Blood Vessels.pdf

THE CARDIOVASCULAR SYSTEM: BIOL172 BLOOD VESSELS Hema Roopnarine STRUCTURE AND FUNCTION OF BLOOD VESSELS 5 main types  Arteries – carry blood AWAY from the heart  Arterioles  Capillaries – site of exchange  Venules  Veins – carry blood TO the heart BASIC STRUCTURE...

THE CARDIOVASCULAR SYSTEM: BIOL172 BLOOD VESSELS Hema Roopnarine STRUCTURE AND FUNCTION OF BLOOD VESSELS 5 main types  Arteries – carry blood AWAY from the heart  Arterioles  Capillaries – site of exchange  Venules  Veins – carry blood TO the heart BASIC STRUCTURE  3 layers or tunics  Tunica interna (intima)  Tunica media  Tunica externa  Modifications account for 5 types of blood vessels and their structural/ functional differences STRUCTURE Tunica interna (intima)  Inner lining in direct contact with blood  Endothelium continuous with endocardial lining of heart  Active role in vessel-related activities Tunica media  Muscular and connective tissue layer  Greatest variation among vessel types  Smooth muscle regulates diameter of lumen Tunica externa  Elastic and collagen fibers  Vasa vasorum  Helps anchor vessel to surrounding tissue ARTERIES  3 layers of typical blood vessel  Thick muscular-to-elastic tunica media  High compliance – walls stretch and expand in response to pressure without tearing  Vasoconstriction – decrease in lumen diameter  Vasodilation – increase in lumen diameter  Arteries have elastic and muscular characteristics which make them suitable to their function MAJOR SYSTEMIC ARTERIES ELASTIC ARTERIES  Largest arteries  Largest diameter but walls relatively thin  Function as pressure reservoir  Help propel blood forward while ventricles relaxing  Also known as conducting arteries – conduct blood to medium-sized arteries ARTERIES Muscular arteries  Tunica media contains more smooth muscle and fewer elastic fibers than elastic arteries  Walls relatively thick  Capable of great vasoconstriction/ vasodilatation to adjust rate of blood flow  Also called distributing arteries Anastomoses  Union of the branches of 2 or more arteries supplying the same body region  Provide alternate routes – collateral circulation ARTERIOLES  Abundant microscopic vessels  Metarteriole has precapillary sphincter which monitors blood flow into capillary  Sympathetic innervation and local chemical mediators can alter diameter and thus blood flow and resistance  Resistance vessels – resistance is opposition to blood flow  Vasoconstriction can raise blood pressure CAPILLARIES  Smallest blood vessels connect arterial outflow and venous return  Microcirculation – flow from metarteriole through capillaries and into postcapillary venule  Exchange vessels – primary function is exchange between blood and interstitial fluid  Lack tunica media and tunica externa  Substances pass through just one layer of endothelial cells and basement membrane  Capillary beds – arise from single metarteriole  Vasomotion – intermittent contraction and relaxation  Throughfare channel – bypasses capillary bed ARTERIES, CAPILLARIES, AND VENULE TYPES OF CAPILLARIES 3 types 1.Continuous  Endothelial cell membranes from continuous tube 2.Fenestrated  Have fenestrations or pores 3.Sinusoids  Wider and more winding  Unusually large fenestrations VEINS & VENULES Portal vein – blood passes through second capillary bed  Hepatic or hypophyseal Venules  Thinner walls than arterial counterparts  Postcapillary venule – smallest venule  Form part of microcirculatory exchange unit with capillaries  Muscular venules have thicker walls with 1 or 2 layers of smooth muscle VEINS  Structural changes not as distinct as in arteries  In general, very thin walls in relation to total diameter  Same 3 layers  Tunica interna thinner than arteries  Tunica interna thinner with little smooth muscle  Tunica externa thickest layer  Not designed to withstand high pressure  Valves – folds on tunica interna forming cusps  Aid in venous return by preventing backflow MAJOR SYSTEMIC VEINS VENOUS VALVES BLOOD DISTRIBUTION  Largest portion of blood at rest is in systemic veins and venules  Blood reservoir  Venoconstriction reduces volume of blood in reservoirs and allows greater blood volume to flow where needed CAPILLARY EXCHANGE Movement of substances between blood and interstitial fluid 3 basic methods 1. Diffusion 2. Transcytosis 3. Bulk flow DIFFUSION  Most important method  Substances move down their concentration gradient  O2 and nutrients from blood to interstitial fluid to body cells  CO2 and wastes move from body cells to interstitial fluid to blood  Can cross capillary wall through intracellular clefts, fenestrations or through endothelial cells  Most plasma proteins cannot cross  Except in sinusoids – proteins and even blood cells leave  Blood-brain barrier – tight junctions limit diffusion TRANSCYTOSIS  Small quantity of material  Substances in blood plasma become enclosed within pinocytotic vessicles that enter endothelial cells by endocytosis and leave by exocytosis  Important mainly for large, lipid-insoluble molecules that cannot cross capillary walls any other way BULK FLOW  Passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction  Based on pressure gradient  Diffusion is more important for solute exchange  Bulk flow more important for regulation of relative volumes of blood and interstitial fluid  Filtration – from capillaries into interstitial fluid  Reabsorption – from interstitial fluid into capillaries CIRCULATORY PATHWAYS HEMODYNAMICS: FACTORS AFFECTING BLOOD FLOW Blood flow – volume of blood that flows through any tissue in a given period of time (in mL/min) Total blood flow is cardiac output (CO)  Volume of blood that circulates through systemic (or pulmonary) blood vessels each minute CO = heart rate (HR) x stroke volume (SV) Distribution of CO depends on  Pressure differences that drive blood through tissue  Flows from higher to lower pressure  Resistance to blood flow in specific blood vessels  Higher resistance means smaller blood flow  BLOOD PRESSURE  Contraction of ventricles generates blood pressure  Systolic BP – highest pressure attained in arteries during systole  Diastolic BP – lowest arterial pressure during diastole  Pressure falls progressively with distance from left ventricle  Blood pressure also depends on total volume of blood  VASCULAR RESISTANCE  Opposition to blood flow due to friction between blood and walls of blood vessels  Depends on  Size of lumen – vasoconstriction males lumen smaller meaning greater resistance  Blood viscosity – ratio of RBCs to plasma and protein concentration, higher viscosity means higher resistance  Total blood vessel length – resistance directly proportional to length of vessel  400 miles of additional blood vessels for each 2.2lb. of fat VENOUS RETURN  Volume of blood flowing back to heart through systemic veins  Occurs due to pressure generated by constriction of left ventricle  Small pressure difference from venule (16 mmHg) to right ventricle (0 mmHg) sufficient SKELETAL MUSCLE PUMP 2 other mechanisms  Skeletal muscle pump – moves blood in 1 direction due to valves  Respiratory pump – due to pressure changes in thoracic and abdominal cavities Proximal valve Distal valve 1 2 3 VELOCITY OF BLOOD FLOW  Speed in cm/sec in inversely related to cross-sectional area  Velocity is slowest where total cross sectional area is greatest  Blood flow becomes slower farther from the heart  Slowest in capillaries  Aids in exchange  Circulation time – time required for a drop of blood to pass from right atrium, through pulmonary and systemic circulation and back to right atrium  Normally 1 minute at rest RELATIONSHIP BETWEEN VELOCITY OF BLOOD FLOW AND TOTAL CROSS-SECTIONED AREA IN DIFFERENT TYPES OF BLOOD VESSELS CONTROL OF BLOOD PRESSURE AND BLOOD FLOW  Interconnected negative feedback systems control blood pressure by adjusting heart rate, stroke volume, systemic vascular resistance, and blood volume  Some act faster that others  Some shorter- or longer-term ROLE OF CARDIOVASCULAR CENTER (CV)  In medulla oblongata  Helps regulate heart rate and stroke volume  Also controls neural, hormonal, and local negative feedback systems that regulate blood pressure and blood flow to specific tissues  Groups of neurons regulate heart rate, contractility of ventricles, and blood vessel diameter  Cardiostimulatory and cardioinhibitory centers  Vasomotor center control blood vessel diameter  Receives input from both higher brain regions and sensory receptors CV CENTER 3 MAIN TYPES OF SENSORY RECEPTORS  Proprioceptors – monitor movements of joints and muscles to provide input during physical activity  Baroreceptors – monitor pressure changes and stretch in blood vessel walls  Chemoreceptors – monitor concentration of various chemicals in the blood Output from CV flows along neurons of ANS  Sympathetic (stimulatory) opposes parasympathetic (inhibitory) NEURAL REGULATION OF BLOOD PRESSURE  Negative feedback loops from 2 types of reflexes 1. Baroreceptor reflexes  Pressure-sensitive receptors in internal carotid arteries and other large arteries in neck and chest  Carotid sinus reflex helps regulate blood pressure in brain  Aortic reflex regulates systemic blood pressure  When blood pressure falls, baroreceptors stretched less, slower rate of impulses to CV  CV decreases parasympathetic stimulation and increases sympathetic stimulation NEURAL REGULATION OF BLOOD PRESSURE 2. Chemoreceptor reflexes  Receptors located close to baroreceptors of carotid sinus (carotid bodies) and aortic arch (aortic bodies)  Detect hypoxia (low O2), hypercapnia (high CO2), acidosis (high H+) and send signals to CV  CV increases sympathetic stimulation to arterioles and veins, producing vasoconstriction and an increase in blood pressure  Receptors also provide input to respiratory center to adjust breathing rate HORMONAL REGULATION OF BLOOD PRESSURE  Renin-angiotensin-aldosterone (RAA) system  Renin (released by kidney when blood volume falls or blood flow decreases) and angiotensin converting enzyme (ACE) act on substrates to produce active hormone angiotensin II  Raises BP by vasoconstriction and secretion of aldosterone (increases water reabsorption in kidneys to raise blood volume and pressure) HORMONAL REGULATION OF BLOOD PRESSURE  Epinephrine and norepinephrine  Adrenal medulla releases in response to sympathetic stimulation  Increase cardiac output by increasing rate and force of heart contractions  Antidiuretic hormone (ADH) or vasopressin  Produced by hypothalamus, released by posterior pituitary  Response to dehydration or decreased blood volume  Causes vasoconstriction which increases blood pressure ATRIAL NATRIURETIC PEPTIDE (ANP)  Released by cells of atria  Lowers blood pressure by causing vasodilation and promoting loss of salt and water in urine  Reduces blood volume AUTOREGULATION OF BLOOD PRESSURE  Ability of tissue to automatically adjust its blood flow to match metabolic demands  Demand of O2 and nutrients can rise tenfold during exercise in heart and skeletal muscles  Also controls regional blood flow in the brain during different mental and physical activities  2 general types of stimuli  Physical – temperature changes, myogenic response  Vasodilating and vasoconstricting chemicals alter blood vessel diameter CIRCULATION Important difference between pulmonary and systemic circulation in autoregulatory response  Systemic blood vessel walls dilate in response to low O 2 to increase O2 delivery  Walls of pulmonary blood vessels constrict under low O 2 to ensure most blood flows to better ventilated areas of lung