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Chapter 5

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NURS 201
Marywyatt Sindlinger

Chapter 5 STRUCTURES AND FUNCTIONS  The heart is a four-chambered organ that lies in the mediastinal space in the thorax.  The heart is divided by the septum, forming the right and left atrium and the right and left ventricle.  Valves separate the chambers of the heart: o Mitral valve separates the left atrium and the left ventricle. o Aortic valve separates the left ventricle and the aorta. o Tricuspid valve separates the right atrium and the right ventricle. o Pulmonic valve separates the right ventricle and the pulmonary artery.  The heart is: o Composed of three layers: endocardium, myocardium, and epicardium. o Surrounded by a fibroserous sac called the pericardium.  The right side of the heart receives blood from the body (via the vena cava) and pumps it to the lungs where it is oxygenated. Blood returns to the left side of the heart (via the pulmonary arteries) and is pumped to the body via the aorta.  The coronary circulation provides blood to the myocardium. The right and left coronary arteries are the first branches of the aorta.  The conduction system consists of specialized cells that create and transport electrical impulses. These electrical impulses initiate depolarization (contraction) of the myocardium and ultimately a cardiac contraction.  Each electrical impulse starts at the SA node (located in the right atrium), travels to the AV node (located at the atrioventricular junction), through the bundle of His, down the right and left bundle branches (located in the ventricular septum), terminating in the Purkinje fibers.  The electrical activity of the heart is recorded on the electrocardiogram (ECG).  Systole, contraction of the myocardium, results in ejection of blood from the ventricles. Relaxation of the myocardium, or diastole, allows for filling of the ventricles.  Cardiac output (CO) is the amount of blood pumped by each ventricle in 1 minute. It is calculated by multiplying the amount of blood ejected from the ventricle with each heartbeat, the stroke volume (SV), by the heart rate (HR) per minute: CO = SV  HR.  Factors affecting SV are preload, afterload, and contractility. Preload is the volume of blood in the ventricles at the end of diastole, and afterload represents the peripheral resistance against which the left ventricle must pump.  Cardiac reserve refers to the heart’s ability to alter the CO in response to an increase in demand (e.g., exercise, hypovolemia).  Stimulation of the sympathetic nervous system increases HR, speed of conduction through the AV node, and force of atrial and ventricular contractions, whereas stimulation of the parasympathetic nervous system decreases HR.  Baroreceptors, located in the aortic arch
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