Major structural parts of the heart include the muscular wall (myocardium), the inner lining (endothelium), the outer lining (pericardium), and the blood vessels supply¬ing the heart with blood. The coronary arteries and veins run along the surface of the heart through the pericardium and traverse the muscular wall. Anatomical features of the heart and circulation are illustrated in Figs. 1–10.

C. Chambers of the Heart
The thin-walled top chambers are called the right and left atrium, see Fig. 3. Oxygenated blood passively enters the left atrium from the lungs (Fig. 4) and deoxygenated blood returns from the lower extremities, abdomen, trunk, and the head region and enters passively into the right atrium. The lower chambers are called the right and left ventricles. The left ventricle is thick-walled because the strong muscle is needed to pump the blood to all parts of the body. The right ventricle is thin-walled because only a small amount of force is required to pump the blood from the right ventricle through the lungs and return the blood to theleft atrium. The right and left atria are separated from each other by a thin membrane called the atrial septum. The right and left ventricles are separated by a thicker muscular structure called the interventricular septum (see Figs. 2 and 4). The right and left atria are separated from the right and left ventricles by the tricuspid and mitral valves (mitral valve ¼ bicuspid valve in Figs. 3 and 5), respectively. The outflow from the left ventricle is separated from the aorta by the aortic valve (Figs. 2, 5 and 9).

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