Chapter 19 Lecture Animation Outline The Circulatory System

Chapter 19 Lecture Animation Outline The Circulatory System: The Heart

Introduction • Chinese, Egyptian, Greek, Roman scholars – “The heart is a pump for filling vessels with blood” • Thirteenth-century physician—Ibn an-Nafis – Described the role of the coronary blood vessels in nourishing the heart • Sixteen-century—Vesalius – Greatly improved knowledge of cardiovascular anatomy, the study of the heart, and treatment of its disorders— cardiology 2 -2

Introduction Cont. • Twentieth century – Discovery of nitroglycerin, improves coronary circulation; digitalis, abnormal heart rhythms; coronary bypass, valve replacement, among others • Cardiology is one of today’s most dramatic attention-getting fields in medicine! 2 -3

Overview of the Cardiovascular System • Expected Learning Outcomes – Define and distinguish between the pulmonary and systemic circuits. – Describe the general location, size, and shape of the heart. – Describe the pericardial sac that encloses the heart. 2 -4

Overview of the Cardiovascular System • Cardiovascular system – Heart and blood vessels • Circulatory system – Heart, blood vessels, and the blood 19 -5

The Pulmonary and Systemic Circuits • Major divisions of circulatory system – Pulmonary circuit: right side of heart • Carries blood to lungs for gas exchange and back to heart – Systemic circuit: left side of heart • Supplies oxygenated blood to all tissues of the body and returns it to the heart 19 -6

The Pulmonary and Systemic Circuits Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. CO 2 • Left side of heart Pulmonary circuit O 2 -poor, CO 2 -rich blood O 2 -rich, CO 2 -poor blood Systemic circuit CO 2 Figure 19. 1 – Fully oxygenated blood arrives from lungs via pulmonary veins – Blood sent to all organs of the body via aorta • Right side of heart – Lesser oxygenated blood arrives from inferior and superior venae cavae – Blood sent to lungs via pulmonary trunk 19 -7

Position, Size, and Shape of the Heart Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. • Heart located in mediastinum, between lungs • Base—wide, superior portion of heart, blood vessels attach here • Apex—inferior end, tilts to the left, tapers to point • 3. 5 in. wide at base, 5 in. from base to apex, and 2. 5 in. anterior to posterior; weighs 10 ounces Aorta Pulmonary trunk Superior vena cava Right lung Base of heart Parietal pleura (cut) Pericardial sac (cut) Apex of heart Diaphragm (c) Figure 19. 2 c 19 -8

Position, Size, and Shape of the Heart Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Sternum Posterior 3 rd rib Lungs Diaphragm Thoracic vertebra Pericardial cavity Left ventricle Right ventricle (a) Interventricular septum Sternum (b) Anterior Figure 19. 2 a, b 19 -9

Gross Anatomy of the Heart • Expected Learning Outcomes – Describe three layers of the heart wall. – Identify the four chambers of the heart. – Identify the surface features of the heart and correlate them with its internal four-chambered anatomy. – Identify the four valves of the heart. – Trace the flow of blood through the four chambers and valves of the heart and adjacent blood vessels. – Describe the arteries that nourish the myocardium and the veins that drain it. 19 -10

The Heart Wall • Pericardium—double-walled sac (pericardial sac) that encloses the heart – Allows heart to beat without friction, provides room to expand, yet resists excessive expansion – Anchored to diaphragm inferiorly and sternum anteriorly • Parietal pericardium—outer wall of sac – Superficial fibrous layer of connective tissue – Deep, thin serous layer 19 -11

The Heart Wall • Visceral pericardium (epicardium)—heart covering – Serous lining of sac turns inward at base of heart to cover the heart surface • Pericardial cavity—space inside the pericardial sac filled with 5 to 30 m. L of pericardial fluid • Pericarditis—inflammation of the membranes – Painful friction rub with each heartbeat 19 -12

Pericardium and Heart Wall Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Pericardial cavity Pericardial sac: Fibrous layer Serous layer Epicardium Myocardium Endocardium Epicardium Pericardial sac Figure 19. 3 19 -13

Human Cadaver Heart Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Fat in interventricular sulcus Left ventricle Right ventricle Anterior interventricular artery (a) Anterior view, external anatomy Superior vena cava Base of heart Inferior vena cava Right atrium Interatrial septum Left atrium Opening of coronary sinus Right AV valve Left AV valve Trabeculae carneae Coronary blood vessels Tendinous cords Right ventricle Papillary muscles Left ventricle Epicardial fat Endocardium Myocardium Interventricular septum Epicardium Apex of heart Figure 19. 4 a, b (b) Posterior view, internal anatomy © The Mc. Graw-Hill Companies, Inc. 19 -14

The Heart Wall • Epicardium (visceral pericardium) – Serous membrane covering heart – Adipose in thick layer in some places – Coronary blood vessels travel through this layer • Endocardium – Smooth inner lining of heart and blood vessels – Covers the valve surfaces and is continuous with endothelium of blood vessels 19 -15

The Heart Wall • Myocardium – Layer of cardiac muscle proportional to work load • Muscle spirals around heart which produces wringing motion – Fibrous skeleton of the heart: framework of collagenous and elastic fibers • Provides structural support and attachment for cardiac muscle and anchor for valve tissue • Electrical insulation between atria and ventricles; important in timing and coordination of contractile activity 19 -16

The Chambers • Four chambers Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. – Right and left atria • Two superior chambers • Receive blood returning to heart • Auricles (seen on surface) enlarge chamber Aorta Right pulmonary artery Left pulmonary artery Superior vena cava Pulmonary trunk Right pulmonary veins Left pulmonary veins Pulmonary valve Interatrial septum Right atrium Left atrium Aortic valve Left AV (bicuspid) valve Left ventricle Fossa ovalis Pectinate muscles Right AV (tricuspid) valve Papillary muscle Interventricular septum Tendinous cords Endocardium Trabeculae carneae Right ventricle Inferior vena cava Myocardium Epicardium – Right and left ventricles • Two inferior chambers • Pump blood into arteries Figure 19. 7 19 -17

The Chambers Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Ligamentum arteriosum Aortic arch Ascending aorta Superior vena cava Left pulmonary artery Branches of the right pulmonary artery • Atrioventricular sulcus —separates atria and ventricles Pulmonary trunk Left pulmonary veins Right pulmonary veins Left auricle Right atrium Coronary sulcus Anterior interventricular sulcus Right ventricle • Interventricular sulcus —overlies the interventricular septum that divides the right ventricle from the left Inferior vena cava Left ventricle Apex of heart (a) Anterior view Figure 19. 5 a • Sulci contain coronary arteries 19 -18

The Chambers Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Aorta Left pulmonary artery Superior vena cava Right pulmonary artery Left pulmonary veins Right pulmonary veins Left atrium Coronary sulcus Right atrium Coronary sinus Inferior vena cava Fat Posterior interventricular sulcus Left ventricle Apex of heart Figure 19. 5 b Right ventricle (b) Posterior view 19 -19

The Chambers • Interatrial septum – Wall that separates atria • Pectinate muscles – Internal ridges of myocardium in right atrium and both auricles • Interventricular septum – Muscular wall that separates ventricles • Trabeculae carneae – Internal ridges in both ventricles 19 -20

The Chambers Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Aorta Right pulmonary artery Left pulmonary artery Superior vena cava Pulmonary trunk Right pulmonary veins Left pulmonary veins Interatrial septum Right atrium Fossa ovalis Pectinate muscles Right AV (tricuspid) valve Tendinous cords Trabeculae carneae Right ventricle Inferior vena cava Pulmonary valve Left atrium Aortic valve Left AV (bicuspid) valve Left ventricle Papillary muscle Interventricular septum Endocardium Myocardium Epicardium Figure 19. 7 19 -21

The Valves • Valves ensure a one-way flow of blood through the heart • Atrioventricular (AV) valves—control blood flow between atria and ventricles – Right AV valve has three cusps (tricuspid valve) – Left AV valve has two cusps (mitral or bicuspid valve) – Chordae tendineae: cords connect AV valves to papillary muscles on floor of ventricles • Prevent AV valves from flipping inside out or bulging into the atria when the ventricles contract 19 -22

The Valves • Semilunar valves—control flow into great arteries; open and close because of blood flow and pressure – Pulmonary semilunar valve: in opening between right ventricle and pulmonary trunk – Aortic semilunar valve: in opening between left ventricle and aorta 19 -23

The Valves Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Left AV (bicuspid) valve Right AV (tricuspid) valve Fibrous skeleton Openings to coronary arteries Aortic valve Pulmonary valve (a) Figure 19. 8 a 19 -24

The Valves: Endoscopic View Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. (b) © Manfred Kage/Peter Arnold, Inc. Figure 19. 8 b 19 -25

The Valves Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Tendinous cords Papillary muscle Figure 19. 8 c (c) © The Mc. Graw-Hill Companies, Inc. 19 -26

Blood Flow Through the Chambers • Ventricles relax – Pressure drops inside the ventricles – Semilunar valves close as blood attempts to back up into the ventricles from the vessels – AV valves open – Blood flows from atria to ventricles 19 -27

Blood Flow Through the Chambers • Ventricles contract – AV valves close as blood attempts to back up into the atria – Pressure rises inside of the ventricles – Semilunar valves open and blood flows into great vessels 19 -28

Blood Flow Through the Chambers Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 10 1 Blood enters right atrium from superior and inferior venae cavae. Aorta Left pulmonary artery 11 5 5 9 Superior vena cava Right pulmonary veins Pulmonary trunk 4 6 6 Left atrium 1 Aortic valve 7 3 Right atrium 8 2 Right AV (tricuspid) valve 3 Contraction of right ventricle forces pulmonary valve open. 4 Blood flows through pulmonary valve into pulmonary trunk. 5 Blood is distributed by right and left pulmonary arteries to the lungs, where it unloads CO 2 and loads O 2. 6 Blood returns from lungs via pulmonary veins to left atrium. Left AV (bicuspid) valve 7 Blood in left atrium flows through left AV valve into left ventricle. Left ventricle 8 Contraction of left ventricle (simultaneous with step 3 ) forces aortic valve open. 9 Blood flows through aortic valve into ascending aorta. Right ventricle Inferior vena cava Left pulmonary veins 2 Blood in right atrium flows through right AV valve into right ventricle. 10 Blood in aorta is distributed to every organ in the body, where it unloads O 2 and loads CO 2. 11 11 Blood returns to heart via venae cavae. Figure 19. 9 • Blood pathway travels from the right atrium through the body and back to the starting point 19 -29

The Coronary Circulation • 5% of blood pumped by heart is pumped to the heart itself through the coronary circulation to sustain its strenuous workload – 250 m. L of blood per minute – Needs abundant O 2 and nutrients 19 -30

Arterial Supply • Left coronary artery (LCA) branches off the ascending aorta – Anterior interventricular branch (left ant descending) • Supplies blood to both ventricles and anterior twothirds of the interventricular septum – Circumflex branch • Passes around left side of heart in coronary sulcus • Gives off left marginal branch and then ends on the posterior side of the heart • Supplies left atrium and posterior wall of left 19 -31

Arterial Supply • Right coronary artery (RCA) branches off the ascending aorta – Supplies right atrium and sinoatrial node (pacemaker) – Right marginal branch • Supplies lateral aspect of right atrium and ventricle – Posterior interventricular branch • Supplies posterior walls of ventricles 19 -32

Arterial Supply Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Ligamentum arteriosum Aortic arch Ascending aorta Superior vena cava Left pulmonary artery Branches of the right pulmonary artery Pulmonary trunk Left pulmonary veins Right pulmonary veins Left auricle Right atrium Coronary sulcus Anterior interventricular sulcus Right ventricle Inferior vena cava Figure 19. 5 a Left ventricle Apex of heart (a) Anterior view 19 -33

Arterial Supply Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Aorta Left pulmonary artery Superior vena cava Right pulmonary artery Left pulmonary veins Right pulmonary veins Left atrium Coronary sulcus Right atrium Coronary sinus Inferior vena cava Fat Posterior interventricular sulcus Left ventricle Apex of heart Right ventricle (b) Posterior view Figure 19. 5 b 19 -34

Arterial Supply • Myocardial infarction (MI)—heart attack – Interruption of blood supply to the heart from a blood clot or fatty deposit (atheroma) can cause death of cardiac cells within minutes – Some protection from MI is provided by arterial anastomoses which provides an alternative route of blood flow (collateral circulation) within the myocardium • Blood flow to the heart muscle during ventricular contraction is slowed, unlike the rest of the body 19 -35

Arterial Supply • Three reasons – Contraction of the myocardium compresses the coronary arteries and obstructs blood flow – Opening of the aortic valve flap during ventricular systole covers the openings to the coronary arteries blocking blood flow into them – During ventricular diastole, blood in the aorta surges back toward the heart and into the openings of the coronary arteries • Blood flow to the myocardium increases during ventricular relaxation 19 -36

Angina and Heart Attack • Angina pectoris—chest pain from partial obstruction of coronary blood flow – Pain caused by ischemia of cardiac muscle – Obstruction partially blocks blood flow – Myocardium shifts to anaerobic fermentation, producing lactic acid and thus stimulating pain 19 -37

Angina and Heart Attack • Myocardial infarction—sudden death of a patch of myocardium resulting from long-term obstruction of coronary circulation – Atheroma (blood clot or fatty deposit) often obstructs coronary arteries – Cardiac muscle downstream of the blockage dies – Heavy pressure or squeezing pain radiating into the left arm – Some painless heart attacks may disrupt electrical conduction pathways, leading to fibrillation and cardiac arrest • Silent heart attacks occur in diabetics and the elderly – MI responsible for about half of all deaths in the United States 19 -38

Venous Drainage • 5% to 10% drains directly into heart chambers— right atrium and right ventricle—by way of thebesian veins • The rest returns to right atrium by the following routes: – Great cardiac vein • Travels alongside anterior interventricular artery • Collects blood from anterior portion of heart • Empties into coronary sinus 19 -39

Venous Drainage • The rest returns to right atrium by the following routes (cont. ): – Middle cardiac vein (posterior interventricular) • Found in posterior sulcus • Collects blood from posterior portion of heart • Drains into coronary sinus – Left marginal vein • Empties into coronary sinus • Coronary sinus – Large transverse vein in coronary sulcus on posterior side of heart – Collects blood and empties into right atrium 19 -40

Cardiac Muscle and the Cardiac Conduction System • Expected Learning Outcomes – Describe the unique structural and metabolic characteristics of cardiac muscle. – Explain the nature and functional significance of the intercellular junctions between cardiac muscle cells. – Describe the heart’s pacemaker and internal electrical conduction system. – Describe the nerve supply to the heart and explain its role. 19 -41

Structure of Cardiac Muscle • Cardiocytes—striated, short, thick, branched cells, one central nucleus surrounded by light-staining mass of glycogen • Intercalated discs—join cardiocytes end to end – Interdigitating folds: folds interlock with each other, and increase surface area of contact 19 -42

Structure of Cardiac Muscle Cont. – Mechanical junctions tightly join cardiocytes • Fascia adherens—broad band in which the actin of the thin myofilaments is anchored to the plasma membrane – Each cell is linked to the next via transmembrane proteins • Desmosomes—weldlike mechanical junctions between cells – Prevents cardiocytes from being pulled apart 19 -43

Structure of Cardiac Muscle • Electrical junctions (gap junctions) allow ions to flow between cells; can stimulate neighbors – Entire myocardium of either two atria or two ventricles acts like single, unified cell • Repair of damage of cardiac muscle is almost entirely by fibrosis (scarring) 19 -44

Structure of Cardiac Muscle Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Striations Nucleus Intercalated discs (a) Striated myofibril Glycogen Nucleus Mitochondria Intercalated discs (b) Intercellular space Desmosomes Gap junctions (c) a: © Ed Reschke Figure 19. 11 a–c 19 -45

Metabolism of Cardiac Muscle • Cardiac muscle depends almost exclusively on aerobic respiration used to make ATP – Rich in myoglobin and glycogen – Huge mitochondria: fill 25% of cell • Adaptable to organic fuels used – Fatty acids (60%); glucose (35%); ketones, lactic acid, and amino acids (5%) – More vulnerable to oxygen deficiency than lack of a specific fuel • Fatigue resistant because it makes little use of anaerobic fermentation or oxygen debt mechanisms – Does not fatigue for a lifetime 19 -46

The Conduction System • Coordinates the heartbeat – Composed of an internal pacemaker and nervelike conduction pathways through myocardium • Generates and conducts rhythmic electrical signals in the following order: – Sinoatrial (SA) node: modified cardiocytes • Pacemaker initiates each heartbeat and determines heart rate • Pacemaker in right atrium near base of superior vena cava – Signals spread throughout atria 19 -47

The Conduction System Cont. – Atrioventricular (AV) node • Located near the right AV valve at lower end of interatrial septum • Electrical gateway to the ventricles • Fibrous skeleton—insulator prevents currents from getting to ventricles from any other route – Atrioventricular (AV) bundle (bundle of His) • Bundle forks into right and left bundle branches • Branches pass through interventricular septum toward apex – Purkinje fibers • Nervelike processes spread throughout ventricular myocardium • Signal passes from cell to cell through gap junctions 19 -48

Structure of Cardiac Muscle • Cardiocytes—striated, short, thick, branched cells, one central nucleus surrounded by light-staining mass of glycogen • Intercalated discs—join cardiocytes end to end – Interdigitating folds: folds interlock with each other, and increase surface area of contact – Mechanical junctions tightly join cardiocytes • Fascia adherens—broad band in which the actin of the thin myofilaments is anchored to the plasma membrane – Each cell is linked to the next via transmembrane proteins • Desmosomes—weldlike mechanical junctions between cells – Prevents cardiocytes from being pulled apart 19 -49

Structure of Cardiac Muscle Cont. – Mechanical junctions tightly join cardiocytes • Fascia adherens—broad band in which the actin of the thin myofilaments is anchored to the plasma membrane – Each cell is linked to the next via transmembrane proteins • Desmosomes—weldlike mechanical junctions between cells – Prevents cardiocytes from being pulled apart 19 -50

The Conduction System Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1 SA node fires. Right atrium 2 Excitation spreads through atrial myocardium. 2 1 Sinoatrial node (pacemaker) Left atrium 2 Atrioventricular node Atrioventricular bundle Purkinje fibers 3 Bundle branches 4 5 3 AV node fires. 4 Excitation spreads down AV bundle. 5 Purkinje fibers distribute excitation through ventricular myocardium. Purkinje fibers Figure 19. 12 19 -51

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Nerve Supply to the Heart • Sympathetic nerves (raise heart rate) – Sympathetic pathway to the heart originates in the lower cervical to upper thoracic segments of the spinal cord – Continues to adjacent sympathetic chain ganglia – Some pass through cardiac plexus in mediastinum 19 -53

Nerve Supply to the Heart Cont. – Some pass through cardiac plexus in mediastinum – Continue as cardiac nerves to the heart – Fibers terminate in SA and AV nodes, in atrial and ventricular myocardium, as well as the aorta, pulmonary trunk, and coronary arteries • Increase heart rate and contraction strength • Dilates coronary arteries to increase myocardial blood flow 19 -54

Nerve Supply to the Heart • Parasympathetic nerves (slows heart rate) – Pathway begins with nuclei of the vagus nerves in the medulla oblongata – Extend to cardiac plexus and continue to the heart by way of the cardiac nerves – Fibers of right vagus nerve lead to the SA node – Fibers of left vagus nerve lead to the AV node – Little or no vagal stimulation of the myocardium • Parasympathetic stimulation reduces the heart rate 19 -55

Electrical and Contractile Activity of the Heart • Expected Learning Outcomes – Explain why the SA node fires spontaneously and rhythmically. – Explain how the SA node excites the myocardium. – Describe the unusual action potentials of cardiac muscle and relate them to the contractile behavior of the heart. – Interpret a normal electrocardiogram. 19 -56

Electrical and Contractile Activity of the Heart • Cycle of events in heart—special names – Systole: atrial or ventricular contraction – Diastole: atrial or ventricular relaxation 19 -57

The Cardiac Rhythm • Sinus rhythm—normal heartbeat triggered by the SA node – Set by SA node at 60 to 100 bpm – Adult at rest is 70 to 80 bpm (vagal tone) • Ectopic focus—another part of heart fires before the SA node – Caused by hypoxia, electrolyte imbalance, or caffeine, nicotine, and other drugs 19 -58

The Cardiac Rhythm • Spontaneous firing from some part of heart; not the SA node – Ectopic foci: region of spontaneous firing • Nodal rhythm—if SA node is damaged, heart rate is set by AV node, 40 to 50 bpm • Intrinsic ventricular rhythm—if both SA and AV nodes are not functioning, rate set at 20 to 40 bpm – Requires pacemaker to sustain life • Arrhythmia—any abnormal cardiac rhythm – Failure of conduction system to transmit signals (heart block) • Bundle branch block • Total heart block (damage to AV node) 19 -59

Cardiac Arrhythmias • Atrial flutter—ectopic foci in atria – Atrial fibrillation – Atria beat 200 to 400 times per minute • Premature ventricular contractions (PVCs) – Caused by stimulants, stress, or lack of sleep 19 -60

Cardiac Arrhythmias • Ventricular fibrillation – Serious arrhythmia caused by electrical signals reaching different regions at widely different times • Heart cannot pump blood and no coronary perfusion – Kills quickly if not stopped • Defibrillation—strong electrical shock whose intent is to depolarize the entire myocardium, stop the fibrillation, and reset SA nodes to sinus rhythm 19 -61

Pacemaker Physiology • SA node does not have a stable resting membrane potential – Starts at − 60 m. V and drifts upward from a slow inflow of Na+ • Gradual depolarization is called pacemaker potential – Slow inflow of Na+ without compensating outflow of K+ 19 -62

Pacemaker Physiology Cont. – When it reaches threshold of − 40 m. V, voltagegated fast Ca 2+ and Na+ channels open • Faster depolarization occurs peaking at 0 m. V • K+ channels then open and K+ leaves the cell – Causing repolarization – Once K+ channels close, pacemaker potential starts over 19 -63

Pacemaker Physiology • Each depolarization of the SA node sets off one heartbeat – At rest, fires every 0. 8 second or 75 bpm • SA node is the system’s pacemaker 19 -64

Pacemaker Physiology Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Membrane potential (m. V) +10 0 – 10 Fast K+ outflow Fast Ca 2+–Na+ inflow – 20 Action potential Threshold – 30 – 40 Pacemaker potential – 50 – 60 Slow Na+ inflow – 70 0 Figure 19. 13 . 4 . 8 1. 2 1. 6 Time (sec) 19 -65

Impulse Conduction to the Myocardium • Signal from SA node stimulates two atria to contract almost simultaneously – Reaches AV node in 50 ms • Signal slows down through AV node – Thin cardiocytes have fewer gap junctions – Delays signal 100 ms which allows the ventricles to fill 19 -66

Impulse Conduction to the Myocardium • Signals travel very quickly through AV bundle and Purkinje fibers – Entire ventricular myocardium depolarizes and contracts in near unison • Papillary muscles contract an instant earlier than the rest, tightening slack in chordae tendineae • Ventricular systole progresses up from the apex of the heart – Spiral arrangement of cardiocytes twists ventricles slightly; like someone wringing out a towel 19 -67

Electrical Behavior of the Myocardium • Cardiocytes have stable resting potential of − 90 m. V • Depolarize only when stimulated – Depolarization phase (very brief) • Stimulus opens voltage-regulated Na+ gates (Na+ rushes in), membrane depolarizes rapidly • Action potential peaks at +30 m. V • Na+ gates close quickly – Plateau phase lasts 200 to 250 ms, sustains contraction for expulsion of blood from heart • Ca 2+ channels are slow to close and SR is slow to remove Ca 2+ from the cytosol 19 -68

Electrical Behavior of the Myocardium • Depolarize only when stimulated (cont. ) – Repolarization phase: Ca 2+ channels close, K+ channels open, rapid diffusion of K+ out of cell returns it to resting potential • Has a long absolute refractory period of 250 ms compared to 1 to 2 ms in skeletal muscle – Prevents wave summation and tetanus which would stop the pumping action of the heart 19 -69

Electrical Behavior of the Myocardium 1. Na+ gates open Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 3 3. Na+ gates close 4. Slow Ca 2+ channels open 5. Ca 2+ channels close, K+ channels open (repolarization) Membrane potential (m. V) 2. Rapid depolarization Plateau +20 4 0 Action potential 2 Na+ inflow depolarizes the membrane and triggers the opening of still more Na+ channels, creating a positive feedback cycle and a rapidly rising membrane voltage. 3 Na+ channels close when the cell depolarizes, and the voltage peaks at nearly +30 m. V. 4 Ca 2+ entering through slow Ca 2+ channels prolongs depolarization of membrane, creating a plateau. Plateau falls slightly because of some K+ leakage, but most K+ channels remain closed until end of plateau. 5 Ca 2+ channels close and Ca 2+ is transported out of cell. K+ channels open, and rapid K+ outflow returns membrane to its resting potential. Myocardial relaxation 2 Myocardial contraction – 60 – 80 Voltage-gated Na+ channels open. 5 – 20 – 40 1 Absolute refractory period 1 0 . 15 Time (sec) . 30 Figure 19. 14 19 -70

The Electrocardiogram Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 0. 8 second R R Millivolts +1 PQ segment ST segment T wave P wave 0 PR Q interval S QT interval QRS interval • Electrocardiogram (ECG or EKG) – Composite of all action potentials of nodal and myocardial cells detected, amplified and recorded by electrodes on arms, legs, and chest – 1 Atria contract Ventricles contract Figure 19. 15 19 -71

The Electrocardiogram • P wave – SA node fires, atria depolarize and contract – Atrial systole begins 100 ms after SA signal • QRS complex – Ventricular depolarization – Complex shape of spike due to different thickness and shape of the two ventricles • ST segment—ventricular systole – Plateau in myocardial action potential • T wave – Ventricular repolarization and relaxation 19 -72

The Electrocardiogram Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 0. 8 second R R +1 Millivolts PQ segment ST segment T wave P wave 0 PR interval Q S QT interval QRS interval – 1 Atria contract Ventricles contract Atria contract Figure 19. 15 Ventricles contract 19 -73

The Electrocardiogram 1. 2. Atrial depolarization begins Atrial depolarization complete (atria contracted) Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Key Wave of depolarization Wave of repolarization R P P Q S 4 Ventricular depolarization complete. 1 Atria begin depolarizing. 3. 4. 5. 6. Ventricles begin to depolarize at apex; atria repolarize (atria relaxed) Ventricular depolarization complete (ventricles contracted) Ventricles begin to repolarize at apex Ventricular repolarization complete (ventricles relaxed) R T P P Q S 2 Atrial depolarization complete. 5 Ventricular repolarization begins at apex and progresses superiorly. R R T P P Q 3 Ventricular depolarization begins at apex and progresses superiorly as atria repolarize. Q S 6 Ventricular repolarization complete; heart is ready for the next cycle. Figure 19. 16 19 -74

ECGs: Normal and Abnormal Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. (a) Sinus rhythm (normal) • Abnormalities in conduction pathways • Myocardial infarction • Heart enlargement (b) Nodal rhythm—no SA node activity • Electrolyte and hormone imbalances Figure 19. 17 a, b 19 -75

Blood Flow, Heart Sounds, and the Cardiac Cycle • Expected Learning Outcomes – Explain why blood pressure is expressed in millimeters of mercury. – Describe how changes in blood pressure operate the heart valves. – Explain what causes the sounds of the heartbeat. – Describe in detail one complete cycle of heart contraction and relaxation. – Relate the events of the cardiac cycle to the volume of blood entering and leaving the heart. 19 -76

Blood Flow, Heart Sounds, and the Cardiac Cycle • Cardiac cycle—one complete contraction and relaxation of all four chambers of the heart • Atrial systole (contraction) occurs while ventricles are in diastole (relaxation) • Atrial diastole occurs while ventricles in systole • Quiescent period— all four chambers relaxed at same time • Questions to consider: How does pressure affect blood flow? How are heart sounds produced? 19 -77

Principles of Pressure and Flow • Two main variables govern fluid movement – Pressure causes a fluid to flow (fluid dynamics) • Pressure gradient—pressure difference between two points • Measured in mm Hg with a manometer or sphygmomanometer Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1 Volume increases 2 Pressure decreases 3 Air flows in P 1 P 2>P 1 Pressure gradient P 2 (a) 1 Volume decreases 2 Pressure increases – Resistance opposes fluid flow • Great vessels have positive blood pressure • Ventricular pressure must rise above this resistance for blood to flow into great vessels 3 Air flows out P 1 P 2<P 1 Pressure gradient (b) P 2 Figure 19. 18 19 -78

Pressure Gradients and Flow • Fluid flows only if it is subjected to more pressure at one point than another which creates a pressure gradient – Fluid flows down its pressure gradient from high pressure to low pressure 19 -79

Pressure Gradients and Flow • Events occurring on left side of heart – When ventricle relaxes and expands, its internal pressure falls – If bicuspid valve is open, blood flows into left ventricle – When ventricle contracts, internal pressure rises – AV valves close and the aortic valve is pushed open and blood flows into aorta from left ventricle • Opening and closing of valves are governed by these pressure changes – AV valves limp when ventricles relaxed – Semilunar valves under pressure from blood in vessels when ventricles relaxed 19 -80

Operation of the Heart Valves Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Atrium Atrioventricular valve Ventricle Atrioventricular valves open Atrioventricular valves closed (a) Figure 19. 19 Aorta Pulmonary artery Semilunar valves open (b) Semilunar valves closed 19 -81

Valvular Insufficiency Disorders • Valvular insufficiency (incompetence)—any failure of a valve to prevent reflux (regurgitation), the backward flow of blood – Valvular stenosis: cusps are stiffened and opening is constricted by scar tissue • Result of rheumatic fever, autoimmune attack on the mitral and aortic valves • Heart overworks and may become enlarged • Heart murmur—abnormal heart sound produced by regurgitation of blood through incompetent valves 19 -82

Valvular Insufficiency Disorders Cont. – Mitral valve prolapse: insufficiency in which one or both mitral valve cusps bulge into atria during ventricular contraction • Hereditary in 1 out of 40 people • May cause chest pain and shortness of breath 19 -83

Heart Sounds • Auscultation—listening to sounds made by body • First heart sound (S 1), louder and longer “lubb, ” occurs with closure of AV valves, turbulence in the bloodstream, and movements of the heart wall • Second heart sound (S 2), softer and sharper “dupp, ” occurs with closure of semilunar valves, turbulence in the bloodstream, and movements of the heart wall • S 3—rarely heard in people over 30 • Exact cause of each sound is not known with certainty 19 -84

Phases of the Cardiac Cycle • • • Ventricular filling Isovolumetric contraction Ventricular ejection Isovolumetric relaxation All the events in the cardiac cycle are completed in less than 1 second! 19 -85

Phases of the Cardiac Cycle • Ventricular filling – During diastole, ventricles expand – Their pressure drops below that of the atria – AV valves open and blood flows into the ventricles • Ventricular filling occurs in three phases – Rapid ventricular filling: first one-third • Blood enters very quickly – Diastasis: second one-third • Marked by slower filling • P wave occurs at the end of diastasis – Atrial systole: final one-third • Atria contract 19 -86

Phases of the Cardiac Cycle • End-diastolic volume (EDV)—amount of blood contained in each ventricle at end of ventricular filling (130 m. L) • Isovolumetric contraction – Atria repolarize and relax – Remain in diastole for the rest of the cardiac cycle – Ventricles depolarize, create the QRS complex, and begin to contract 19 -87

Phases of the Cardiac Cycle • AV valves close as ventricular blood surges back against the cusps • Heart sound S 1 occurs at the beginning of this phase • “Isovolumetric” because even though the ventricles contract, they do not eject blood – Because pressure in the aorta (80 mm Hg) and in the pulmonary trunk (10 mm Hg) is still greater than in the ventricles • Cariocytes exert force, but with all four valves closed, the blood cannot go anywhere 19 -88

Phases of the Cardiac Cycle • Ventricular ejection of blood begins when the ventricular pressure exceeds arterial pressure and forces semilunar valves open – Pressure peaks in left ventricle at about 120 mm Hg and 25 mm Hg in the right • Blood spurts out of each ventricle rapidly at first— rapid ejection • Then more slowly under reduced pressure— reduced ejection • Ventricular ejections last about 200 to 250 ms – Corresponds to the plateau phase of the cardiac action potential 19 -89

Phases of the Cardiac Cycle • Ventricular ejection (cont. ) • T wave occurs late in this phase • Stroke volume (SV) of about 70 m. L of blood is ejected of the 130 m. L in each ventricle – Ejection fraction of about 54% – As high as 90% in vigorous exercise • End-systolic volume (ESV)—the 60 m. L of blood left behind 19 -90

Phases of the Cardiac Cycle • Isovolumetric relaxation is early ventricular diastole – When T wave ends and ventricles begin to expand • Elastic recoil and expansion would cause pressure to drop rapidly and suck blood into the ventricles – Blood from the aorta and pulmonary trunk briefly flows backward – Filling the semilunar valves and closing the cusps – Creates a slight pressure rebound that appears as the dicrotic notch of the aortic pressure curve 19 -91

Phases of the Cardiac Cycle • Isovolumetric relaxation (cont. ) – Heart sound S 2 occurs as blood rebounds from the closed semilunar valves and the ventricle expands – “Isovolumetric” because semilunar valves are closed and AV valves have not yet opened • Ventricles are therefore taking in no blood – When AV valves open, ventricular filling begins again 19 -92

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Phases of the Cardiac Cycle • In a resting person – Atrial systole lasts about 0. 1 second – Ventricular systole lasts about 0. 3 second – Quiescent period, when all four chambers are in diastole, 0. 4 second • Total duration of the cardiac cycle is therefore 0. 8 second in a heart beating 75 bpm 19 -94

Events of the Cardiac Cycle Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Diastole Pressure (mm Hg) 120 Systole Diastole Aortic pressure 100 Aortic valve opens 80 Left ventricular pressure 60 AV valve closes 40 Left atrial pressure 20 • Ventricular filling Aortic valve closes (dicrotic notch) AV valve opens • Isovolumetric contraction Ventricular volume (m. L) 0 End-diastolic volume 120 90 60 End-systolic volume R R T P P • Ventricular ejection ECG Q Q S S Heart sounds S 2 S 3 Phase of cardiac cycle 1 a 0 S 1 1 b. 2 1 c. 4 Ventricular filling 1 a Rapid filling 1 b Diastasis 1 c Atrial systole 2 S 2 3 4 S 3 1 a . 6. 8 Time (sec) 2 Isovolumetric contraction Figure 19. 20 S 1 1 b. 2 1 c 2 . 4 3 Ventricular ejection 4 Isovolumetric relaxation • Isovolumetric relaxation 19 -95

Overview of Volume Changes End-systolic volume (ESV) Passively added to ventricle during atrial diastole Added by atrial systole Total: End-diastolic volume (EDV) Stroke volume (SV) ejected by ventricular systole Leaves: End-systolic volume (ESV) 60 m. L +30 m. L +40 m. L 130 m. L − 70 m. L 60 m. L Both ventricles must eject same amount of blood 19 -96

Overview of Volume Changes Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1 Right ventricular output exceeds left ventricular output. 2 Pressure backs up. 3 Fluid accumulates in pulmonary tissue. 1 2 3 • If the left ventricle pumps less blood than the right, the blood pressure backs up into the lungs and causes pulmonary edema (a) Pulmonary edema Figure 19. 21 a 19 -97

Overview of Volume Changes Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1 Left ventricular output exceeds right ventricular output. 2 Pressure backs up. 3 Fluid accumulates in systemic tissue. • If the right ventricle pumps less blood than the left, pressure backs up in the systemic circulation and causes systemic edema 1 2 (b) Systemic edema 3 Figure 19. 21 b 19 -98

Overview of Volume Changes • Congestive heart failure (CHF)—results from the failure of either ventricle to eject blood effectively – Usually due to a heart weakened by myocardial infarction, chronic hypertension, valvular insufficiency, or congenital defects in heart structure 19 -99

Overview of Volume Changes • Left ventricular failure—blood backs up into the lungs causing pulmonary edema – Shortness of breath or sense of suffocation • Right ventricular failure—blood backs up in the vena cava causing systemic or generalized edema – Enlargement of the liver, ascites (pooling of fluid in abdominal cavity), distension of jugular veins, swelling of the fingers, ankles, and feet • Eventually leads to total heart failure 19 -100

Cardiac Output • Expected Learning Outcomes – Define cardiac output and explain its importance. – Identify the factors that govern cardiac output. – Discuss some of the nervous and chemical factors that alter heart rate, stroke volume, and cardiac output. – Explain how the right and left ventricles achieve balanced output. – Describe some effects of exercise on cardiac output. 19 -101

Cardiac Output • Cardiac output (CO)—the amount ejected by ventricle in 1 minute • Cardiac output = heart rate x stroke volume – About 4 to 6 L/min at rest – A RBC leaving the left ventricle will arrive back at the left ventricle in about 1 minute – Vigorous exercise increases CO to 21 L/min for a fit person and up to 35 L/min for a world-class athlete 19 -102

Cardiac Output Cont. • Cardiac reserve—the difference between a person’s maximum and resting CO – Increases with fitness, decreases with disease • To keep cardiac output constant as we increase in age, the heart rate increases as the stroke volume decreases 19 -103

Heart Rate • Pulse—surge of pressure produced by each heart beat that can be felt by palpating a superficial artery with the fingertips – – Infants have HR of 120 bpm or more Young adult females average 72 to 80 bpm Young adult males average 64 to 72 bpm Heart rate rises again in the elderly 19 -104

Heart Rate • Tachycardia—resting adult heart rate above 100 bpm – Stress, anxiety, drugs, heart disease, or fever – Loss of blood or damage to myocardium • Bradycardia—resting adult heart rate of less than 60 bpm – In sleep, low body temperature, and endurance-trained athletes • Positive chronotropic agents—factors that raise the heart rate • Negative chronotropic agents—factors that lower heart rate 19 -105

Chronotropic Effects of the Autonomic Nervous System • Autonomic nervous system does not initiate the heartbeat, it modulates rhythm and force • Cardiac centers in the reticular formation of the medulla oblongata initiate autonomic output to the heart • Cardiostimulatory effect—some neurons of the cardiac center transmit signals to the heart by way of sympathetic pathways • Cardioinhibitory effect—others transmit parasympathetic signals by way of the vagus nerve 19 -106

Chronotropic Effects of the Autonomic Nervous System • Sympathetic postganglionic fibers are adrenergic – They release norepinephrine – Bind to β-adrenergic fibers in the heart – Activate c. AMP second-messenger system in cardiocytes and nodal cells – Lead to opening of Ca 2+ channels in plasma membrane 19 -107

Chronotropic Effects of the Autonomic Nervous System • Sympathetic postganglionic fibers are adrenergic (cont. ) – Increased Ca 2+ inflow accelerated depolarization of SA node – c. AMP accelerates the uptake of Ca 2+ by the sarcoplasmic reticulum allowing the cardiocytes to relax more quickly – By accelerating both contraction and relaxation, norepinephrine and c. AMP increase the heart rate as high as 230 bpm – Diastole becomes too brief for adequate filling – Both stroke volume and cardiac output are reduced 19 -108

Chronotropic Effects of the Autonomic Nervous System • Parasympathetic vagus nerves have cholinergic, inhibitory effects on the SA and AV nodes – Acetylcholine (ACh) binds to muscarinic receptors – Opens K+ gates in the nodal cells – As K+ leaves the cells, they become hyperpolarized and fire less frequently – Heart slows down – Parasympathetics work on the heart faster than sympathetics • Parasympathetics do not need a second-messenger system 19 -109

Chronotropic Effects of the Autonomic Nervous System • Without influence from the cardiac centers, the heart has an intrinsic “natural” firing rate of 100 bpm • Vagal tone—holds down this heart rate to 70 to 80 bpm at rest – Steady background firing rate of the vagus nerves 19 -110

Chronotropic Effects of the Autonomic Nervous System • Cardiac centers in the medulla receive input from many sources and integrate it into the “decision” to speed or slow the heart • Higher brain centers affect heart rate – Cerebral cortex, limbic system, hypothalamus • Sensory or emotional stimuli 19 -111

Chronotropic Effects of the Autonomic Nervous System • Medulla also receives input from muscles, joints, arteries, and brainstem – Proprioceptors in the muscles and joints • Inform cardiac center about changes in activity, HR increases before metabolic demands of muscle arise 19 -112

Chronotropic Effects of the Autonomic Nervous System Cont. – Baroreceptors signal cardiac center • Pressure sensors in aorta and internal carotid arteries • Blood pressure decreases, signal rate drops, cardiac center increases heart rate • If blood pressure increases, signal rate rises, cardiac center decreases heart rate 19 -113

Chronotropic Effects of the Autonomic Nervous System • Medulla also receives input from muscles, joints, arteries, and brainstem (cont. ) – Chemoreceptors • In aortic arch, carotid arteries, and medulla oblongata • Sensitive to blood p. H, CO 2 and O 2 levels • More important in respiratory control than cardiac control – If CO 2 accumulates in blood or CSF (hypercapnia), reacts with water and causes increase in H+ levels – H+ lowers the p. H of the blood possibly creating acidosis (p. H < 7. 35) 19 -114

Chronotropic Effects of the Autonomic Nervous System Cont. • Hypercapnia and acidosis stimulate the cardiac center to increase heart rate • Also respond to hypoxemia—oxygen deficiency in the blood that usually slows down the heart – Chemoreceptors (cont. ) • Chemoreflexes and baroreflexes—responses to fluctuation in blood chemistry—are both negative feedback loops 19 -115

Chronotropic Effects of Chemicals • Chemicals affect heart rate as well as neurotransmitters from cardiac nerves – Blood-borne adrenal catecholamines (NE and epinephrine) are potent cardiac stimulants • Drugs that stimulate heart – Nicotine stimulates catecholamine secretion – Thyroid hormone increases number of adrenergic receptors on heart so more responsive to sympathetic stimulation – Caffeine inhibits c. AMP breakdown prolonging adrenergic effect 19 -116

Chronotropic Effects of Chemicals • Electrolytes – K+ has greatest chronotropic effect • Hyperkalemia—excess K+ in cardiocytes – Myocardium less excitable, heart rate slows and becomes irregular • Hypokalemia—deficiency K+ in cardiocytes – Cells hyperpolarized, require increased stimulation – Calcium • Hypercalcemia—excess of Ca 2+ – Decreases heart rate and contraction strength • Hypocalcemia—deficiency of Ca 2+ – Increases heart rate and contraction strength 19 -117

Stroke Volume • The other factor in cardiac output, besides heart rate, is stroke volume (SV) • Three variables govern stroke volume – Preload – Contractility – Afterload • Examples – Increased preload or contractility increases stroke volume – Increased afterload decreases stroke volume 19 -118

Preload • Preload—the amount of tension in ventricular myocardium immediately before it begins to contract – Increased preload causes increased force of contraction – Exercise increases venous return and stretches myocardium – Cardiocytes generate more tension during contraction – Increased cardiac output matches increased venous return 19 -119

Preload • Frank–Starling law of the heart: SV EDV – Stroke volume is proportional to the end diastolic volume – Ventricles eject as much blood as they receive – The more they are stretched, the harder they contract 19 -120

Contractility • Contractility refers to how hard the myocardium contracts for a given preload • Positive inotropic agents increase contractility – Hypercalcemia can cause strong, prolonged contractions and even cardiac arrest in systole – Catecholamines increase calcium levels – Glucagon stimulates c. AMP production – Digitalis raises intracellular calcium levels and contraction strength 19 -121

Contractility • Negative inotropic agents reduce contractility – Hypocalcemia can cause weak, irregular heartbeat and cardiac arrest in diastole – Hyperkalemia reduces strength of myocardial action potentials and the release of Ca 2+ into the sarcoplasm – Vagus nerves have effect on atria but too few nerves to ventricles for a significant effect 19 -122

Afterload • Afterload—the blood pressure in the aorta and pulmonary trunk immediately distal to the semilunar valves – Opposes the opening of these valves – Limits stroke volume • Hypertension increases afterload and opposes ventricular ejection 19 -123

Afterload • Anything that impedes arterial circulation can also increase afterload – Lung diseases that restrict pulmonary circulation – Cor pulmonale: right ventricular failure due to obstructed pulmonary circulation • In emphysema, chronic bronchitis, and black lung disease 19 -124

Exercise and Cardiac Output • Exercise makes the heart work harder and increases cardiac output • Proprioceptors signal cardiac center – At beginning of exercise, signals from joints and muscles reach the cardiac center of brain – Sympathetic output from cardiac center increases cardiac output • Increased muscular activity increases venous return – Increases preload and ultimately cardiac output 19 -125

Exercise and Cardiac Output • Increases in heart rate and stroke volume cause an increase in cardiac output • Exercise produces ventricular hypertrophy – Increased stroke volume allows heart to beat more slowly at rest – Athletes with increased cardiac reserve can tolerate more exertion than a sedentary person 19 -126

Coronary Artery Disease • Coronary artery disease (CAD)—a constriction of the coronary arteries – Usually the result of atherosclerosis: an accumulation of lipid deposits that degrade the arterial wall and obstruct the lumen – Endothelium damaged by hypertension, virus, diabetes, or other causes 19 -127

Coronary Artery Disease Cont. – Monocytes penetrate walls of damaged vessels and transform into macrophages • Absorb cholesterol and fats to be called foam cells – Look like fatty streak on vessel wall – Can grow into atherosclerotic plaques (atheromas) – Platelets adhere to damaged areas and secrete plateletderived growth factor • Attracting immune cells and promoting mitosis of muscle and fibroblasts, and the deposition of collagen • Bulging mass grows to obstruct arterial lumen 19 -128

Coronary Artery Disease • Causes angina pectoris, intermittent chest pain, by obstructing 75% or more of the blood flow • Immune cells of atheroma stimulate inflammation – May rupture, resulting in traveling clots or fatty emboli • Cause coronary artery spasms due to lack of secretion of nitric oxide (vasodilator) • Inflammation transforms atheroma into a hardened complicated plaque called arteriosclerosis 19 -129

Coronary Artery Disease • Major risk factor for atherosclerosis is excess of low-density lipoprotein (LDL) in the blood combined with defective LDL receptors in the arterial walls – Protein-coated droplets of cholesterol, neutral fats, free fatty acids, and phospholipids • Most cells have LDL receptors that take up these droplets from blood by receptor-mediated endocytosis – Dysfunctional receptors in arterial cells accumulate excess cholesterol 19 -130

Coronary Artery Disease • Unavoidable risk factors: heredity, aging, being male • Preventable risk factors: obesity, smoking, lack of exercise, anxious personality, stress, aggression, and diet • Treatment – Coronary bypass surgery • Great saphenous vein – Balloon angioplasty – Laser angioplasty 19 -131