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Lecture 3 The Cardiovascular System: The Heart The Heart • Heart is transport system; two side-by-side pumps • Pulmonary circuit – Pumps to lungs to get rid of CO2, pick up O2, • Systemic circuit – Pumps to body tissues via Figure 18.1 The systemic and pulmonary circuits. Capillary beds of lungs where gas exchange occurs Pulmonary Circuit Pulmonary arteries Aorta and branches Venae cavae Right atrium Right ventricle Oxygen-rich, CO2-poor blood Oxygen-poor, CO2-rich blood © 2013 Pearson Education, Inc. Pulmonary veins Left atrium Heart Left ventricle Systemic Circuit Capillary beds of all body tissues where gas exchange occurs Heart Location • Located in the medial cavity of the thorax – mediastinum – Superior surface of diaphragm – Two-thirds of heart to left of midsternal line – Anterior to vertebral column, posterior to sternum Figure 18.2a Location of the heart in the mediastinum. Midsternal line 2nd rib Sternum Diaphragm © 2013 Pearson Education, Inc. Location of apical impulse Figure 18.2b Location of the heart in the mediastinum. Mediastinum Heart Left lung Body of T7 vertebra Posterior © 2013 Pearson Education, Inc. Figure 18.2c Location of the heart in the mediastinum. Superior vena cava Pulmonary trunk Aorta Parietal pleura (cut) Left lung Pericardium (cut) Apex of heart Diaphragm © 2013 Pearson Education, Inc. Protection for the heart • The heart is enclosed in a double-walled sac called the pericardium • Composed of two layers – fibrous pericardium – serous pericardium • parietal layer • visceral layer The Pericardium • – Fibrous pericardium – superficial layer – protects heart – anchors sac/heart to surrounding structures The Pericardium • Serous Pericardium – thin, slippery, two layer between fibrous peridcardium and heart – Parietal layer lines internal surface of fibrous pericardium – Visceral layer (epicardium) on external surface of heart – Two layers separated by fluid-filled pericardial cavity (decreases friction) Figure 18.3 The pericardial layers and layers of the heart wall. Pulmonary trunk Fibrous pericardium Pericardium Parietal layer of serous pericardium Myocardium Pericardial cavity Epicardium (visceral layer of serous pericardium) Myocardium Endocardium Heart chamber © 2013 Pearson Education, Inc. Heart wall Layers of the Heart Wall • Three layers of heart wall: – Epicardium – serous pericardium – Myocardium – heart muscle – Endocardium – inside the heart Myocardium • Spiral bundles of contractile cardiac muscle cells tethered together by cardiac skeleton • Cardiac skeleton: crisscrossing, interlacing layer of connective tissue – Anchors cardiac muscle fibers – Supports great vessels and valves – Limits spread of action potentials to specific paths Figure 18.4 The circular and spiral arrangement of cardiac muscle bundles in the myocardium of the heart. Cardiac muscle bundles © 2013 Pearson Education, Inc. Endocardium • Inner layer of the heart • Continuous with endothelial lining of blood vessels • Lines heart chambers; covers cardiac skeleton of valves Chambers of the Heart • Four chambers: – Two superior atria – Two inferior ventricles • Interatrial septum – separates atria • Interventricular septum – separates ventricles Atria • Receiving chambers, thin walled – only need to push down to ventricles • Consists of smooth and pectinate muscles • Contribute little to the pumping activity of the heart Right Atrium • Right atrium receives blood from three veins – Superior vena cava – blood above diaphragm – Inferior vena cava – blood below diaphragm – Coronary sinus – blood from myocardium • Holds blood coming back from the body Left Atrium • Left atrium receives blood from four veins – two left pulmonary veins – two right pulmonary veins • Holds blood coming back from the lungs The Ventricles • Makes up most of the heart • Thicker walls than atria • Actual pumps of heart – Right ventricle – Left ventricle Right Ventricle • Forms most of the heart’s anterior surface • Pumps blood into the pulmonary track – blood from heart to lungs for gas exchange Left Ventricle • Dominates the posterior surface • Pumps blood into aorta – the largest artery in the blood – oxygenated blood from heart to body Figure 18.5b Gross anatomy of the heart. Brachiocephalic trunk Superior vena cava Right pulmonary artery Ascending aorta Pulmonary trunk Right pulmonary veins Left common carotid artery Left subclavian artery Aortic arch Ligamentum arteriosum Left pulmonary artery Left pulmonary veins Auricle of left atrium Right atrium Right coronary artery (in coronary sulcus) Anterior cardiac vein Right ventricle Circumflex artery Right marginal artery Great cardiac vein Anterior interventricular artery (in anterior interventricular sulcus) Apex Small cardiac vein Inferior vena cava Anterior view © 2013 Pearson Education, Inc. Left coronary artery (in coronary sulcus) Left ventricle Figure 18.5e Gross anatomy of the heart. Aorta Superior vena cava Right pulmonary artery Pulmonary trunk Right atrium Right pulmonary veins Fossa ovalis Pectinate muscles Tricuspid valve Right ventricle Chordae tendineae Trabeculae carneae Inferior vena cava Frontal section © 2013 Pearson Education, Inc. Left pulmonary artery Left atrium Left pulmonary veins Mitral (bicuspid) valve Aortic valve Pulmonary valve Left ventricle Papillary muscle Interventricular septum Epicardium Myocardium Endocardium The Heart Valves • Ensure unidirectional blood flow through heart • Open and close in response to pressure changes on both sides • Two atrioventricular (AV) valves • Two semilumar (SL) values Atrioventricular Valves • Tricuspid valve – on right atrium • Mitral valve (biscupal) – on left atrium • chordae tendineae – white collagen cords that attach to each valve – anchor valves from moving upward during ventricular pressure Figure 18.7 The atrioventricular (AV) valves. 1 Blood returning to the heart fills atria, pressing against the AV valves. The increased pressure forces AV valves open. Direction of blood flow Atrium 2 As ventricles fill, AV valve flaps hang limply into ventricles. Cusp of atrioventricular valve (open) Chordae tendineae 3 Atria contract, forcing additional blood into ventricles. Ventricle Papillary muscle AV valves open; atrial pressure greater than ventricular pressure Atrium 1 Ventricles contract, forcing blood against AV valve cusps. 2 AV valves close. 3 Papillary muscles contract and chordae tendineae tighten, preventing valve flaps from everting into atria. AV valves closed; atrial pressure less than ventricular pressure © 2013 Pearson Education, Inc. Cusps of atrioventricular valve (closed) Blood in ventricle Figure 18.6c Heart valves. Pulmonary valve Aortic valve Area of cutaway Mitral valve Tricuspid valve Chordae tendineae attached to tricuspid valve flap © 2013 Pearson Education, Inc. Papillary muscle Semilunar Valves • Two semilunar (SL) valves – Prevent backflow into ventricles when ventricles relax – Open and close in response to pressure changes • Named after where they pump blood to: – Aortic semilunar valve – Pulmonary semilunar valve Figure 18.8 The semilunar (SL) valves. Aorta Pulmonary trunk As ventricles contract and intraventricular pressure rises, blood is pushed up against semilunar valves, forcing them open. Semilunar valves open As ventricles relax and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing them to close. Semilunar © 2013 Pearson Education, Inc. valves closed Figure 18.6a Heart valves. Pulmonary valve Aortic valve Area of cutaway Mitral valve Tricuspid valve Myocardium Mitral (left atrioventricular) valve Tricuspid (right atrioventricular) valve Aortic valve Pulmonary valve Cardiac skeleton © 2013 Pearson Education, Inc. Anterior Arteries and Veins • Pulmonary circuit pump – veins carry oxygen rich blood to the heart – arteries carry oxygen poor blood from the heart • Systemic circuit pump – veins carry oxygen poor blood to the heart – arteries carry oxygen rich blood from the heart Figure 18.1 The systemic and pulmonary circuits. Capillary beds of lungs where gas exchange occurs Pulmonary Circuit Pulmonary arteries Aorta and branches Venae cavae Right atrium Right ventricle Oxygen-rich, CO2-poor blood Oxygen-poor, CO2-rich blood © 2013 Pearson Education, Inc. Pulmonary veins Left atrium Heart Left ventricle Systemic Circuit Capillary beds of all body tissues where gas exchange occurs Blood volume and blood pressure • Equal volumes of blood pumped to pulmonary and systemic circuits • Pulmonary circuit – short, low-pressure circulation • Systemic circuit – long, high-friction (resistance) circulation Figure 18.10 Anatomical differences between the right and left ventricles. Left ventricle Right ventricle Interventricular septum © 2013 Pearson Education, Inc. Figure 18.6d Heart valves. Pulmonary valve Aortic valve Area of cutaway Mitral valve Tricuspid valve Opening of inferior vena cava Tricuspid valve Mitral valve Chordae tendineae Myocardium of right ventricle Interventricular septum Papillary muscles © 2013 Pearson Education, Inc. Myocardium of left ventricle The Heart’s blood supply • Nourished by the right and left coronary arteries • Arise from the base of the aorta • Supplies blood when the heart is relaxed – requires 1/20th the blood supply – left ventricle needs the most Heart Muscle Histology • Cardiac muscle is striated and contracts by sliding filament mechanism – similar to skeletal muscle • Cardiac muscle cells, short, branched, fat, and interconnected Heart Cells Interlock • Intercalated discs - junctions between cells – anchor cardiac cells via demsmosomes and gap junctions • Desmosomes prevent cells from separating during contraction • Gap junctions allow ions to pass from cell to cell; electrically couple adjacent cells Figure 18.12a Microscopic anatomy of cardiac muscle. Nucleus Intercalated discs © 2013 Pearson Education, Inc. Cardiac muscle cell Gap junctions Desmosomes Functional Syncytium • Heart cells are electrically coupled together becoming a single coordinated unit • Heart cells have mitochondria that fill 25-35% of the cellular volume Figure 18.12b Microscopic anatomy of cardiac muscle. Cardiac muscle cell Intercalated disc Mitochondrion Nucleus Mitochondrion T tubule Sarcoplasmic reticulum Z disc Nucleus Sarcolemma I band © 2013 Pearson Education, Inc. A band I band Heart Muscle Contraction • Do not need nervous system stimulation • All cardiomyocytes contract as unit, or none do • 1% are autorhythmic that depolarize spontaneously and pace the heart • Bulk is contractile muscle fibers whose action potentials are mediated by Na+, Ca2+ and K+ channels Polarization-Depolarization • Positive feedback cycle • Fast voltage-gated Na+ channels allow extracellular Na+ to enter cell • Slow Ca2+ channels allow extracellular Ca2+ to enter cell • K+ channels allow intracellular K+ out of cell Movement of Action Potential • Action potential (electrical current) passes down T tubules throughout cells causing Ca2+ release from sarcoplasmic reticulum Ion movement creates electrical current Action potential Plateau 20 2 0 Tension development (contraction) –20 –40 3 1 –60 Absolute refractory period –80 0 150 Time (ms) © 2013 Pearson Education, Inc. 300 Tension (g) Membrane potential (mV) Figure 18.13 The action potential of contractile cardiac muscle cells. Slide 4 1 Depolarization is due to Na+ influx through fast voltage-gated Na+ channels. A positive feedback cycle rapidly opens many Na+ channels, reversing the membrane potential. Channel inactivation ends this phase. 2 Plateau phase is due to Ca2+ influx through slow Ca2+ channels. This keeps the cell depolarized because few K+ channels are open. 3 Repolarization is due to Ca2+ channels inactivating and K+ channels opening. This allows K+ efflux, which brings the membrane potential back to its resting voltage. Energy is needed! • Has many mitochondria – Great dependence on aerobic respiration – Little anaerobic respiration ability • Readily switches fuel source for respiration – can use lactic acid from skeletal muscles Intrinsic Cardiac Conduction System • 1% autorhythmic cells set pace for cardiac rhythm – pacemaker cells Membrane potential (mV) Figure 18.14 Pacemaker and action potentials of pacemaker cells in the heart. +10 0 –10 –20 –30 –40 –50 –60 –70 Action potential 2 1 Pacemaker potential This slow depolarization is due to both opening of Na+ channels and closing of K+ channels. Notice that the membrane potential is never a flat line. Threshold 2 Depolarization The action potential begins when the pacemaker potential reaches threshold. Depolarization is due to Ca2+ influx through Ca2+ channels. 2 3 3 1 1 Pacemaker potential Time (ms) © 2013 Pearson Education, Inc. Slide 1 3 Repolarization is due to Ca2+ channels inactivating and K+ channels opening. This allows K+ efflux, which brings the membrane potential back to its most negative voltage. Pacemaker cells are found in nodes • Sinoatrial (SA) node – sets the pace for the heart – the pacemaker • Atrioventricular (AV) node – transmite to atrioventricular (AV) bundles • Both found in right atrium Figure 18.15a Intrinsic cardiac conduction system and action potential succession during one heartbeat. Superior vena cava Right atrium 1 The sinoatrial (SA) node (pacemaker) generates impulses. Internodal pathway 2 The impulses pause (0.1 s) at the atrioventricular (AV) node. 3 The atrioventricular (AV) bundle connects the atria to the ventricles. 4 The bundle branches conduct the impulses through the interventricular septum. Left atrium Subendocardial conducting network (Purkinje fibers) Interventricular septum 5 The subendocardial conducting network depolarizes the contractile cells of both ventricles. Anatomy of the intrinsic conduction system showing the sequence of electrical excitation © 2013 Pearson Education, Inc. Slide 1 Figure 18.15b Intrinsic cardiac conduction system and action potential succession during one heartbeat. Pacemaker potential SA node Atrial muscle AV node Ventricular muscle Pacemaker potential Plateau 0 100 200 300 400 Milliseconds © 2013 Pearson Education, Inc. Comparison of action potential shape at various locations Nervous System Control • Neurons do lie in heart wall and activate SA and AV nodes when needed – can slow down and speed up heartbeat Electrocardiography • A graphic record of heart activity – measurement of deflection waves • Three waves: – P wave – depolarization SA node atria – QRS complex - ventricular depolarization and atrial repolarization – T wave - ventricular repolarization Figure 18.17 An electrocardiogram (ECG) tracing. Sinoatrial node Atrioventricular node QRS complex R Ventricular depolarization Ventricular repolarization Atrial depolarization T P Q P-R Interval 0 © 2013 Pearson Education, Inc. S 0.2 S-T Segment Q-T Interval 0.4 Time (s) 0.6 0.8 Figure 18.18 The sequence of depolarization and repolarization of the heart related to the deflection waves of an ECG tracing. SA node R R T P Q Q S S 4 Ventricular depolarization is complete. R R Q Q S 5 Ventricular repolarization begins at apex, causing the T wave. S 2 With atrial depolarization complete, the impulse is delayed at the AV node. R T P Q S 3 Ventricular depolarization begins at apex, causing the QRS complex. Atrial repolarization occurs. © 2013 Pearson Education, Inc. R T P Q T P T P T P 1 Atrial depolarization, initiated by the SA node, causes the P wave. AV node Slide 1 6 Depolarization S Ventricular repolarization is complete. Repolarization Sitole vs Disitole • Blood flow through heart during one complete heartbeat – atrial systole and diastole followed by ventricular systole and diastole Systole—contraction Diastole—relaxation Figure 18.21 Summary of events during the cardiac cycle. Left heart QRS P Electrocardiogram T 1st Heart sounds Dicrotic notch 120 Pressure (mm Hg) P 2nd 80 Aorta Left ventricle 40 Atrial systole Left atrium 0 Ventricular volume (ml) 120 EDV SV 50 ESV Atrioventricular valves Aortic and pulmonary valves Phase Open Closed Open Closed Open Closed 1 2a 2b 3 1 Left atrium Right atrium Left ventricle Right ventricle Atrial contraction Ventricular filling 1 © 2013 Pearson Education, Inc. Ventricular filling (mid-to-late diastole) Ventricular Isovolumetric contraction phase ejection phase 2a 2b Ventricular systole (atria in diastole) Isovolumetric relaxation 3 Early diastole Ventricular filling Regulation of Heart Rate • Autonomic Nervous System – emotional or physical stressors – release of norepinephrine • Hormones – Epinephrine from adrenal medulla – Thyroxine from thyroid gland Cardiac Output • the amount of blood pumped out of each ventricle in 1 minute CO = HR x SV HR = heart rate (beats/min) SV = stroke volume (mls/beat) Figure 18.21 Summary of events during the cardiac cycle. Left heart QRS P Electrocardiogram T 1st Heart sounds Dicrotic notch 120 Pressure (mm Hg) P 2nd 80 Aorta Left ventricle 40 Atrial systole Left atrium 0 Ventricular volume (ml) 120 EDV SV 50 ESV Atrioventricular valves Aortic and pulmonary valves Phase Open Closed Open Closed Open Closed 1 2a 2b 3 1 Left atrium Right atrium Left ventricle Right ventricle Atrial contraction Ventricular filling 1 © 2013 Pearson Education, Inc. Ventricular filling (mid-to-late diastole) Ventricular Isovolumetric contraction phase ejection phase 2a 2b Ventricular systole (atria in diastole) Isovolumetric relaxation 3 Early diastole Ventricular filling Stoke Volume SV = EDV = ESV • EDV = end diastolic volume – volume of blood in ventricles at rest • ESV = end systolic volume – volume of blood in ventricles after contraction Figure 18.22 Factors involved in determining cardiac output. Exercise (by sympathetic activity, skeletal muscle and respiratory pumps; see Chapter 19) Heart rate (allows more time for ventricular filling) Venous return Physiological response Result © 2013 Pearson Education, Inc. Sympathetic activity Contractility EDV (preload) Initial stimulus Exercise, fright, anxiety Bloodborne epinephrine, thyroxine, excess Ca2+ Parasympathetic activity ESV Stroke volume Heart rate Cardiac output Lab Exercise 30 • Anatomy of the Heart