Chapter 20 The Heart


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Chapter 20 The Heart

  1. 1. Cardiovascular System - The HeartIntroduction to Cardiovascular SystemThe Pulmonary Circuit Carries blood to and from gas exchange surfaces of lungsThe Systemic Circuit Carries blood to and from the bodyBlood alternates between pulmonary circuit and systemic circuitThree Types of Blood Vessels Arteries Carry blood away from heart Veins Carry blood to heart Capillaries Networks between arteries and veinsCapillaries Also called exchange vessels Exchange materials between blood and tissues Materials include dissolved gases, nutrients, wastesFour Chambers of the Heart Right atrium Collects blood from systemic circuit Right ventricle Pumps blood to pulmonary circuit Left atrium Collects blood from pulmonary circuit Left ventricle Pumps blood to systemic circuitAnatomy of the HeartGreat veins and arteries at the basePointed tip is apexSurrounded by pericardial sacSits between two pleural cavities in the mediastinumThe Pericardium Double lining of the pericardial cavity Parietal pericardium Outer layer Forms inner layer of pericardial sac Visceral pericardium Cardiovascular System – The Heart – Page 1
  2. 2. Inner layer of pericardium Pericardial cavity Is between parietal and visceral layers Contains pericardial fluid Pericardial sac Fibrous tissue Surrounds and stabilizes heartSuperficial Anatomy of the Heart Atria Thin-walled Expandable outer auricle (atrial appendage) Sulci Coronary sulcus: divides atria and ventricles Anterior interventricular sulcus and posterior interventricular sulcus: –separate left and right ventricles –contain blood vessels of cardiac muscleThe Heart Wall Epicardium (outer layer) Visceral pericardium Covers the heart Myocardium (middle layer) Muscular wall of the heart Concentric layers of cardiac muscle tissue Atrial myocardium wraps around great vessels Two divisions of ventricular myocardium Endocardium (inner layer) Simple squamous epitheliumCardiac Muscle Tissue Intercalated discs Interconnect cardiac muscle cells Secured by desmosomes Linked by gap junctions Convey force of contraction Propagate action potentialsCharacteristics of Cardiac Muscle Cells Small size Single, central nucleus Branching interconnections between cells Intercalated discsInternal Anatomy and Organization Cardiovascular System – The Heart – Page 2
  3. 3. Interatrial septum: separates atria Interventricular septum: separates ventricles Atrioventricular (AV) valves Connect right atrium to right ventricle and left atrium to left ventricle The fibrous flaps that form bicuspid (2) and tricuspid (3) valves Permit blood flow in one direction: atria to ventriclesThe Right Atrium Superior vena cava Receives blood from head, neck, upper limbs, and chest Inferior vena cava Receives blood from trunk, viscera, and lower limbs Coronary sinus Cardiac veins return blood to coronary sinus Coronary sinus opens into right atrium Foramen ovale Before birth, is an opening through interatrial septum Connects the two atria Seals off at birth, forming fossa ovalis Pectinate muscles Contain prominent muscular ridges On anterior atrial wall and inner surfaces of right auricleThe Right Ventricle Free edges attach to chordae tendineae from papillary muscles of ventricle Prevent valve from opening backward Right atrioventricular (AV) Valve Also called tricuspid valve Opening from right atrium to right ventricle Has three cusps Prevents backflow Trabeculae carneae Muscular ridges on internal surface of right (and left) ventricle Includes moderator band: –ridge contains part of conducting system –coordinates contractions of cardiac muscle cellsThe Pulmonary Circuit Conus arteriosus (superior end of right ventricle) leads to pulmonary trunk Pulmonary trunk divides into left and right pulmonary arteries Blood flows from right ventricle to pulmonary trunk through Cardiovascular System – The Heart – Page 3
  4. 4. pulmonary valve Pulmonary valve has three semilunar cuspsThe Left Atrium Blood gathers into left and right pulmonary veins Pulmonary veins deliver to left atrium Blood from left atrium passes to left ventricle through left atrioventricular (AV) valve A two-cusped bicuspid valve or mitral valveThe Left Ventricle Holds same volume as right ventricle Is larger; muscle is thicker and more powerful Similar internally to right ventricle but does not have moderator band Systemic circulation Blood leaves left ventricle through aortic valve into ascending aorta Ascending aorta turns (aortic arch) and becomes descending aortaStructural Differences between the Left and Right Ventricles Right ventricle wall is thinner, develops less pressure than left ventricle Right ventricle is pouch-shaped, left ventricle is roundThe Heart Valves Two pairs of one-way valves prevent backflow during contraction Atrioventricular (AV) valves Between atria and ventricles Blood pressure closes valve cusps during ventricular contraction Papillary muscles tense chordae tendineae: prevent valves from swinging into atria Semilunar valves Pulmonary and aortic tricuspid valves Prevent backflow from pulmonary trunk and aorta into ventricles Have no muscular support Three cusps support like tripodAortic Sinuses At base of ascending aorta Sacs that prevent valve cusps from sticking to aorta Origin of right and left coronary arteriesConnective Tissues and the Cardiac (Fibrous) Skeleton Physically support cardiac muscle fibers Distribute forces of contraction Cardiovascular System – The Heart – Page 4
  5. 5. Add strength and prevent overexpansion of heart Elastic fibers return heart to original shape after contractionThe Cardiac (Fibrous) Skeleton Four bands around heart valves and bases of pulmonary trunk and aorta Stabilize valves Electrically insulate ventricular cells from atrial cellsThe Blood Supply to the Heart = Coronary Circulation Coronary arteries and cardiac veins Supplies blood to muscle tissue of heartThe Coronary Arteries Left and right Originate at aortic sinuses High blood pressure, elastic rebound forces blood through coronary arteries between contractionsRight Coronary Artery Supplies blood to Right atrium Portions of both ventricles Cells of sinoatrial (SA) and atrioventricular nodes Marginal arteries (surface of right ventricle) Posterior interventricular arteryLeft Coronary Artery Supplies blood to Left ventricle Left atrium Interventricular septumTwo main branches of left coronary artery Circumflex artery Anterior interventricular arteryArterial Anastomoses Interconnect anterior and posterior interventricular arteries Stabilize blood supply to cardiac muscleThe Cardiac Veins Great cardiac vein Drains blood from area of anterior interventricular artery into coronary sinus Anterior cardiac veins Empties into right atrium Posterior cardiac vein, middle cardiac vein, and small Cardiovascular System – The Heart – Page 5
  6. 6. cardiac vein Empty into great cardiac vein or coronary sinusThe Conducting SystemHeartbeat A single contraction of the heart The entire heart contracts in series First the atria Then the ventriclesTwo Types of Cardiac Muscle Cells Conducting system Controls and coordinates heartbeat Contractile cells Produce contractions that propel bloodThe Cardiac Cycle Begins with action potential at SA node Transmitted through conducting system Produces action potentials in cardiac muscle cells (contractile cells) Electrocardiogram (ECG) Electrical events in the cardiac cycle can be recorded on an electrocardiogram (ECG)A system of specialized cardiac muscle cells Initiates and distributes electrical impulses that stimulate contractionAutomaticity Cardiac muscle tissue contracts automaticallyStructures of the Conducting System Sinoatrial (SA) node - wall of right atrium Atrioventricular (AV) node - junction between atria and ventricles Conducting cells - throughout myocardiumConducting Cells Interconnect SA and AV nodes Distribute stimulus through myocardium In the atrium Internodal pathways In the ventricles AV bundle and the bundle branchesPrepotential Also called pacemaker potential Cardiovascular System – The Heart – Page 6
  7. 7. Resting potential of conducting cells Gradually depolarizes toward threshold SA node depolarizes first, establishing heart rateHeart Rate SA node generates 80–100 action potentials per minute Parasympathetic stimulation slows heart rate AV node generates 40–60 action potentials per minuteThe Sinoatrial (SA) Node In posterior wall of right atrium Contains pacemaker cells Connected to AV node by internodal pathways Begins atrial activation (Step 1)The Atrioventricular (AV) Node In floor of right atrium Receives impulse from SA node (Step 2) Delays impulse (Step 3) Atrial contraction beginsThe AV Bundle In the septum Carries impulse to left and right bundle branches Which conduct to Purkinje fibers (Step 4) And to the moderator band Which conducts to papillary musclesPurkinje Fibers Distribute impulse through ventricles (Step 5) Atrial contraction is completed Ventricular contraction beginsAbnormal Pacemaker Function Bradycardia: abnormally slow heart rate Tachycardia: abnormally fast heart rate Ectopic pacemaker Abnormal cells Generate high rate of action potentials Bypass conducting system Disrupt ventricular contractionsElectrocardiogram (ECG or EKG) A recording of electrical events in the heart Obtained by electrodes at specific body locations Abnormal patterns diagnose damageFeatures of an ECG P wave Cardiovascular System – The Heart – Page 7
  8. 8. Atria depolarize QRS complex Ventricles depolarize T wave Ventricles repolarizeTime Intervals Between ECG Waves P–R interval From start of atrial depolarization To start of QRS complex Q–T interval From ventricular depolarization To ventricular repolarizationContractile Cells Purkinje fibers distribute the stimulus to the contractile cells, which make up most of the muscle cells in the heart Resting Potential Of a ventricular cell: about –90 mV Of an atrial cell: about –80 mVRefractory Period Absolute refractory period Long Cardiac muscle cells cannot respond Relative refractory period Short Response depends on degree of stimulusTiming of Refractory Periods Length of cardiac action potential in ventricular cell 250–300 msecs: –30 times longer than skeletal muscle fiber –long refractory period prevents summation and tetanyThe Role of Calcium Ions in Cardiac Contractions Contraction of a cardiac muscle cell is produced by an increase in calcium ion concentration around myofibrils 20% of calcium ions required for a contraction Calcium ions enter plasma membrane during plateau phase Arrival of extracellular Ca2+ Triggers release of calcium ion reserves from sarcoplasmic reticulum As slow calcium channels close Intracellular Ca2+ is absorbed by the SR Cardiovascular System – The Heart – Page 8
  9. 9. Or pumped out of cell Cardiac muscle tissue Very sensitive to extracellular Ca2+ concentrationsThe Energy for Cardiac Contractions Aerobic energy of heart From mitochondrial breakdown of fatty acids and glucose Oxygen from circulating hemoglobin Cardiac muscles store oxygen in myoglobin The Cardiac CycleCardiac cycle = The period between the start of one heartbeat andthe beginning of the nextIncludes both contraction and relaxationPhases of the Cardiac Cycle Within any one chamber Systole (contraction) Diastole (relaxation)Blood Pressure In any chamber Rises during systole Falls during diastole Blood flows from high to low pressure Controlled by timing of contractions Directed by one-way valvesCardiac Cycle and Heart Rate At 75 beats per minute Cardiac cycle lasts about 800 msecs When heart rate increases All phases of cardiac cycle shorten, particularly diastoleEight Steps in the Cardiac Cycle 1.Atrial systole Atrial contraction begins Right and left AV valves are open 2.Atria eject blood into ventricles Filling ventricles 3.Atrial systole ends AV valves close Ventricles contain maximum blood volume Known as end-diastolic volume (EDV) 4.Ventricular systole Isovolumetric ventricular contraction Pressure in ventricles rises Cardiovascular System – The Heart – Page 9
  10. 10. AV valves shut 5.Ventricular ejection Semilunar valves open Blood flows into pulmonary and aortic trunks Stroke volume (SV) = 60% of end-diastolic volume 6.Ventricular pressure falls Semilunar valves close Ventricles contain end-systolic volume (ESV), about 40% of end-diastolic volume 7.Ventricular diastole Ventricular pressure is higher than atrial pressure All heart valves are closed Ventricles relax (isovolumetric relaxation) 8.Atrial pressure is higher than ventricular pressure AV valves open Passive atrial filling Passive ventricular filling Cardiac cycle endsHeart Sounds S1 Loud sounds Produced by AV valves S2 Loud sounds Produced by semilunar valves S3, S4 Soft sounds Blood flow into ventricles and atrial contractionHeart Murmur Sounds produced by regurgitation through valves CardiodynamicsThe movement and force generated by cardiac contractions End-diastolic volume (EDV) End-systolic volume (ESV) Stroke volume (SV) SV = EDV – ESV Ejection fraction The percentage of EDV represented by SV Cardiac output (CO) The volume pumped by left ventricle in 1 minute Cardiovascular System – The Heart – Page 10
  11. 11. Cardiac OutputCO = HR X SVCO = cardiac output (mL/min)HR = heart rate (beats/min)SV = stroke volume (mL/beat)Factors Affecting Cardiac Output Cardiac output Adjusted by changes in heart rate or stroke volume Heart rate Adjusted by autonomic nervous system or hormones Stroke volume Adjusted by changing EDV or ESVFactors Affecting the Heart Rate Autonomic innervation Cardiac plexuses: innervate heart Vagus nerves (X): carry parasympathetic preganglionic fibers to small ganglia in cardiac plexus Cardiac centers of medulla oblongata: –cardioacceleratory center controls sympathetic neurons (increases heart rate) –cardioinhibitory center controls parasympathetic neurons (slows heart rate)Autonomic Innervation Cardiac reflexes Cardiac centers monitor: –blood pressure (baroreceptors) –arterial oxygen and carbon dioxide levels (chemoreceptors) Cardiac centers adjust cardiac activity Autonomic tone Dual innervation maintains resting tone by releasing ACh and NE Fine adjustments meet needs of other systemsEffects on the SA Node Sympathetic and parasympathetic stimulation Greatest at SA node (heart rate) Membrane potential of pacemaker cells Lower than other cardiac cells Rate of spontaneous depolarization depends on Resting membrane potential Rate of depolarization ACh (parasympathetic stimulation) Slows the heart Cardiovascular System – The Heart – Page 11
  12. 12. NE (sympathetic stimulation) Speeds the heartAtrial Reflex Also called Bainbridge reflex Adjusts heart rate in response to venous return Stretch receptors in right atrium Trigger increase in heart rate Through increased sympathetic activityHormonal Effects on Heart Rate Increase heart rate (by sympathetic stimulation of SA node) Epinephrine (E) Norepinephrine (NE) Thyroid hormoneFactors Affecting the Stroke Volume The EDV: amount of blood a ventricle contains at the end of diastole Filling time: –duration of ventricular diastole Venous return: –rate of blood flow during ventricular diastolePreload The degree of ventricular stretching during ventricular diastole Directly proportional to EDV Affects ability of muscle cells to produce tensionThe EDV and Stroke Volume At rest EDV is low Myocardium stretches less Stroke volume is low With exercise EDV increases Myocardium stretches more Stroke volume increasesThe Frank–Starling Principle As EDV increases, stroke volume increasesPhysical Limits Ventricular expansion is limited by Myocardial connective tissue The cardiac (fibrous) skeleton The pericardial sac Cardiovascular System – The Heart – Page 12
  13. 13. End-Systolic Volume (ESV) The amount of blood that remains in the ventricle at the end of ventricular systole is the ESVThree Factors That Affect ESV Preload Ventricular stretching during diastole Contractility Force produced during contraction, at a given preload Afterload Tension the ventricle produces to open the semilunar valve and eject bloodContractility Is affected by Autonomic activity HormonesEffects of Autonomic Activity on Contractility Sympathetic stimulation NE released by postganglionic fibers of cardiac nerves Epinephrine and NE released by suprarenal (adrenal) medullae Causes ventricles to contract with more force Increases ejection fraction and decreases ESV Parasympathetic activity Acetylcholine released by vagus nerves Reduces force of cardiac contractionsHormones Many hormones affect heart contraction Pharmaceutical drugs mimic hormone actions Stimulate or block beta receptors Affect calcium ions (e.g., calcium channel blockers)Afterload Is increased by any factor that restricts arterial blood flow As afterload increases, stroke volume decreasesHeart Rate Control Factors Autonomic nervous system Sympathetic and parasympathetic Circulating hormones Venous return and stretch receptorsStroke Volume Control Factors EDV Filling time Rate of venous return Cardiovascular System – The Heart – Page 13
  14. 14. ESV Preload Contractility AfterloadCardiac Reserve The difference between resting and maximal cardiac outputThe Heart and Cardiovascular System Cardiovascular regulation Ensures adequate circulation to body tissues Cardiovascular centers Control heart and peripheral blood vessels Cardiovascular system responds to Changing activity patterns Circulatory emergencies Cardiovascular System – The Heart – Page 14