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Cardiovascular System: Blood Vessels Chapter 19 Blood Vessels • Closed system starting and ending at heart • 3 major types – Arteries carry blood away from the heart • Systemic are O2 rich • Pulmonary are O2 poor – Capillaries have direct contact with tissues – Veins carry blood toward the heart • Systemic are O2 poor • Pulmonary are O2 rich Blood Vessel Walls • 3 layers or tunics – Tunica intima • Simple squamous endothelia layer – Tunica media • Smooth muscle and elastin fibers • Sympathetic NS controls vasodilation and -constriction • Thicker in arteries to regulate pressure and flow – Tunica externa • Collagen fibers for support and reinforcement • Larger vessels with vasa vasorum • Central lumen contains the blood Arteries • Elastic arteries are largest and nearest the heart – Large lumen why? – Tunica media w/ more elastin than smooth why? – Continuous, constant blood flow • Muscular arteries deliver blood to organs – Tunica media primarily smooth muscle why? • Arterioles lead to capillary beds – Tunica media scattered smooth muscle and little elastin – Vasodilation and –constriction alter capillary flow Capillaries • Thin tunica intima w/basement membrane – Pericytes, similar to smooth muscle, to stabilize – Cells move single file – Accessible to most tissues for exchange • Types – Continuous abundant in skin and muscle • Endothelium uninterrupted • Tight junctions link cells – Fenestrated where rapid fluid exchange occurs • Endothelium with numerous pores • Pores thinly covered, but more permeable – Sinusoids in liver, bone marrow, and lymphoid tissue • Larger pores and lumens • Slower blood flow Capillary Beds • Vascular shunts – Terminal arteriole – Metateriole – Thoroughfare channel – Postcapillary venule • True capillaries – Precapillary spinchters at junction with shunts • Rings of smooth muscle • Regulate flow based on chemical input and needs – Digestive tract: before and after a meal – Skeletal muscles: exercising and relaxing Veins • Venules – Smallest are entirely endothelia and extremely porous – Larger with thin tunica media and tunica externa • Veins – 3 thinner tunicas • Media has little smooth muscle or elastin • Externa thickest – Larger lumens offer little blood flow resistance • Pressure lower • Hold ~65% of blood volume – Venous valves in limbs • Formed by tunica intima • Prevent blood backflow due to gravity Vascular Anastomoses • Fusing of blood vessels in a given region • Provide multiple paths to/from organs/tissue – Near joints, also in heart and brain – Lacking in retina, kidneys, and spleen • Ensures uninterrupted blood flow – Metarteriole-thoroughfare channel is an example Physiology of Circulation Terminology • Blood flow (ml/min) – Amount of blood moving at a given time – Equivalent to cardiac output (CO) – Constant at rest; varies w/organ needs • Blood pressure (mm Hg) – Force exerted on a vascular wall • Systemic arteries used when measured – Keeps blood moving from high to low Physiology of Circulation Terminology (cont.) • Peripheral resistance – Opposition to blood flow from vessel friction – Sources • Viscosity: thickness of blood – Changes in RBC numbers can increase/decrease, • Vessel length – Longer vessel = more resistance – More vessels = more total length • Vessel diameter – Smaller arterioles can constrict/enlarge – Flow is slowed along walls larger diameter = less wall contact – Turbulence, from additional wall resistance, increases Basic Circulation Physiology • Blood flow = blood pressure/resistance SO – Increase pressure = increase flow BUT – Increase resistance = decrease flow • Resistance affects local flow more than pressure – Vasoconstriction/-dilation in an organ – Pressure basically unchanged overall Systemic Blood Pressure • Highest at heart – Decreases w/increased distance • Steepest change in arterioles – Resistance highest • Flow maintained by pressure gradient Arterial Pressure • Pressure near heart is pulsatile – Systolic pressure is max during ventricular contraction • Added blood supply stretches arteries – Diastolic pressure is minimum during ventricular relaxation • Recoil of arteries to maintain pressure – Difference creates pulse pressure, measured as our pulse • Increases w/ arteriosclerosis b/c elasticity decreases • Mean arterial pressure (MAP) is pressure moving blood to tissues – MAP = diastolic + 1/3 pulse pressure – Accounts for changes in arterial BP and longer diastole Capillary Pressure • Significantly lower than arterial • Beneficial to capillary structure – Vessels are thin = fragile – Minimum needed to force filtrates out Venous Pressure • Minimal changes in a cardiac cycle • Adaptations to compensate – Muscular pump: skeletal muscles • ‘Cankles’ and mats for standing – Respiratory pump: pressure created from inhalation – Valves – Smooth muscle in tunics Monitoring Blood Pressure • Short-term – Cardiac output (CO) • Increase CO = increase BP • Decrease CO = decrease BP • See fig. 19.7 – Peripheral resistance (R) • Increase vasoconstriction = increase BP • Increase vasodilation = decrease BP • Long-term – Blood volume • Increase blood volume = increase BP • Decrease blood volume = decrease BP Vasomotor Center • Monitors blood vessel diameter from medulla – SNS innervation (NE and ACh) – Minimizes moment specific changes to BP • Types of neural controls – Baroreceptors respond to vessel stretch • Carotid arteries and aortic arch • Inhibits vasomotor center vasodilation – Chemoreceptors respond to O2 and pH drop (CO2 rise) • Close to baroreceptors • Stimulates vasomotor center vasoconstriction – Hypothalamus and cerebral cortex input to medulla Hormonal Control of BP • Vasodilators – Atrial natriuretic peptide (ANP): increase Na+ and H2O excretion – Nitric oxide (NO): brief and localized – Inflammatory: histamines and prostocyclin – Alcohol: inhibits ADH • Vasoconstrictors – Adrenal medulla hormones: NE and Epi – Antidiuretic hormone (ADH): stimulates H2O conservation – Angiotensin II: renin from kidneys catalyzes production Renal Regulation of BP • Monitors blood volume from kidneys – Increased BP stimulates H2O loss = decrease BP – Decreased BP stimulates H2O retention = increase BP • Excessive salt intake • Indirect mechanisms – Renin-angiotensin mechanism – Angiotensin II: vasoconstriction, aldosterone to resorb Na+/H2O, ADH release to resorb water • Direct mechanisms – Fluid speed increased to kidneys – Less absorption = more excretion • Figs 19.10 and 19.11 Measuring Circulation • Pulse – Measured at pressure points – Can be used to slow/halt distal blood flow • Blood pressure – Ausculatory method at brachial artery – First sound = systolic, no sounds = diastolic • Along with respiratory rate and body temp constitute vital signs Blood Flow • Functions – – – – Deliver O2 and nutrients; remove CO2 and wastes Exchange gases Absorb nutrients Form urine • Rate dictated by needs (rest) – – – – – – Brain (13%) Heart (4%) Kidneys (20%) Abdominal organs (24%) Skeletal muscles (20%) All others (19%) Blood Flow Velocity • Inversely related to cross-sectional area – More area (more vessels) = slower flow • Slowest in capillaries = more exchange time Short-Tem Autoregulation • Adjustments of blood flow to tissue needs – Local, short-term intrinsic control – MAP and CO unchanged, but diameter of arterioles not • Metabolic controls (vasodilation) – Hb carries O2 and NO to tissues when O2 levels drop – K+, H+, adenosine, histamine too • Myogenic controls – Stretch receptors in smooth muscle – Less pressure signals vasodilation (how change flow?) • Fig 19.15 Long-Term Autoregulation • Angiogenesis increases blood vessel number and diameter – Short-term fails to meet needs – Occurs over weeks or months • Coronary occlusions or high-altitude living conditions Capillary Diffusion • Via [gradient] • Nutrients and gases • Means vary based on molecule properties Capillary Fluid Flow • Capillary hydrostatic pressure (HPc) force fluid out, more at arteriole than venule end • Capillary colloid osmotic pressure (OPc) draws fluid in, constant at both ends • Net filtration pressure (NFP) tells of net fluid loss or gain – NFP = (HPc – HPif) – (OPc – OPif) • More fluid enter tissue than return to blood, but lymphatic system returns Circulatory Shock • Hypovolemic – From large loss of blood volume – HR up, vasoconstriction to increase venous return – Temporary fix; fluid replacement needed ASAP • Vascular – Extreme vasodilation drops resistance • Anaphylactic shock, septicemia, or ANS failure – Blood volume normal, but poor circulation • Cardiac – Heart fails Blood Vessel Imbalances • Varicose veins: stretching veins due to leaky valves • Arteriosclerosis: hardening of arteries – Contributes to hypertension: BP > 140/90 mmHg – Most common is atherosclerosis: plaque build up of tunica intima • Hypotension: BP < 100/80 mm Hg – Less serious and often good – Can increase fainting/dizziness • Aneurysm: ballooning of a blood vessel • Phlebitis: inflammation of a vein