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Anatomy of the Heart and CirculationI. Anatomic FeaturesII. Circulation of BloodGLOSSARYatherosclerosis same as atheroma, raised plaques filled withcholesterol, calcium, and other substances on the inner wall ofarteries that obstruct the lumen and the flow of blood; theplaque of atheroma hardens the artery, hence the termatherosclerosis (sclerosis ¼ hardening).capillaries minute, thin-walled blood vessels which connect thearterioles and the venules, forming a network in nearly allorgans and tissues of the body.coronary arteries the arteries that supply the heart muscle andother parts of the heart with blood.heart the size of a closed fist, it lies within the chest cavity,directly under the breastbone (sternum); the shape of the heartis conical with the apex pointing downward to the left edge ofthe diaphragm.myocardial infarction death of an area of heart muscle due toblockage of a coronary artery by blood clot and atheroma;medical term for a heart attack or coronary thrombosis.myocardium the heart muscle.tissues an aggregation of specialized cells which togetherperform certain special functions.I. ANATOMIC FEATURESMajor structural parts of the heart include the muscularwall (myocardium), the inner lining (endothelium), theouter lining (pericardium), and the blood vessels supplyingthe heart with blood. The coronary arteries andveins run along the surface of the heart through thepericardium and traverse the muscular wall. Anatomicalfeatures of the heart and circulation are illustrated inFigs. 1–10.A. Muscle Wall/MyocardiumThe wall of the heart consists of three layers: (1) a middlelayer of muscle fibers called the myocardium; (2) anouter thin covering, the pericardium (Fig. 2) and (3) aninternal lining called the endocardium that is smoothas silk. The blood of the heart rests momentarily on theendocardium before it is ejected from the heart into theaorta.B. PericardiumThe muscle wall of the heart is surrounded by a thin butdouble membrane, the pericardium. The innermost orvisceral layer of the pericardium is a secretory membranethat is closely attached to the surface of the heart muscle.The outer or parietal layer of the pericardium consistsof a thin but tough layer. Figure 1 shows the parietalpericardium (cut). The cells of the pericardium secreteminute quantities of a lubricating fluid that separatethe two layers of the pericardium. This pericardialfluid may be increased by diseases creating a pericardialeffusion.C. Chambers of the HeartThe thin-walled top chambers are called the right and leftatrium, see Fig. 3. Oxygenated blood passively enters theleft atrium from the lungs (Fig. 4) and deoxygenated bloodreturns from the lower extremities, abdomen, trunk, andthe 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 muscleis needed to pump the blood to all parts of the body. Theright ventricle is thin-walled because only a small amountof force is required to pump the blood from the rightventricle through the lungs and return the blood to the13