CHAPTER 7: CARDIOVASCULAR SYSTEM

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II. The Heart The walls of veins and venules have the same 3 layers as arteries, but they are much thinner. Thinner walls allow veins to expand to a greater extent. Veins often have valves which allow blood to flow only toward the heart when open and prevent the backward flow of blood when closed. A. The heart is a cone-shaped, muscular organ about the size of a fist. B. It is located between the lungs directly behind the sternum (breast bone) and is tilted so the apex (pointed end) is directed left. C. Myocardium is the major portion of the heart consisting of cardiac muscle tissue. D. Heart lies within a pericardium sac that contains pericardial fluid which provides cushioning. E. Endocardium lines inner surface of the heart. F. Internal wall called the septum separates heart into right and left halves. G. Heart has 2 upper, thin-walled atria and 2 lower, thick-walled ventricles. Atria receive blood from the venous portion of the cardiovascular system. Atria are much smaller and weaker than muscular ventricles, but hold the same volume of blood. Ventricles pump blood into arterial portion of cardiovascular system. H. Heart valves direct flow of blood and prevent backward movement. Valves are supported by strong, fibrous tendons attached to muscular projections of ventricular walls. They support the valves and prevent them from inverting when the heart contracts. Atrioventricular valves occur between atria and ventricles and prevent back flow of blood from ventricle to atrium. Right atrioventricular or tricuspid valve on right side of heart consists of 3 cusps or flaps. Left atrioventricular or bicuspid (mitral) valve on left side of heart consists of 2 cusps or flaps.
Semilunar valves have flaps that resemble half-moons and are located between the ventricles and their attached vessels. III. Passage of Blood through the Heart The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk. The aortic semilunar valve lies between the left ventricle and the aorta. A. Oxygen-poor (deoxygenated) blood enters right atrium from both superior and inferior vena cava. B. Right atrium sends blood through right atrioventricular valve (tricuspid valve) to right ventricle. C. Right ventricle sends blood through pulmonary semilunar valve into pulmonary trunk. The pulmonary trunk divides into two pulmonary arteries that go to the lungs. D. Oxygen-rich (oxygenated) blood returns from lungs through pulmonary veins and is delivered to the left atrium. E. Left atrium sends blood through left atrioventricular valve (bicuspid or mitral valve) to left ventricle. F. Left ventricle sends blood through aortic semilunar valve into the aorta to the body proper through arteries. G. Oxygen-poor blood never mixes with oxygen-rich blood. H. Blood must go through the lungs in order to pass from the right side to the left side of the heart. I. The heart is a double pump serving the lungs and the body circulations simultaneously. J. Since the left ventricle has the harder job of pumping blood throughout the body, its walls are thicker than those of the right ventricle. K. The Heartbeat Each heartbeat is called a cardiac cycle. The heart contracts or beats about 70 times a minute. Each heartbeat lasts about 0.85 seconds. A normal adult heart rate at rest can vary from 60 to 80 beats per minute.
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CHAPTER 7: CARDIOVASCULAR SYSTEM

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