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Copyright EMAP Publishing 2018 This article is not for distribution

Clinical Practice Keywords chambers/Circulation/ Pulse/Cardiac output/Stroke volume Systems of life This article has been double-blind peer reviewed In this article... ● Functioning of the heart and its components – chambers, valves, and ● The conduction system, heart rhythm and (diastole and ) ● Cardiac examination – inspection, palpation and auscultation

Cardiac system 1: anatomy and physiology

Key points Author Selina Jarvis is a research nurse and former Mary Seacole development The heart is a scholar at Kingston and St George’s University of London and Kings Health muscle that Partners (Guys and St Thomas’ NHS Foundation trust); Selva Saman is consultant, contracts and Margate Health Consortium, Margate Netcare Hospital, Margate, South Africa. relaxes, pumping blood through Abstract The heart is a complex organ that pumps blood through the body with an the body intricate system of muscle layers, chambers, valves and nodes. It has its own circulation system and receives electric impulses that make it contract and relax, The heart cavity is which triggers a sequence of events forming the cardiac cycle. A solid and divided into two methodical understanding of how the heart works is key to understanding what atria and two can go wrong with it. This first article in a two-part series covers anatomy and ventricles separated physiology, and the second part discusses pathophysiology. by cardiac valves Citation Jarvis S, Saman S (2018) Cardiac system 1: anatomy and physiology. Nursing Blood supply goes Times [online]; 114: 2, 34-37. into the heart via the and is drained via he heart is the key organ of the . The inside of the heart (heart the coronary veins cardiovascular system – the cavity) is divided into four chambers – two body’s transport system for atria and two ventricles – separated by car- The heart has its Tblood. A muscle that contracts diac valves that regulate the passage of blood. own conduction rhythmically and autonomously, it works The heart is enclosed in a sac, the peri- system, the in conjunction with an extensive network , which protects it and prevents it sinoatrial node of blood vessels running throughout the from over-expanding, anchoring it inside being its natural body. Basically, the heart is a pump the thorax. The is attached to pacemaker ensuring the continuous circulation of the diaphragm and inner surface of the blood in the body. This article describes sternum, and is made up of: During the cardiac the heart’s anatomy and physiology. ● The fibrous pericardium, composed cycle, the chambers of a loosely fitting but dense layer of of the heart contract What is the heart? ; and relax, and blood The heart weighs around 350g and is ● The serous pericardium or epicardium, flows from high- roughly the size of an adult’s clenched fist. composed of the parietal and visceral pressure to low- It is enclosed in the mediastinal cavity of layers; pressure areas the thorax between the lungs, and extends ● A film of serous fluid between the downwards on the left between the second fibrous and serous pericardium and fifth intercostal space (Fig 1). If one that allows them to glide smoothly draws an imaginary line from the middle against each other. of the left clavicle down to below the nipple, this is where the most forceful part Atria and ventricles of the heart, the apex beat, can be felt. The atria receive blood returning to the The heart has a middle muscular layer, heart, while the ventricles receive blood the myocardium, made up of from the atria – via the atrioventricular cells, and an inner lining called the valves – and pumps it into the lungs and

Nursing Times [online] February 2018 / Vol 114 Issue 2 34 www.nursingtimes.net Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Systems of life the rest of the body (Fig 2a). The left (LA) and left (LV) are separated Fig 1. Location of the heart and landmarks from the right atria (RA) and right ventricle Mid clavicular line (RV) by a band of tissue called the septum. The RA receives deoxygenated blood 2nd intercostal space from the head and neck and from the rest of the body via the superior and inferior vena cava, respectively. The RV then 1 2 pumps blood into the lungs (through the 5th rib pulmonary trunk, which divides into the right and left pulmonary arteries), where it 3 5th intercostal space is oxygenated. The oxygenated blood is 4 returned to the LA via the pulmonary veins and passes into the LV through the cardiac valves. From the LV, it is delivered to the whole body through the . Apex beat The RV does not need a huge amount of force to pump blood into the lungs, com- pared with the LV, which has to pump Particular valves can be best heard in certain positions: 1 = ; 2 = pulmonary blood into the rest of the body. The LV has a valve’ 3 ; 4 = thicker wall and its cavity is circular, while the RV cavity is crescent-shaped with a Fig 2a and 2b. Heart chambers and cardiac valves thinner wall (Marieb and Hoehn, 2015). ab Cardiac valves When working correctly, the cardiac valves Tricuspid vave Mitral valve (Fig 2b) ensure a one-way system of blood flow. They have projections (cusps) held in place by strong tendons (chordae tendinae) LA LA RA attached to the inner walls of the heart by small papillary muscles. RA The RA and RV are separated by the tri- LV cuspid valve, which has three leaflets. The tricuspid valve allows deoxygenated blood RV to move from the RA into the RV. From the RV, blood passes through the pulmonary Aortic valve valve (situated between the RV and the pul- monary ), allowing deoxygenated blood to enter the lungs. LA = Left atrium; LV = Left ventricle; RA = Right atrium; RV = Right ventricle On the left side of the heart, oxygenated blood from the lungs enters the LA from the descending artery and the left circumflex coronary arteries is open at that point of pulmonary . The LA is separated from artery. The right coronary artery runs the cardiac cycle. Table 1 shows which the LV by the mitral valve (also called down the right side of the heart dividing regions of the heart are supplied by which bicuspid valve, as it has two leaflets (Fig 2b)) into the marginal artery (lateral part of the coronary arteries. and blood flows through this valve into the right-hand side of the heart) and posterior The venous drainage system of the heart LV. It then passes through the aortic valve descending artery (supplying the posterior uses the coronary veins, which follow a into the aorta, which transports oxygen- part of the heart) (Fig 3). course similar to that of the coronary ated blood throughout the body. The coronary arteries provide an inter- arteries. The coronary sinus is a collection of mittent supply of blood to the heart, pre- coronary veins (small, middle, great and dominantly when the heart is relaxed oblique veins, left marginal vein and left pos- The heart itself requires a richly oxygen- (during diastole), as the entrance to the terior ventricular vein) that drain into the RA ated blood supply to support its activity. This is delivered via the right and left coro- nary arteries, which lie on the epicardium Table 1. Coronary arteries and regional blood supply to the heart and penetrate the myocardium with Coronary artery Anatomical region of the heart supplied deeper branches to supply this highly by it active layer of muscle. The right and left coronary arteries arise Right coronary artery Inferior aspect from vascular openings at the base of the Left anterior descending artery Anteroseptal and anteroapical aspects aorta, called the coronary ostia. The left Left circumflex artery Anterolateral aspect coronary artery runs towards the left side of the heart, dividing into the left anterior Right coronary artery Posterior aspect

Nursing Times [online] February 2018 / Vol 114 Issue 2 35 www.nursingtimes.net Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Systems of life at the posterior aspect of the heart. Two thirds of the cardiac venous blood is Fig 3. Coronary arteries Aorta returned to the heart via the coronary sinus, Pulmonary trunk while one third is returned directly into the heart (with the anterior cardiac veins opening directly into the RA and the smallest coronary veins into all four chambers). Left coronary artery The conduction system and heart rhythm Right coronary The cardiac muscle has the ability to artery undergo depolarisation (change in the Left circumflex artery excitation of a ), which leads to a con- traction of the muscle cells. Posterior In the heart, the electrical changes descending artery needed to generate a cardiac impulse are Left anterior descending regulated by its own conduction system, Right marginal artery artery which starts with a sequence of excitation in a specialised area of cardiac cells, the sinoatrial node (SAN), situated in the right Fig 4. Conduction system of the heart atrium. This is the heart’s natural pace- maker. When working properly, it sets the heart rhythm () and initiates impulses that act on the myocardium, stim- ulating cardiac contraction. The cardiac impulse passes from the SAN into the atria, which starts to contract, and the impulse is Sinoatrial node transmitted to another mass of specialised cells, the (AVN). The AVN is situated in the inter-atrial septum, a band of tissue between the RA and LA that provides a pathway of conduc- Purkinje fibres tion between the atria and the ventricles. There is a slight delay (of 0.1 seconds) of the impulse at the AVN because the fibres of Atrioventricular node the AVN are smaller, which gives the atria time to contract and empty into the ventri- cles before ventricular contraction occurs. regulating blood flow between the cham- the atrial chambers and push residual The impulse then travels down into a bers. Pressures in the left side of the heart blood out into the ventricles. large bundle of specialised tissue, the Bundle are around five times higher than in the This signifies the last part of the ventric- of His, which conducts it down the ventri- right side, but the same volume of blood is ular resting phase (diastole) and the blood cles. The Bundle of His subsequently splits pumped per cardiac beat. within the ventricles is referred to as the end into the right and left bundles in the inter- The cardiac cycle can be broken down diastolic volume (EDV). The atria then relax ventricular septum. Purkinje fibres then into a sequence of events based on the and then the electrical impulse is trans- continue down to the inferior aspect of the principle that any blood flow through the mitted to the ventricles, which undergo heart, before looping upwards and travelling chambers depends on pressure changes, as depolarisation (QRS wave on an ECG). in the lateral aspects of the RV and LV (Fig 4). blood will always flow from a high-pres- sure to a low-pressure area (Marieb and Ventricular systole Cardiac cycle Hoehn, 2015). The process is shown in Fig 5 At this point, the atria are relaxed and the ven- The chambers of the heart contract and and described below. tricles begin to contract. This contraction of relax in a coordinated fashion. The con- the ventricles leads to an increase in ventric- traction phase is referred to as ‘systole’ and Atrial systole and ventricular filling ular pressures within the cavity. As the pres- the relaxation phase, when the heart fills At this part of the cardiac cycle, the pres- sure rises, it exceeds the pressure within the up again, as ‘diastole’. The RA and LA syn- sure in the heart is low and the blood from arteries, forcing the opening of the aortic chronise during atrial systole and diastole, the circulation passively fills the atria on and pulmonary valves as blood is ejected while the RV and LV synchronise during both sides. This culminates in the opening from ventricles and into these large vessels. ventricular systole and diastole. One com- of the atrioventricular valves and blood plete cycle of these events is referred to as moving into the ventricles. Around 70% of Isovolumetric relaxation the cardiac cycle. ventricular filling occurs during this At this point, the ventricles relax and any During the cardiac cycle, the pressure phase. After depolarisation of the atria blood remaining in the chamber is called in the cardiac chambers increases or falls, (P wave on an electrocardiogram [ECG]), end systolic volume (ESV). The ventricular affecting valve opening or closure, thereby the atria contract compressing blood in pressure precipitously drops and as this

Nursing Times [online] February 2018 / Vol 114 Issue 2 36 www.nursingtimes.net Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice For more articles on cardiac nursing, go to Systems of life nursingtimes.net/cardiology occurs, blood within the aorta and pulmo- nary trunk momentarily backflow and the Fig 5. Cardiac cycle aortic and pulmonary valves close. This Atrial systole Ventricular Isovolumetric backflow causes a brief rise in the pressure systole relaxation Dicrotic notch Aortic valve in the aorta giving a characteristic change 120 opens in the pressure of the cardiac cycle called 100 80 Aortic the dicrotic notch. While the ventricles have valve 60 Mitral valve closes been in systole, the atria are in diastole and 40 closes fill again ready for the next cardiac cycle. 20

Pressure (mmHg) 0 Cardiac output and stroke volume Mitral valve 130 Cardiac output (CO) is the amount of blood opens 90 pumped out by the heart in one minute. CO QRS 50 P

can be calculated using a simple equation: Volume (ml) the stroke volume (SV) – the volume of T blood pumped by the ventricles with each heart beat – multiplied by the . S1 Systole S2 Diastole S1 Systole S2 Diastole First, one needs to calculate the SV – the Closure of Closure of mitral and aortic and difference between the EDV (the volume of tricuspid pulmonary blood left in the ventricles during diastole) valves valves and the ESV (the volume of blood remaining in the ventricles after it has contracted). If the EDV is 120ml and the ESV is 50ml, that enhance contractility – such as adren- create what can be described as a ‘lub-dup’ the SV will be: aline and thyroxine – are said to have a pos- sound heard between pauses. ● 120ml (EDV) - 50ml (ESV) = 70 ml/beat (SV) itive inotropic effect. Conversely, factors In certain physiological states, ausculta- Once the SV has been determined, the that decrease contractility – such as cal- tion may reveal extra heart sounds that may CO can be calculated. If the SV is 70mls and cium channel blockers – are said to have a require further investigation. Listening to the heart rate is 70bpm, the CO will be: negative inotropic effect on the heart each of the valves of the heart can reveal ● 70ml (SV) x 70bpm (heart rate) (Marieb and Hoehn, 2015). useful information; for example, incompe- = 4,900ml/min (CO) tence or narrowing of the valves will cause a The CO can vary; for example, it will Clinical examination ‘whooshing’ sound, referred to as a murmur. increase in response to metabolic demands Clinical examination of the heart requires Any failure of the pump function of the such as exercise or pregnancy. In patholog- several steps (Douglas et al, 2013) in an orderly heart (for example, in heart failure) can lead ical states such as heart failure, the CO may sequence of inspection, palpation and aus- fluid to back up into the lungs, in which not be sufficient to support simple activi- cultation, starting from the patient’s hands. case auscultation may reveal crackles. The ties of daily living or to increase in response There should be careful assessment of legs should also be assessed for any signs of to demands such as mild-to-moderate the pulse (whether it is strong/weak/slow fluid accumulation (peripheral oedema) exercise (Jarvis and Saman, 2017). rising), its rate per minute, its character (Jarvis and Saman, 2017). and rhythm (regular or irregular). The Frank–Starling law venous pressure in the neck veins (jugular Conclusion A physiological principle underpinning pressure) should be assessed to help Cardiac anatomy and physiology is com- cardiac function is the Frank–Starling law, understand fluid status; it can reveal signs plex. A useful resource on this topic is which proposes that the critical factor of heart failure or valve disease. available online (www.cvphysiology.com). affecting SV is the preload – that is, the Palpating the anterior chest wall The second article in this series, covering blood that goes from the returning circula- (precordium) allows clinicians to assess cardiac pathophysiology, will show how tion into the heart during its filling. the force of the heart: a failing valve can be using a methodical system can facilitate a The amount of preload determines the felt as a thrill, while hypertrophy of the more comprehensive understanding of volume of blood that can leave the heart (the heart muscle may lead to a heave. The apex what goes wrong in cardiac diseases. NT CO) and influences the stretch and tension beat should be felt to ensure it is where it on the individual muscle cells which make should be – on the mid-clavicular line at References Douglas G et al (2013) Macleod’s Clinical up the cardiac fibres. The SV increases in the fifth intercostal space (Fig 1); this is Examination, 13th edn. Edinburgh: Churchill response to preload. As a result of the filling, part of the routine clinical examination of Livingstone. increased pressure in the ventricles increases the heart (Douglas et al, 2013). Jarvis S, Saman S (2017) Heart failure 1: the stretching of the cardiac muscle fibres. During the cardiac cycle, there are two pathogenesis, presentation and diagnosis. Nursing This stretch culminates in increased con- sounds associated with each heart beat and Times; 113: 9, 49-53. Marieb EN, Hoehn KN (2015) Human Anatomy and tractility of the heart and increased CO. Up these are audible with a stethoscope (Fig 5). Physiology (10th edn). London: Pearson. to a certain physiological limit, the preload Both signal the closure of the cardiac and contractility of the heart are positively valves: the first heart sound (S1) represents correlated. This explains how exercise can the closure of the mitral and tricuspid Cardiac system series Date improve cardiac performance. valves, and the second heart sound (S2) is Part 1: Anatomy and physiology Jan Many hormones and chemicals can due to the closure of the aortic and pulmo- Part 2: Pathophysiology Feb affect the contractility of the heart. Factors nary valves (Fig 5). These two heart sounds

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