The Heart: Anatomy, Physiology and Exercise Physiology

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The Heart: Anatomy, Physiology and Exercise Physiology pdf

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The Heart: Anatomy, Physiology and Exercise Physiology - page 1
Chapter The Heart: Anatomy, Physiology and Exercise Physiology Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe 1 Contents 1.1 3 3 4 4 6 6 9 10 11 11 11 11 13 16 16 17 17 18 19 19 20 20 Introduction 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5 1.3.6 1.3.7 1.3.8 1.4 1.4.1 1.4.2 1.5 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Anatomy of the Heart . . . . . . . . . . . . . . . . . . Chamber and Valves . . . . . . . . . . . . . . . . . . . Cardiac Cell and Cardiac Muscle . . . . . . . . . Coronary Arteries and Cardiac Veins . . . . . Venous Circulation . . . . . . . . . . . . . . . . . . . . . Nerve Supply of the Heart . . . . . . . . . . . . . . . Conduction System of the Heart . . . . . . . . . Physiology of the Heart . . . . . . . . . . . . . . . . Circulatory System: Systemic and Pulmonary Circulation . . . . . . . . . . . . . . Conduction System of the Heart (Excitation Sequence) . . . . . . . . . . . . . . . . . . Action Potential (AP) . . . . . . . . . . . . . . . . . . . Mechanism of Excitation and Contraction Coupling of Cardiac Myocytes . . . . . . . . . . . Autonomic Nervous System and Heart . . . . Cardiac Cycle . . . . . . . . . . . . . . . . . . . . . . . . . Physiology of Coronary Circulation . . . . . . Coronary Collaterals . . . . . . . . . . . . . . . . . . . Exercise Physiology . . . . . . . . . . . . . . . . . . . . Gender and Exercise Performance . . . . . . . Age and Exercise Performance . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . The impact of anatomy on medicine was first recognised by Andreas Vesalius during the 16th century [1] and from birth to death, the heart is the most talked about organ of the human body. It is the centre of attraction for people from many lifestyles, such as philosophers, artists, poets and physicians/surgeons. The heart is one of the most efficient organs in the human body and heart disease is one of the commonest causes of mor- bidity and mortality in both developing and developed countries. Understanding the anatomy and pathophysi- ology is very important and challenging. With innova- tive changes in the imaging world, the perception of these has changed radically and applied anatomy and physiology plays an important role in understanding structure and function. 1.2 Anatomy of the Heart The heart is located in the chest, directly above the dia- phragm in the region of the thorax called mediastinum, specifically the middle mediastinum. The normal hu- man heart varies with height and weight (Table 1.1). The tip (apex) of the heart is pointed forward, downward, and toward the left. The (inferior) diaphragmatic sur- face lies directly on the diaphragm. The heart lies in a double walled fibroserous sac called the pericardial sac, which is divided into (a) fibrous pericardium, and (b) serous pericardium. The fibrous pericardium envelops the heart and attaches onto the great vessels [2]. The se- rous pericardium is a closed sac consisting of two lay- ers – a visceral layer or epicardium forming the outer lining of the great vessels and the heart, and a parietal layer forming an inner lining of the fibrous pericardium [2–4]. The two layers of the serous pericardium contain the pericardial fluid, which prevents friction between the heart and the pericardium [2–4].
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The Heart: Anatomy, Physiology and Exercise Physiology - page 2
4 Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe Table 1.1 Anatomical facts about the human heart Normal human heart varies with height and weight Weighs approximately 300–350 grams in males Weighs approximately 250–300 grams in females Right ventricle thickness is 0.3–0.5 cm Left ventricle thickness is 1.3–1.5 cm Divided into four distinct chambers Composed of three layers (epicardium, myocardium and endocardium) Contains two atria (left and right) Contains two ventricles (left and right) Contains four valves (aortic, mitral, tricuspid, pulmonary) 1.2.1 Chamber and Valves The heart is divided into four distinct chambers with muscular walls of different thickness [2, 4, 9]. The left atrium (LA) and right atrium (RA) are small, thin- walled chambers located just above the left ventricle (LV) and right ventricle (RV), respectively. The ventricles are larger thick-walled chambers that perform most of the work [2, 4, 9] (Table 1.2). The atria receive blood from the venous system and lungs and then contract and eject the blood into the ventricles. The ventricles then pump the blood throughout the body or into the lungs. The heart contains four valves and the fibrous skeleton of the heart contains the annuli of the four valves, membranous septum, aortic intervalvular, right, and left fibrous trigo- nes [3, 4, 6, 10, 11] (Fig. 1.2, Table 1.3). The right trigone and the membranous septum together form the central fibrous body, which is penetrated by the bundle of His [3, 4, 6, 10, 11]. The fibrous skeleton functions not only to provide an electrophysiological dissociation of atria and the ventricles but also provides structural support to the heart [8, 12, 13]. Each of the four valves has a distinctive role in maintaining physiological stability [3]. 1.2.2 The wall of the heart is composed of three layers: (a) epicardium; (b) myocardium; and (c) endocardium (Fig. 1.1) [5, 6]. The epicardium is the outer lining of the car- diac chambers and is formed by the visceral layer of the serous pericardium. The myocardium is the intermedi- ate layer of the heart and is composed of three discern- able layers of muscle [5, 6] that are seen predominantly in the left ventricle and inter-ventricular septum alone and includes a subepicardial layer, a middle concentric layer and a subendocardial layer. The rest of the heart is composed mainly of the subepicardial and suben- docardial layers [7, 8]. The myocardium also contains important structures such as excitable nodal tissue and the conducting system. The endocardium the innermost layer of the heart is formed of the endothelium and sub- endothelial connective tissue [5, 6]. Cardiac Cell and Cardiac Muscle The cardiac cell contains bundles of protein strands called myofibrils. These myofibrils are surrounded by sarcoplasmic reticulum, which contains cysternae (di- lated terminals) [6, 10, 11, 14–16]. The sarcomeres are the contractile unit of myofibrils and the T tubules are continuations of the cell membrane located near the Z-lines, which conduct the action potential (AP) to the interior of the cell [6, 14]. The T tubules connect the sar- colemma to the sarcoplasmic reticulum in the skeletal muscle and the cardiac muscle [14, 15]. Cardiac muscle is an involuntary striated muscle, which is mononucleated and has cross-striations formed by alternate segments of thick and thin protein filaments, which are anchored by segments called Z-lines. Cardiac muscle is relatively shorter than skeletal muscle [6, 10, 11, 14–16] and actin and myosin are the primary struc- tural proteins. When the cardiac muscle is observed by a light microscope, the thinner actin filaments appear as lighter bands, while thicker myosin filaments appear as darker bands [8, 12, 13, 15]. The dark bands are actu- ally the region of overlap between the actin and myosin filaments and the light bands are the region of actin fila- ments [8, 12, 13, 15]. The thinner actin flaments contain two others proteins called troponin and tropomyosin, which play an important role in contraction [6, 14, 15]. Cardiac muscle also contains dense bands (specialised Parietal pericardium Endocardium Pericardial cavity Visceral pericardium (epicardium) Myocardium Fig 1.1 Layers of the heart
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Chapter 1 The Heart: Anatomy, Physiology and Exercise Physiology 5 Table 1.2 Cardiac atrial and ventricular chambers [2, 4–6, 9] Left ventricle (LV) 1. Made of an inlet portion comprised of mitral valve apparatus, subaortic outflow portion and a trabeculated apical zone 2. Three times thicker than the RV and most muscular 3. Thickest towards the base and thinnest towards the apex 4. LV free wall and septal thickness is three times the thickness of the RV free wall 5. Mitral and aortic valves share fibrous continuity 6. LV apex is relatively less trabeculated than the RV apex Right ventricle (RV) 1. Comprised of inlet and outflow segments 2. Inlet extends from tricuspid annulus to the insertions of the papillary muscles 3. Apical trabecular zone extends inferiorly beyond the papillary muscle attachment toward the ventricular apex and halfway along the anterior wall. 4. Outflow portion (conus) is a muscular subpulmonary channel 5. Arch shaped muscular ridge separates the tricuspid and pulmonary valves Right atrium (RA) 1. Thinnest walls of the four chambers 2. Forms the right border of the heart 3. Gives off the right auricular appendage 4. Receives the superior vena cava, inferior vena cava and coronary sinus 5. Discharges into right ventricle through the tricuspid valve Left atrium (LA) 1. Forms base of the heart (posterior surface) 2. Gives off the left auricular appendage. 3. Receives two right pulmonary veins (sometimes three) and two left pulmonary veins (sometimes one) 4. Discharges into the left ventricle through the mitral valve Ventricular septum 1. Intracardiac partition having four parts (inlet, membranous, trabecular and infundibular) 2. Divided into muscular and membranous septum 3. Membranous septum lies beneath the right and posterior aortic cusps and contact mitral and tricuspid annuli Atrial septum 1. Composed of interatrial and atrioventricular regions when viewed from right 2. Composed of entirely interatrial regions when viewed from the left 3. Interatrial region is characterised by fossa ovalis 4. Atrioventricular portion separates the right atrium from the left ventricle
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6 Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe Pulmonary valve Right atrium Aortic valve Left atrium Tricuspid valve Right ventricle Mitral valve Left ventricle Interventricular septum Fig 1.2 Cardiac chambers and valves cell junctions) called intercalated discs that separate in- dividual cells from one another at their ends [6, 14, 15] and these discs consist of a transverse and a lateral por- tion. The transverse portion of the disc acts as a zone of firm adhesion and a route of transmission of contractile force and the lateral portion of the disc acts as a gap junction across which propagation of electrical impulses between the adjacent cardiac cells occurs [6, 14, 15]. This in effect allows the individual cells of the heart to act as a syncytium [8, 12–15]. 1.2.3 portion of the right ventricle, the inferior left ventricular wall, and the PDA). In the majority (80–90%) of cases, the RCA supplies the atrioventricular node (AV node). Finally, the coronary arteries branch into small arteries and arterioles. These vessels terminate in end arteries that supply the myocardial tissue with blood [2, 3, 6–8]. In general, the RCA is dominant in 60–65% of cases because it gives off a PDA branch (balanced coronary circulation) [2, 3, 6–8]. In about 10–15% of cases, the LCx gives rise to the PDA (left predominant circulation). In 20- 25% of cases, the RCA, in addition to supplying the PDA, crosses the posterior interventricular septum to reach as far as the left marginal artery and thereby supply the diaphragmatic surface of the left ventricle (right pre- dominance) [2, 3, 8]. However, this term does not distin- guish this condition from balanced coronary circulation (Table 1.5) [7]. 1.2.4 Venous Circulation The venous circulation of the heart is from the coronary sinus, anterior cardiac veins and the lesser cardiac (the- besian) veins [6]. The coronary sinus receives most of the venous return from the epicardium and myocardium [6] and it opens into the right atrium between the open- ing of the inferior vena cava and the right AV valve [2, 3, 5, 6]. The coronary sinus gives rise to tributaries such as the great cardiac vein, middle cardiac vein, smaller car- diac vein and oblique vein. The great cardiac vein drains the anterior portion of the interventricular septum and anterior aspects of both ventricles [2–6]. The middle cardiac vein drains the posterior portion of the interventricular septum and posterior aspect of Coronary Arteries and Cardiac Veins The heart receives blood from left coronary arteries (LCA) and right coronary arteries (RCA) [17] (Fig. 1.3, Table 1.4). The left coronary artery arises from the left aortic sinus (at an acute angle from the aorta) [2, 3, 6, 8] as a single short main artery (left mainstem). The LCA bifurcates to form the left anterior descend- ing artery (LAD) and left circumflex (LCx) [2, 3, 6]. The LAD anastomoses with the posterior descending artery (PDA) a branch of the right coronary artery (RCA) [2, 3, 6]. The LAD supplies the interventricular septum (an- terior two-thirds), the apex, and the anterior aspects of the right and left ventricle. The LCx has a major branch, the left marginal artery, and in around 10–15% of the population, the LCx anastomoses with the RCA to give rise to the PDA [2, 3, 6–8]. In general, the LCx supplies the posterior aspect of the left atrium and superior por- tion of the left ventricle [2, 3, 6–8]. The RCA arises from the right aortic sinus and has major branches such as the PDA (supplying the poste- rior third of the interventricular septum and AV node [6, 7], the nodal artery (supplying the right atrium and the SA node), and the right marginal artery (supplying a SA node Right coronary artery Left coronary artery (left mainstem) Left circumflex artery Left ventricle Left anterior descending branch (LAD) Septum AV node Bundle of His Right ventricle Fig 1.3 Coronary arteries
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Chapter 1 The Heart: Anatomy, Physiology and Exercise Physiology 7 Table 1.3 Cardiac valves and cardiac skeleton [2, 4–6, 8, 9] Tricuspid valve 1. Tricuspid valve (right atrioventricular valve) connects the right atrium and the right ventricle 2. Composed of five components (annulus, leaflets, commissures, chordae tendineae, papillary muscles) 3. Anterior tricuspid leaflet is the largest (most mobile) and the posterior leaflet is the smallest 4. Tricuspid valve has a triangular orifice Mitral valve 1. Mitral valve (left atrioventricular valve) connects the left atrium to the left ventricle 2. Composed of five components (annulus, leaflets, commissures, chordae tendineae, papillary muscles) 3. Composed of two leaflets only 4. Anterior tricuspid leaflet is large, semicircular and twice the height of posterior leaflet 5. Posterior leaflet is rectangular and is divided into three scallops 6. Mitral valve has an elliptical orifice Aortic valve 1. Aortic valve (semilunar valve) opens between the left ventricle and the aorta 2. Composed of three components (annulus, cusps, commissures) 3. Composed of three semilunar cusps Pulmonary valves 1. Pulmonary valve (semilunar valve) regulates flow between the right ventricle and the pulmonary artery 2. Composed of three components (annulus, cusps, commissures) Cardiac grooves 1. Atrioventricular (AV) groove separates the atria from the ventricle 2. Anterior and posterior interventricular (IV) grooves separate the ventricles 3. Right coronary arteries (RCA) travel in the right atrioventricular groove 4. Circumflex artery (Cx) travels in the left atrioventricular groove 5. The left anterior descending artery (LAD) travels along the anterior interventricular groove 6. Posterior descending artery (PDA) travels along posterior interventricular groove Cardiac crux (external and internal) 1. External cardiac crux is the intersection between the AV, posterior IV and interatrial (IA) grooves 2. Internal cardiac crux is the posterior intersection between the mitral and tricuspid annuli and the atrial and ventricular septa Cardiac margins 1. Acute margin – junction between anterior and inferior wall of the right ventricle 2. Obtuse margin – rounded lateral wall of left ventricle
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8 Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe Table 1.4 Coronary blood supply [2, 3, 6, 8] Coronary arterial circulation A. Left coronary artery (LCA) 1. Ostium of the left coronary artery originates from the left aortic sinus. 2. LCA arises at an acute angle from the aorta. 3. LCA courses to the left and anteriorly (Left anterior descending artery – LAD) and after variable length gives rise to left circumflex artery (LCx). 4. Diagonal artery may arise between LAD and LCx or from the LAD. 5. LAD continues towards the septum and gives rise to septal perforator branches. B. Right coronary artery 1. Arises from the right aortic sinus. 2. Major branches are nodal, right marginal, PDA. 3. Supplies the RA, SA node, part of RV, posterior third of the interventricular septum, AV node and right branch of the AV bundle (of His). C. Left circumflex artery (LCx) 1. Originates from the LAD and course is variable. 2. May terminate into one or more large obtuse marginal branches. 3. May continue as a large artery and give rise to posterior descending artery (PDA). 4. When the left circumflex artery supplies the major PDA, it is referred to as a dominant artery. Coronary venous circulation 1. Composed of the coronary sinus, cardiac veins, and thebesian venous system. 2. Great cardiac vein and other cardiac veins (left posterior and middle) drain into the coronary sinus and finally empties into the right atrium. 3. Rarely, the coronary sinus drains directly into the left atrium. Coronary collaterals 1. Provide communication between major coronary arteries and branches. 2. May dilate and provide blood supply beyond the obstructed/stenosed epicardial vessel. 3. May develop between the terminal extension of two arteries, between side branches of two arteries, between branches of same artery or within the same branch. 4. Most common in the ventricular septum, ventricular apex, anterior right ventricular free wall, anterolateral left ventricle free wall, cardiac crux and atrial surfaces. Cardiac lymphatics 1. Lymphatics drain towards the epicardial surface and they merge to form the right and left channels. 2. Left and right channels travel in a retrograde fashion with their respective coronary arteries. 3. The left and right channels travel along the ascending aorta and merge before draining into the pretracheal lymph node. 4. The merged single lymphatic chain travels through a cardiac lymph node and finally empties into the right lymphatic duct.
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Chapter 1 The Heart: Anatomy, Physiology and Exercise Physiology 9 Table 1.4 (continued) Coronary blood supply [2, 3, 6, 8] Great vessels 1. Subclavian and internal jugular veins join together bilaterally to form right and left innominate (brachiocephalic) veins. 2. Right and left (longer) innominate veins join together to form superior vena cava (SVC). 3. SVC receives azygous vein before draining into the right atrium. 4. Thoracic aorta arises at the level of aortic valve and is made up of the ascending aorta (sinus and tubular portions), aortic arch and descending aorta. 5. Aortic arch gives rise to the innominate, left common carotid and subclavian arteries. 6. Descending aorta lies adjacent to the left atrium, oesophagus and vertebral column. Table 1.5 Regions supplied by coronary arteries [7] Right coronary artery Right ventricle Right atrium Diaphragmatic surface/inferior wall of left ventricle (LV) Posterior wall of left ventricle (90%) Posterior third of interventricular septum (90%) SA node (55–60%) AV node and bundle of His (80–90%) Left coronary artery (Left anterior descending) Anterior and lateral wall of LV Most of the left ventricle Interventricular septum (anterior 2/3 rd ) Right and left bundle branches Left coronary artery (Left circumflex artery) SA node (40–45%) Left atrium AV node and bundle of His (10%) Lateral wall of LV Posterior wall of left ventricle (10%) Posterior third of interventricular septum (10%) both ventricles [4, 6] and the smaller cardiac vein drains the marginal aspect of the right ventricle [11]. The the- besian veins drain the endocardium and the innermost layers of the myocardium directly into the underlying chamber [11]. 1.2.5 Nerve Supply of the Heart The sympathetic and parasympathetic autonomic ner- vous supplies to the heart form the cardiac plexus, which is located close to the arch of the aorta. The fibres from the cardiac plexus accompany the coronary arter- ies and reach the heart, with most of them terminating at the SA node, AV node and a much less dense supply to the atrial and ventricular myocardium [11]. In general, the parasympathetic vagal fibres are inhibitory and re- duce the heart rate and stroke volume. The sympathetic nerves act as accelatory nerves increasing both the heart rate and stroke volume [11]. The afferent nerves run along sympathetic pathways via both cardiac accelera- tor nerves and thoracic splanchnic nerves to reach the intermediolateral horn of T1–T4 of the spinal cord [11]. The noradrenergic or the sympathetic nervous system is mainly involved with increasing the heart rate (chronot- ropy), contractility (ionotropy) and the speed of con- duction (dromotropy) in the cardiac muscle fibres and the conduction tissue; and the transmitter involved is mainly nor-epinephrine [6]. The SA node receives most of it nerve fibres from the right-sided thoracic sympa- thetic ganglia and the right vagus [8]. The AV nodes and ventricles receive their nerve supply form the left- sided thoracic sympathetic ganglia and the left vagus, which is mainly because SA node develops from the structures on the right side of the embryo and the AV node develops from the structures on the left side of the embryo [8].
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10 Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe The sympathetic effects are mediated mainly by the adrenergic receptors, which includes β-1 and β-2 adren- ergic receptors [2, 11]. β-1 receptors are found mainly in the SA node and AV node, and the ventricular myo- cardium acts via activation of adenylate cyclase and an increase in cAMP (cyclic adenosine monophosphate) concentration in the cell to mediate the above men- tioned sympathetic effects [2, 11]. β-2 receptors are mainly found in the vascular smooth muscles in addi- tion to the bronchial smooth muscle and wall of the GI tract and the bladder. The mechanism of action is same as that of β-1 receptors, i.e., increase in cAMP levels but they cause relaxation of the vascular smooth muscle and are involved in regulation of blood flow and systemic blood pressure [2, 11]. The cholinergic or the parasympathetic nervous sys- tem effects in the heart are opposite to the ones men- tioned above and the transmitter involved is mainly acetylcholine [8, 12, 13]. The vagi supply the parasym- pathetic fibres to the heart via the cardiac plexuses. The parasympathetic effects are mediated via the muscara- nic receptors, which act by inhibition of adenylate cy- clase and hence decrease the intracellular cAMP levels and result in a decrease in heart rate, contraction and conduction velocity [8, 12, 13]. The autonomic centres in the CNS, mainly the vaso- motor centre of medulla and the hypothalamus regulate the balance between the level of sympathetic and the parasympathetic output to the cardiovascular system, depending on the afferent inputs from the periphery and the CNS [7, 8, 12, 13]. There is normally a tonic vagal discharge in humans, which overrides the mod- erate tonic discharge in the cardiac sympathetic nerves [7, 8, 12, 13]. Both the sympathetic and the parasympa- thetic fibres in the splanchnic thoracic nerves and the vagi carry afferent input mediated via baroreceptors and chemoreceptors to the autonomic centres in the CNS, in addition to the efferent output from the CNS. These afferents and efferents are involved in mediation of car- diovascular reflexes as baroreceptor and chemoreceptor reflexes [7, 8, 12, 13]. The receptors of the autonomic nervous system to the heart are the target of numerous drugs used in the treatment of various cardiovascular disorders in both acute and chronic settings [8, 12, 13]. 1.2.6 include P-cells (Pale/Pacemaker-cells), transitional cells and Purkinje cells [19, 20]. These are important in main- taining the heart’s electrical activity in an orderly fash- ion. The conduction system consists of sinus node, in- Table 1.6 Conduction system of the heart [2, 6, 9] Sinoatrial (SA)/sinus node 1. Heart’s normal pacemaker automatically initiates impulses/contraction cycle at a rate of approximately 72 depolarisations per minute 2. Located by the right atrium (cista terminalis) near the superior vena cava 3. Supplied by the nodal branch of RCA 4. Innervation is principally by the parasympathetic nervous system (slows the autorhythmicity) 5. Blood supply arises from RCA in 55–60% of people (close contact with right atrial appendage and SVC) 6. Blood supply arises from left circumflex in 40–45% of people (lies close to left atrial appendage) Atrioventricular (AV) node/node of Tawara 1. Located in the right atrium along the lower part of the inter-atrial septum 2. Autorythmic with approximately 40 depolarisations per minute 3. In majority, supplied by RCA 4. It gives rise to the AV bundle AV Bundle of His 1. Band of nerve fibres that originates from AV node and cross the A-V ring 2. AV bundle is closely related to the annuli of aortic, mitral and tricuspid valves 3. AV bundle receives dual blood supply (AV nodal artery and first septal perforator of LAD) 4. Divides into right and left bundle branches that are continuations of the bundle of His 5. These right and left bundles extend along the right and left sides of the inter-ventricular septum to the tips of the two ventricles Purkinje fibres 1. Terminal branching of the right and left bundle (thou- sands of fibrils extending between myocardial fibres). Conduction System of the Heart The cardiac conduction system consists of highly speci- alised cells, which are mainly involved in the conduction of impulses to the different regions of the myocardium [19, 20]. It has been seen to be composed of three types of morphologically and functionally distinct cells, which
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Chapter 1 The Heart: Anatomy, Physiology and Exercise Physiology Others (525ml) 10% Abdominal organs (1200ml) 24% Heart (215ml) Skin (490ml) 4% 10% Brain (650ml) 13% 11 Kidney (950ml) 19% Skeletal muscle (1030ml) 20% Fig 1.4 Distribution of systemic blood flow to various organs of the body during rest (adapted from Widmaier et al. [14]) ternodal tracts, AV node, AV (His) bundle, right and left bundle branches and Purkinje fibres [4, 18] (Table 1.6). 1.3 1.3.2 Conduction System of the Heart (Excitation Sequence) The cardiac myocytes have a unique ability of automatic impulse generation, which results in automatic rhyth- micity. Normally the electrical impulse begins in the SA node, as it has the fastest impulse generation ability and hence drives the heart. The impulse then spreads to the rest of the right atrial walls directly, to the left atrium by the interatrial conducting fibres and then to the AV node (AV junctional tissue) [6] (Fig. 1.6). The AV node conducts the impulse with a delay and propagates through the ventricular myocardium via the AV bundle of His and Purkinje fibres [1, 3, 6, 8–13, 18]. From the Purkinje fibres, the excitation impulse then continues through myocardial cells outside the specialised con- duction pathway to reach the subendocardial surface. This rapid, simultaneous and coordinated spread of ex- citation through the ventricles produces a coordinated contraction of both ventricles, thus ensuring efficient pumping of blood to the pulmonary and systemic circu- lations [1, 3, 6, 8–13, 18]. 1.3.3 Physiology of the Heart 1.3.1 Circulatory System: Systemic and Pulmonary Circulation The cardiovascular system delivers oxygen and nutri- ents to the tissues and carries away waste materials to be eliminated by organs such as lungs, liver and kidneys [1, 4] (Fig. 1.4). This system is required to function un- der various normalised and diseased conditions. The pulmonary and systemic circulations together help in fulfilling this role. Pulmonary circulation is a low resis- tance, high capacitance bed, and systemic circulation, in comparison, is a relatively high resistance vascular bed [4, 11, 14, 21] (Fig. 1.5). The deoxygenated blood from the superior vena cava (from upper extremities, head, and chest wall), infe- rior vena cava (trunk, abdominal organs and lower ex- tremities) and the coronary sinuses (from myocardium) reaches the RA [1, 4, 6, 11]. The RA is filled with deoxy- genated blood, increasing pressure in the atrial chamber. When the atrial pressure exceeds the pressure in the RV, the tricuspid valve opens allowing this blood to enter the RV [1, 4, 6, 11]. As a result of this filling, and as the RV starts to contract the pressure in the RV builds up forcing the tricuspid valve to close and the pulmonic valve to open, thereby ejecting the blood into the pul- monary arteries and lungs [1, 4, 6, 11]. The oxygenated blood from the lungs reaches the LA via the pulmonary veins and as a result, pressure in LA builds up and when it exceeds that of the LV, the mitral valve opens, allowing the blood to enter the LV [1, 4, 6, 11]. When the blood fills the LV, and as the LV starts to contract, the LV chamber pressure increase forces the mitral valve to close and aortic valve to open, thus eject- ing blood into the aorta, to be distributed throughout the body [1, 4, 6, 11]. Action Potential (AP) Ventricles, atria and the Purkinje system have a stable resting membrane potential of about –90 mV, deter- mined mainly by K + conductance [12, 13]. The action potential is of long duration about 300 ms, which is pro- longed in comparison with the action potential of the rest of the cells in the body. The action potential of the myocardial cells excluding the nodal tissue is initiated by a sudden transient inward increase in the membrane conductance of the Na + ion, referred to as the upstroke of the action potential or phase 0 (Figs. 1.7 and 1.8) [12, 13]. This is followed by a brief transient outward increase in K + ion membrane conductance resulting in a brief period of initial repolarisation. The decreasing Na + ion conductance also plays a part in this initial repolari- sation phase and is referred to as phase 1 of action po-
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12 Syed Shah, Gopinath Gnanasegaran, Jeanette Sundberg-Cohon, and John R Buscombe Pulmonary trunk SA node Pulmonary arteries Left atrium and the AV junctional tissue Lung capillaries Pulmonary veins AV node Pulmonary valve Right ventricle Right AV valve Right atrium Left atrium Bundle of His Heart Left AV valve Left ventricle Aortic valve Right and left bundle branches Terminal Purkinje fibers Aorta Arteries (various organs) Arterioles (various organs) Capillaries (various organs) Venules (various organs) Veins (various organs) Vena cava Subendocardial surface Fig 1.6 Conducting system of the heart Fig 1.5 Circulatory pathway of the cardiovascular system [8] tential [12, 13]. After this phase comes the plateau phase or phase 2 of action potential, which is characterised by the transient increase in inward Ca ++ ion conductance, accompanied by an increase in outward K + ion conduc- tance [12, 13]. These outward and inward currents are such that they maintain the membrane potential in the plateau phase. The plateau phase is followed by the repo- larisation phase of the action potential or phase 3, which results from the declining inward Ca ++ ion conductance and an increase in the outward K + ion conductance [12, 13]. This outward K + ion conductance hyperpolarises the membrane towards K + ion equilibrium and brings about the repolarisation or phase 4 [12, 13]. The SA node action potential is different from the rest of the conducting system and the myocardium. It is characterised by an unstable resting membrane potential or phase 4. This results from an increased Na + ion con- ductance resulting in inward Na + current [12, 13]. The inward Na + current is triggered by the repolarisation of the preceding action potential. In addition, the phase 0 of the action potential in the SA node is the result of in- ward Ca ++ ion conductance in contrast to Na + ion, as in the rest of the myocardium [12, 13]. In addition, the SA node action potential lacks the plateau or phase 2 of the myocardial cell action potential [12, 13]. The upstroke of the action potential in the AV node is also due to the Ca ++ ion conductance as in the SA node. The conduction velocity is fastest in the Purkinje system and slowest in the AV node [12, 13].
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