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Cardiovascular System Heart General Shape o Approximately the size of a person’s fist o Hollow, cone-shaped Location o Apex rests on the diaphragm; leans to the left o Base is at the level of the second rib o Located between lungs in the mediastinum Structure Atria: receiving chambers Ventricles: discharging chambers Coronary sulcus: deep groove, furrow separating atria from ventricles Interventricular septum: divides the heart into right and left parts Valves o Allow blood to flow only in one direction through ventricles and out arteries o AV (atrioventricular valves): between atria and ventricles Bicuspid (mitral) valve Left AV valve Has two cusps (flaps) Tricuspid valve Right AV valve Has three cusps o Semilunar valves: three cusps Pulmonary valve Between right ventricle and pulmonary artery Aortic valve Between left ventricle and aortic arch Coronary arteries and veins: blood supply to the heart Coverings o Enclosed by double sac of serous membrane called pericardium o Serous fluid produced by pericardial membranes to decrease friction as heart contracts Walls o Epicardium: outer layer o Myocardium: thick bundles of cardiac muscle o Endocardium: sheet of endothelium that lines the chambers of the heart, continuous with linings of blood vessels entering and leaving the heart Cardiac Conduction System General process of blood circulation o Superior and inferior vena cava empty blood into right atrium o Blood moves from right atrium through tricuspid valve to the right ventricle o Blood from the right ventricle moves through the pulmonary semilunar valve to the pulmonary trunk o Pulmonary trunk splits into right and left pulmonary veins o Blood travels to lungs o From the lungs, blood comes back to the heart through right and left pulmonary arteries o Right and left pulmonary arteries empty into the left atrium o Blood moves from the left atrium through the bicuspid valve into the left ventricle o The left ventricle contracts to send blood up through the aortic semilunar valve into the aorta o Blood travels from the aorta into body tissues Pulmonary circulation o Blood vessels carry blood to and from the lungs o Arteries carry deoxygenated blood from right side of the heart to lungs for gas exchange o Veins carry oxygenated blood from lungs to left side of the heart System circulation o Blood vessels carry blood to and from all body tissues o Arteries carry oxygenated blood from left side of heart to body tissues o Veins carry deoxygenated blood from body tissues to right side of heart Blood vessels Layers of tissue o Tunica intima (tunica interna) Innermost layer Includes endothelium Forms smooth, friction-reducing lining o Tunica media Middle layer Mostly circularly-arranged smooth muscle cells and sheets of elastin Elastin allows vessels to stretch and recoil Muscles Contracting/relaxing allows vasoconstriction (narrowing vessels) or vasodilation (enlargement of vessels) o Tunica adventitia (tunica externa) Outermost layer Composed mainly of loosely woven collagen fibers Protect blood vessel and anchor it to surround structures Infiltrated with nerve fibers and lymphatic vessels Types of blood vessels o Arteries Transport blood away from heart Have thickest tunica media Elastic Closest to heart Experience greatest pressure Have greatest amount of elastin enabling them to expand Recoil of elastic arteries helps propel blood forward Contain smooth muscle but generally do not vasoconstrict o Muscular Tunica media composed mainly of smooth muscle Vasomotor fibers of sympathetic nervous system regulate activity More active than elastic arteries in vasoconstriction Less elastin than elastic arteries Arterioles Smallest arteries Range from all three layers to single layer of smooth muscle Control blood flow into capillary beds Capillaries Smallest blood vessels Contain only tunica interna Average diameter is 8-10 micrometers (little larger than diameter of red blood cell) Exchange materials (gases, nutrients, hormones) Pre-capillary sphincter Cuff of smooth muscle that surrounds each true capillary Regulates blood flow into capillary Blood flow regulated by vasomotor nerves and local chemical conditions Can bypass or flood capillary bed Continuous Abundant in skin and muscles Most common Continuous endothelial cells with gaps called intercellular clefts; allow passage of small solutes and fluids in/out of capillary Brain capillaries DO NOT have intercellular clefts o BLOOD-BRAIN BARRIER Fenestrated Some endothelial cells have pores (fenestrations) in them More permeable to small solutes and fluids Found where active absorption or filtration occurs o (small intestine, endocrine organs, kidneys) Sinusoid Leaky capillaries only in liver, bone marrow, lymphoid tissues and some endocrine organs Large, irregular shaped lumens and usually fenestrated Larger intercellular clefts than continuous Allow large molecules and blood cells to pass between blood and tissues o Veins Muscular pump Skeletal muscle contraction pushes on adjacent veins Propels blood upward toward heart Venous valves Folds of tunica interna Prevent blood from flowing backwards Respiratory pump Diaphragm contracts to decrease volume in abdominal cavity o Increase in pressure o Squeezing of local veins that force blood toward heart Diaphragm contracts to increase volume of thoracic cavity o Pressure decreases o Increase in diameter of veins in thoracic cavity o Speeds blood entry into right atrium Venules Smallest veins Formed when capillaries unite Extremely porous Allow fluid and white blood cells to move through their walls Primarily endothelium Veins Have all three tunics Thinner walls and larger lumens than arteries Can accommodate large volume of blood Thin tunica media Little smooth muscle or elastin Tunica externa is heaviest layer Superior/inferior vena cava have longitudinal bands of smooth muscle around tunica externa Blood Pressure Force per unit area exerted on the wall of a blood vessel by its contained blood o Expressed in mmHg o Measured in reference to system arterial blood pressure in large arteries near heart Taken using sphygmomanometer and stethoscope Recorded as fraction o Systolic pressure over diastolic pressure Systolic pressure: blood pressure when heart contracts Diastolic pressure: blood pressure when heart relaxes o Pulse pressure: difference between systolic and diastolic pressure Influences o Peripheral resistance o Vessel elasticity o Blood volume o Cardiac output Peripheral resistance Blood cells and plasma encounter resistance when they come in contact with blood vessel walls Sources of peripheral resistance: blood vessel diameter, blood viscosity, total vessel length Blood vessel diameter o Diameter becomes smaller, greater proportion of fluid in contact with wall, resistance to flow is increased, pressure rises Larger diameter, same volume, less pressure Smaller diameter, same volume, more pressure o Diameter regulated by vasomotor fibers Sympathetic nerve fibers that innervate vessel’s smooth muscle layer Release vasoconstrictors: substances that cause blood vessels to constrict Norepinephrine Epinephrine Angiotensin II Vasopressin o Diameter can decrease due to buildup of fatty deposits (plaques) Arteriosclerosis Blood viscosity o Related to thickness of a fluid o Greater viscosity, less easily molecules slide past one another o Greater viscosity; greater resistance to flow; greater pressure required to pump same volume of fluid Total vessel length o Increased fatty tissue requires more blood vessels and adds to total vessel length in body o Longer total vessel length, greater resistance encountered, greater blood pressure Vessel elasticity Healthy elastic artery expands to absorb shock of systolic pressure In arteriosclerosis, arteries become calcified and rigid: cannot expand Arteries experience higher pressures Blood volume Greater volume of fluid; more fluid presses against walls; greater pressure Less volume, less pressure Reduced blood volume could be due to sweating Increased blood volume could be due to excessive salt intake Cardiac output CARDIAC OUTPUT = STROKE VOLUME X HEART RATE o Cardiac output: volume of blood pumped by one ventricle per minute o Stroke volume: amount of blood ejected from ventricles with each beat Normal resting heart o Heart rate (pulse): 75 beats/min o Stroke volume: 70 mL/min o Average cardiac output: 5.25 L/min o Ventricle can hold 120 mL of blood (EDV) o 50 mL remains in ventricle after contraction (ESV) Cardiac reserve o Heart exceeding normal cardiac output o In aerobic exercise, cardiac reserve is four times normal cardiac output Stroke volume o Difference between EDV and ESV o Preload: degree of stretch of cardiac muscle before it contracts o Stretching increases number of cross bridge interactions Increases force of heart contraction Achieved by increasing amount of venous blood return to heart Heart Imbalances Ectopic focus: abnormal pacemaker Incompetent valve: allows backflow of blood Arrhythmia: abnormal heartbeat o Bradycardia: slow heart beat (below 60 beats/min) o Tachycardia: rapid heart rate (about 100 beats/min) Fibrillation: heart is uncoordinated and useless as a pump Angina pectoris: chest pain resulting from ischemia of myocardium Congestive heart failure: circulation is inadequate to meet tissue needs due to cardiac decompensation Heart block: damage to AV node, releasing ventricles from control of SA node Myocardial infarction: Results from prolonged coronary blockage Pulmonary congestion: result of initial failure of left side of heart