Chapter 6: Cardiovascular System

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 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 heart has two upper chambers, the right and left atria, which receive blood and 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 pumps blood and therefore 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 one-way 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.

The largest part of the heart, the lower left area, is known as the apex. This site is best for auscultating (listening to) sounds ...