Fundamentals of medical surgical nursing TEst bank notes

and ventricles on both sides of the heart. The tricuspid valve is located on the right-hand side, and the bicuspid, often called the mitral valve, is located on the left. The other two valves are located where the aorta (aortic valve) and pulmonary artery (pulmonary valve) join the heart, and are referred to as the semilunar valves. The purpose of heart valves is to ensure that the blood flows through the heart in one direction by opening to let blood flow through, and then closing to prevent it flowing backwards. The heart requires a constant supply of blood to maintain its cellular activity. This is delivered via the coronary arteries. Two coronary arteries arise from the aorta immediately above the aortic valve, each of which has several branches (Figure 13.2). The heart contains a small proportion of specialised muscle fibres that are self-excitable, meaning that they are able to spontaneously generate an electrical impulse. These cells form the conduction system (Figure 13.3). In normal circumstances, the impulse is generated in the sinoatrial node (the pacemaker). The impulse travels down the rest of the conduction system and then to different regions of the heart, producing a coordinated contraction of the four chambers of the heart (Tortora & Derrickson 2009). The sequence of electrical events can be captured and recorded on an electrocardiogram (ECG). Figure 13.2 The heart: coronary circulation. Reproduced from Tortora, G.J., Derrickson, B. (2011) Principles of Anatomy and Physiology, with kind permission of Wiley Blackwell. Arch of aorta LEFT CORONARY Pulmonary trunk POSTERIOR INTERVENTRICULAR BRANCH Left auricle Ascending aorta RIGHT CORONARY Right atrium MARGINAL BRANCH Right ventricle CIRCUMFLEX BRANCH ANTERIOR INTERVENTRICULAR BRANCH Left ventricle Anterior view of coronary arteries Nursing care of conditions related to the circulatory system Chapter 13 213 Figure 13.3 The heart: conduction system. Reproduced from Tortora, G.J., Derrickson, B. (2011) Principles of Anatomy and Physiology, with kind permission of Wiley Blackwell. Action potential Threshold + 10 mV – 60 mV Membrane potential Pacemaker potential Time (sec) 0 0.8 1.6 2.4 (b) Pacemaker potentials and action potentials in autorhythmic fibres of SA node Frontal plane Sinoatrial (SA) node Atrioventricular (AV) node Right atrium Right ventricle Left atrium Left ventricle (a) Anterior view of frontal section Atrioventricular (AV) Bundle (bundle of his) Right and left Bundle branches Purkinje fibres Part 2 Adult Medical and Surgical Nursing 214 Blood flow through the heart Deoxygenated blood enters the right atrium of the heart via the inferior and superior venae cavae and coronary sinus (Figure 13.4). Blood then flows to the right ventricle through the tricuspid valve. The ventricle is stimulated to contract, pumping blood to the lungs through the pulmonary valve and pulmonary arteries, at which point gaseous exchange occurs. Carbon dioxide is exchanged for oxygen. Oxygen-rich red blood cells are then transported back to the heart (left atrium) to be distributed to the rest of the body. Blood flows through the mitral valve into the left ventricle. Oxygenated blood is then pumped into the aorta via the aortic valve to be transported around the body. Deoxygenated body returns to the right atrium and the process is repeated. Regulation of the heart The sinoatrial node is responsible for initiating the impulse and the heart rate. The heart rate may, however, be influenced by the autonomic nervous system. The sympathetic nerve supply is responsible for increasing the heart rate, whereas the parasympathetic (vagus) nerve decreases the heart rate. The nerve supply originates in the cardiorespiratory centre, which is located in the medulla oblongata. Other factors that affect the heart rate include hormones, stress, drugs, body temperature, electrolyte imbalance and circulating blood volume. Blood vessels There are several types of blood vessel within the body (Figure 13.5): • Arteries and arterioles carry oxyg