressure and Resistance • Total Capillary Blood Flow • Equals cardiac output • Is determined by: • Pressure (P) and resistance (R) in the cardiovascular system • Pressure (P) • The heart generates P to overcome resistance • Absolute pressure is less important than pressure gradient • The Pressure Gradient (∆P) • Circulatory pressure • The difference between: • Pressure at the heart and pressure at peripheral capillary beds • Flow (F) • Proportional to the pressure difference (∆P) • Divided by R • Measuring Pressure • Blood pressure (BP) • Arterial pressure (mm Hg) • Capillary hydrostatic pressure (CHP) • Pressure within the capillary beds • Venous pressure • Pressure in the venous system • Circulatory Pressure • ∆P across the systemic circuit (about 100 mm Hg) • Circulatory pressure must overcome total peripheral resistance • R of entire cardiovascular system • Total Peripheral Resistance • Vascular resistance • Blood viscosity • Turbulence • Vascular Resistance • Due to friction between blood and vessel walls • Depends on vessel length and vessel diameter • Adult vessel length is constant • Vessel diameter varies by vasodilation and vasoconstriction • R increases exponentially as vessel diameter decreases • Viscosity • R caused by molecules and suspended materials in a liquid • Whole blood viscosity is about four times that of water • Turbulence • Swirling action that disturbs smooth flow of liquid • Occurs in heart chambers and great vessels • Atherosclerotic plaques cause abnormal turbulence
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