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Chapter 12 The Circulatory System

HEART Location, size, and position Triangular organ located in mediastinum with two thirds of the mass to the left of the body midline and one third to the right; the apex on the diaphragm; shape and size of a closed fist (Figure 12-1) Cardiopulmonary resuscitation (CPR)—the heart lies between the sternum in front and the bodies of the thoracic vertebrae behind; rhythmic compression of the heart between the sternum and vertebrae can maintain blood flow during cardiac arrest; if combined with artificial respiration procedure, it can be life saving

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HEART Anatomy Heart chambers (Figure 12-2) Two upper chambers called atria (receiving chambers)—right and left atria Two lower chambers called ventricles (discharging chambers)—right and left ventricles Wall of each heart chamber is composed of cardiac muscle tissue called myocardium Endocardium—smooth lining of heart chambers—inflammation of endocardium called endocarditis Covering sac, or pericardium Pericardium is a two-layered fibrous sac with a lubricated space between the two layers Inner layer called visceral pericardium or epicardium Outer layer called parietal pericardium

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HEART Anatomy Heart action Heart valves (Figure 12-3) Contraction of the heart is called systole Relaxation is called diastole Heart valves (Figure 12-3) Valves keep blood flowing through the heart and prevent backflow Consist of two atrioventricular, or AV, and two semilunar (SL) valves Tricuspid—at the opening of the right atrium into the ventricle Bicuspid (mitral)—at the opening of the left atrium into the ventricle Pulmonary semilunar—at the beginning of the pulmonary artery Aortic semilunar—at the beginning of the aorta

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HEART Heart sounds Two distinct heart sounds in every heartbeat or cycle—“lub-dub” First sound (lub) is caused by the vibration and closure of AV valves during contraction of the ventricles Second sound (dub) is caused by the closure of the semilunar valves during relaxation of the ventricles

HEART Blood flow through the heart (Figure 12-4) The heart acts as two separate pumps—the right atrium and ventricle performing different functions from the left atrium and ventricle Sequence of blood flow: venous blood enters the right atrium through the superior and inferior venae cavae—passes from the right atrium through the tricuspid valve to the right ventricle; from the right ventricle it passes through the pulmonary semilunar valve to the pulmonary artery to the lungs—blood moves from the lungs to the left atrium, passing through the bicuspid (mitral) valve to the left ventricle; blood in the left ventricle is pumped through the aortic semilunar valve into the aorta and is distributed to the body as a whole

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