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Cardiac System 1: Anatomy and Physiology

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Cardiac System 1: Anatomy and Physiology Copyright EMAP Publishing 2018 This article is not for distribution Clinical Practice Keywords Heart chambers/Circulation/ Pulse/Cardiac output/Stroke volume Systems of life This article has been Cardiology double-blind peer reviewed In this article... ● Functioning of the heart and its components – chambers, valves, arteries and veins ● The conduction system, heart rhythm and cardiac cycle (diastole and systole) ● 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 coronary arteries and is drained via he heart is the key organ of the endocardium. 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 cardium, 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 pericardium 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? connective tissue; 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 cardiac muscle 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 atrium (LA) and left ventricle (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 aorta. 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 = aortic valve; 2 = pulmonary blood into the rest of the body. The LV has a valve’ 3 tricuspid valve; 4 = mitral valve 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- Pulmonary valve monary artery), 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 vein. 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 Coronary circulation 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 Superior vena cava 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 cell), 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 (sinus 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 Bundle of His Sinoatrial node transmitted to another mass of specialised cells, the atrioventricular node (AVN).
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