Blood Vessels PDF

Blood Vessels Blood vessels Arteries • carry oxygenated blood • Compliance- ability of a blood vessel to adjust the blood pressure and increase the volume of blood that it can hold Veins • carry deoxygenated blood • Capacitance -Veins are capable of storing a significantly larger volume of blood...

Blood Vessels Blood vessels Arteries • carry oxygenated blood • Compliance- ability of a blood vessel to adjust the blood pressure and increase the volume of blood that it can hold Veins • carry deoxygenated blood • Capacitance -Veins are capable of storing a significantly larger volume of blood than arteries due to their large lumen and high compliance Structure and function of blood vessels 5 main types • • • • • Arteries – carry blood AWAY from the heart Arterioles – provide blood to the organs Capillaries – site of exchange Venules – smallest veins receive blood from the capillaries Veins – carry blood TO the heart Structure of Blood Vessels • Tunica Interna • Tunica Media • Tunica Externa Blood Vessels Structure Tunica Interna (intima) • • • • • Inner lining in direct contact with blood Endothelium- composed of simple squamous epithelium Has a base membrane Covered by an internal elastic lamina Has a lumen, only layer in touch with blood Blood vessel structure Tunica Media • Smooth Muscle layer and external elastic lamina (connective tissue) • Arrange in rings around the lumen • High compliance (walls stretch or expand easily) • Smooth muscle regulates diameter of lumen Blood Vessels Structure Tunica externa (Adventitia) • Elastic and collagen fibers • Vasa vasorum • Helps anchor vessel to surrounding tissue Arteries 3 layers of typical blood vessel 1. Thick muscular-to-elastic tunica media 2. High compliance – walls stretch and expand in response to pressure without tearing 3. Vasoconstriction – decrease in lumen diameter Vasodilation – increase in lumen diameter Elastic Arteries 1. Largest arteries 2. Largest diameter but walls relatively thin 3. Function as pressure reservoir 4. Help propel blood forward while ventricles relaxing 5. Also known as conducting arteries – conduct blood to medium-sized 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 • precapillary sphincter monitors blood flow into capillary • • Sympathetic innervation and local chemical mediators can alter diameter and thus blood flow and resistance Resistance vessels – resistance is opposite to blood flow • Vasoconstriction can raise blood pressure Arteries Arterioles Capillaries EXCHANGE Venules Capillaries • Smallest blood vessels • Connect arterial outflow and venous return • Microcirculation flow from metarteriole through capillaries and into postcapillary venule • Are known as exchange vessels – primary function is exchange between blood and interstitial fluid (nutrients and waste products) Arteries, Capillaries, and Venule Types of Capillaries 3 types • Continuous Endothelial cell membranes from continuous tube • Fenestrated Have fenestrations or pores • Sinusoids Wider and more winding large fenestrations Veins • • Very thin walls in relation to total diameter Same 3 layers (thickness of the layers are different) • • • • Tunica interna thinner than arteries Tunica interna thinner with little smooth muscle Tunica externa thickest layer Not designed to withstand high pressure • (absence of internal and elastic layers) • Larger lumen • Valves – folds on tunica interna forming Flaplike-cusps • Aid in venous return by preventing backflow Blood Distribution Largest portion of blood (64%) 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 Diffusion Transcytosis 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 • • Dynamics of Capillary Exchange NFP (Net Filtration Pressure) Net filtration pressure (NFP) • balance of 2 pressures • one promote filtration the other promote reabsorption 2 pressures promote filtration • Blood hydrostatic pressure (BHP) generated by pumping action of heart • Falls over capillary bed from 35 to 16 mmHg • Interstitial fluid osmotic pressure (IFOP) • 1 mmHg NFP = (BHP +IFOP) – (BCOP + IFHP) NFP = (BHP + IFOP) – (BCOP + IFHP 2 pressures promote reabsorption • Blood colloid osmotic pressure (BCOP) • Due to presence of blood plasma proteins to large to cross walls • Averages 36 mmHg • Interstitial fluid hydrostatic pressure (IFHP) • Close to zero mmHg https://www.youtube.com/watch?app=desktop&v=6ecmOuCIoNc Starling’s Law Nearly as much reabsorbed as filtered • At the arterial end, net outward pressure of 10 mmHg and fluid leaves capillary (filtration) • At the venous end, fluid moves in (reabsoprtion) due to -9 mmHg • On average, about 85% of fluid filtered in reabsorpbed • Excess enters lymphatic capillaries (about 3L/ day) to be eventually returned to blood 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 blood vessels each minute • Stroke Volume (SV) – volume of blood pumped by the ventricles each heartbeat CO = heart rate (HR) x stroke volume (SV) Hemodynamics: Factors affecting blood flow 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 makes 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 Larger the vessel generates greater friction, therefore, more resistance Venous return • Volume of blood flowing back to heart through systemic veins • Occurs due to pressure generated by constriction of left ventricle Venous Return milking 2 other mechanisms • Skeletal muscle pump – milks blood in 1 direction due to valves • Respiratory pump – due to pressure changes in thoracic and abdominal cavities Milking - Calf muscle contraction drives venous blood toward the heart Velocity of blood flow • Speed in cm/sec is inversely related to cross- sectional area • Velocity is slowest where total crosssectional 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 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 • Is the main region for nervous system regulation of heart rate, force of the contractions and vasodilation or vasoconstrcion of blood vessels • Vasomotor center control blood vessel diameter Cardiovascular Center Receives input from both higher brain regions and sensory receptors Sensory receptors 1. Proprioceptors – monitor movements of joints and muscles to provide input during physical activity 2. Baroreceptors – monitor pressure changes and stretch in blood vessel walls 3. Chemoreceptors – monitor concentration of various chemicals in the blood ( H+, CO2, O2) Neural regulation of blood pressure Negative feedback loops from 2 types of Reflexes 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 Chemoreceptor reflexes • Receptors located close to baroreceptors of carotid sinus (carotid bodies) and aortic arch (aortic bodies) • Detect • hypoxia • Hypercapnia • acidosis CV increases sympathetic stimulation to arterioles and veins, producing vasoconstriction and an increase in blood pressure 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 • Vasodilation • Loss of salt and water in urine • Reduces blood volume