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Structure and Function

The cardiovascular system includes a complex network of arteries, veins, capillaries, and the key structure, the heart, which pumps blood throughout your entire body.

The heart is a hollow, muscular organ about the size of a closed fist that pumps oxygen-rich blood and nutrients to the trillions of cells of the body (Fig. 6-1). To accomplish this, it beats an average of 60 to 100 times a minute for your entire lifetime. Your heart is located in the center of your chest, slightly to the left, in an area called the mediastinum. It has three layers: the outer lining, called the epicardium; the middle muscular layer, called the myocardium; and the inner lining, called the endocardium. The heart is enclosed in a fibrous membrane called the pericardium, or pericardial 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.


The average person’s heart beats 104,000 times per day, which adds up to 38,000,000 times every year!

The heart has two upper chambers, the right and left atria, which perform about 30% of the work, and two larger, lower chambers, the right and left ventricles, which perform the other 70% of the work. The left ventricle is the largest and most muscular chamber, because it works harder than the others. The right and left sides of the heart are divided by a thick layer of muscle tissue called the septum.

There are four valves in the heart that open and close to regulate blood flow. The tricuspid valve exits the right atrium into the right ventricle, and the mitral, or bicuspid valve, exits the left atrium into the left ventricle. The pulmonary valve exits the right ventricle into the pulmonary arteries, and the aortic valve exits the left ventricle into the aorta.

image Learning Style Tip

Verbally describe photos or illustrations in this book to a real or imaginary friend who cannot see them. Provide all of the details they need to accurately visualize what you are describing. Record your voice as you do this so that you can study, by listening, while doing other things such as household chores or driving.

The largest part of the heart, the lower left area, is known as the apex. 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 1 full minute is considered the most accurate method of measuring heart rate and is the preferred method in situations where accuracy is very important.

Blood flows through both sides of the heart at the same time. ...

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