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Anatomy for Nursing Students – Ali A. Mahdi Lec .(3) The Cardiovascular System The term circulatory system refers to the heart, blood vessels, and blood. The term cardiovascular system, however, refers only to the passages through which the blood flows—the heart, a four-chambered muscular pump; arteries, the vessels that carry blood away from the heart; veins, the vessels that carry blood back to the heart; and capillaries, microscopic blood vessels that connect the smallest arteries to the smallest veins .The cardiovascular system has two major divisions:a pulmonary circuit, which carries blood to thelungs for gas exchange and then returns it to the heart,and a systemic circuit, which supplies blood to everyorgan of the body . 1 Anatomy for Nursing Students – Ali A. Mahdi Lec .(3) Size, Shape, and Position of the Heart The heart is located in the thoracic cavity in the mediastinum,the area between the lungs. About twothirds ofit lies to the left of the median plane .Its inferior end, the apex, tilts to the left and tapers to ablunt point . The adult heart is about9 cm wide at the base, 13 cm from base toapex, and 6 cm from anterior to posterior at itsthickest point-roughly the size of a fist. It weighs about300 g . The Pericardium The heart is enclosed in a double-walled sac called the pericardium,which is anchored to the diaphragm below andto the connective tissue of the great vessels above the heart. The parietal pericardium (pericardial sac) consistsof a tough fibrous layer and a thin, smooth serous layer. The serous layerturns inward at the base of the heart and forms the visceral pericardium covering the heart surface. Between the parietaland visceral membranes is a space called the pericardial cavity. It contains 5 to 30 ml of pericardial fluid, anexudate of the serous pericardium that lubricates the membranesand allows the heart to beat almost without friction. In pericarditis—inflammation of the pericardium— themembranes may become dry and produce a painful frictionrub with each heartbeat. In addition to reducing friction, thepericardium isolates the heart from other thoracic organs,allows the heart room to expand, and resists excessiveexpansion. 1 Anatomy for Nursing Students – Ali A. Mahdi Lec .(3) The Heart Wall The heart wall consists of three layers: the epicardium,myocardium, and endocardium. The epicardium(visceral pericardium) is a serous membranecomposed of a simple squamous epithelium. Over much of the heart, it hasthick deposits of fat that fill grooves in the heart surfaceand protect the coronary blood vessels. The myocardium, by far the thickest layer, is composedof cardiac muscle and performs the work of theheart. Its muscle cells spiral around the heart and arebound together by a meshwork of a fibrous skeleton. The fibrous skeletonhas at least three functions: (1) to provide structural supportfor the heart, especially around the valves and theopenings of the great vessels; (2) to give the muscle somethingto pull against; and, as a nonconductor of electricity, (3) tolimit the routes by which electrical excitation travelsthrough the heart. This insulation prevents the atria fromstimulating the ventricles directly and is important in thetiming and coordination of electrical and contractile activity. The endocardium consists of a simple squamousendothelium (epithelium) overlying a thin connective tissue layer. It formsthe smooth inner lining of the chambers and valves and iscontinuous with the endothelium of the blood vessels. The Chambers The heart has four chambers . Blood returningto the heart is received by two superior chambers, theright and left 1 Anatomy for Nursing Students – Ali A. Mahdi Lec .(3) atria (AY-tree-uh; singular atrium). The two inferior chambers, the rightand left ventricles, are the pumps that eject blood into thearteries. The right ventricle constitutes most of the anterioraspect of the heart, while the left ventricle forms theapex and inferoposterior aspect. The atria (sing. Atrium) exhibit thin flaccid walls correspondingto their light workload—all they do is pump blood into theventricles immediately below. They are separated from eachother by a wall, the interatrial septum.A thicker wall, the interventricularseptum, separates the right and left ventricles. The right ventriclepumps blood only to the lungs and back, so its wall isonly moderately thick and muscular. The left ventricle istwo to four times as thick because it bears the greatest workloadof all four chambers, pumping blood through the entirebody. The Valves To pump blood effectively, the heart needs valves thatensure a predominantly one-way flow. There is a valvebetween each atrium and its ventricle and at the exit fromeach ventricle into its great artery . The atrioventricular (AV) valves regulate the openingsbetween the atria and ventricles. The right AV (tricuspid)valve has three cusps and the left AV (bicuspid)valve has two. The left AV valve is also known as themitral (MY-trul) valve after its resemblance to a miter, theheaddress of a catholic bishop. String-like chordaetendineae (COR-dee ten-DIN-ee-ee), like theshroud lines of a parachute, connect the AV valve cusps toconical papillary muscles on the floor of the ventricle. 1 Anatomy for Nursing Students – Ali A. Mahdi Lec .(3) The semilunar valves (pulmonary and aorticvalves) regulate the openings between the ventricles andthe great arteries. The pulmonary valve controls the openingfrom the right ventricle into the pulmonary trunk, andthe aortic valve controls the opening from the left ventricleinto the aorta. Each has three cusps shaped somewhatlike shirt pockets . Valvular Insufficiency Valvular insufficiency (incompetence) refers to any failure of a valveto prevent reflux (regurgitation)-the backward flow of blood. Valvular stenosis is a form of insufficiency in which the cusps arestiffened and the opening is constricted by scar tissue. It frequentlyresults from rheumatic fever. As the valves become scarred and constricted, theheart is overworked by the effort to force blood through the openingsand may become enlarged. Regurgitation of blood through theincompetent valves creates turbulence that can be heard as a heartmurmur. Mitral valve prolapse (MVP) is an insufficiency in which one or bothmitral valve cusps bulge into the atrium during ventricular contraction.It is often hereditary and affects about 1 out of 40 people, especiallyyoung women. In some people it causes chest pain, fatigue, and shortness of breath. 1