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Chapter 6: Cardiovascular System
in an area called the mediastinum . It has three layers: the outer lining, called the epicardium ( epi- = above, upon); the middle muscular layer, called the myocardium ( myo- = muscle); and the inner lining, called the endocardium ( endo- = in, within). The right and left sides of the heart are divided by a thick layer of muscle tissue called the interventricular septum . The heart is enclosed in a fibrous membrane called the pericardium ( peri- = beside, near), or peri- cardial sac, which also contains a small amount of pericardial fluid . This fluid acts as a lubricant that reduces friction as the heart repeatedly contracts and relaxes. Atria and Ventricles The heart has four chambers. The two upper chambers, the right and left atria , receive blood and perform about 30% of the work. The two larger, lower cham- bers, the right and left ventricles , perform the other 70% of the work. The left ventricle is the largest and most muscular chamber because it works harder than the others, pumping blood out to the entire body. The apex is the tip of the left ventricle. This site is best for auscultating (listening to) sounds from the mitral valve and is where the apical pulse is best heard. Listening to the apical pulse for one full minute is considered the most accurate method of measuring heart rate and is the preferred method in situations in which accuracy is very important. Heart Valves Four one-way valves in the heart open and close to prevent the blood from flow- ing backward. The tricuspid valve separates the right atrium and the right ven- tricle, and the mitral, or bicuspid valve , separates the left atrium and the left ventricle. The pulmonary valve separates the right ventricle and the pulmonary arteries ( pulmon/o = lungs), and the aortic valve separates the left ventricle and the aorta. Blood Circulation Through the Heart Blood flows through all four chambers and valves at the same time. Blood that is low in oxygen (O 2 ) but high in carbon dioxide (CO 2 ) returns from the body to the right atrium via the inferior and superior venae cavae . Although both the right and left atria contract at the same time, they each pump blood to a different area. The right atrium pumps oxygen-poor blood and CO 2 downward through the tricuspid valve into the right ventricle. When the right ventricle contracts, it forces blood up and out through the pulmonary valve into the pul- monary arteries. The pulmonary arteries lead to the lungs and are unique in that they are the only arteries in the body that transport oxygen-poor blood. While the blood circulates through the lungs, it gets rid of CO 2 and picks up O 2 . Blood that is now oxygen rich returns through the pulmonary veins to the left atrium (Box 6-1). The pulmonary veins are unique in that they are the only veins in the body that transport oxygen-rich blood. When the left atrium con- tracts, it forces blood downward through the mitral valve into the left ventricle. From there, the blood is pumped by the left ventricle upward and out through the aortic valve into the aorta and out to the tissues and organs. In summary, pulmonary circulation occurs when the right side of the heart receives deoxygen- ated blood from the body and sends it to the lungs. Systemic circulation occurs when the left side of the heart receives oxygenated blood from the lungs and sends it out to the body.
Flashpoint Arteries move blood from the heart to the body while veins move blood from the body back to the heart. With a condition called venous insufficiency, it can help to elevate the legs but with arterial insufficiency, elevating the legs does not help.
Learning Tip Create your own mnemonic, such as “ T ry P erforming B etter A lways,” to remem- ber the order of blood flow through the valves of the heart: tricuspid, pulmonary, bicuspid, aortic.
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