Plate 28 Heart

The cardiovascular system is a transport system that carries blood and lymph to and from the tissues of the body. The cardiovascular system includes the heart, blood vessels, and lymphatic vessels. Blood vessels provide the route by which blood circulates to and from all parts of the body. The heart pumps the blood. Lymphatic vessels carry tissue-derived fluid, called lymph, back to the blood vascular system.

The heart is a four-chambered organ consisting of a right and left atrium and a right and left ventricle. Blood from the body is returned to the right atrium from which it is delivered to the right ventricle. Blood is pumped from the right ventricle to the lungs for oxygenation and returns to the left atrium. Blood from the left atrium is delivered to the left ventricle from which it is pumped to the rest of the body, i.e., the systemic circulation.

The heart, which differentiates from a straight vascular tube in the embryo, has the same basic three-layered structure in its wall as do the blood vessels above the level of capillaries and postcapillary venules. In the blood vessels, the three layers are called the tunica intima, including the vascular endothelium and its underlying connective tissue; the tunica media, a muscular layer that varies in thickness in arteries and veins; and the tunica adventitia, the outermost layer of relatively dense connective tissue. In the heart, these layers are called the endocardium, the myocardium, and the epicardium, respectively.

Figure 1, heart, atrioventricular septum, human, H&E x45; inset x125.

This micrograph of the field shows portions of the atrial (A) and ventricular (V) walls at the level of the atrioventricular septum and the root of the mitral valve (MV). Both chambers and the valve are lined with the squamous endothelium of the endocardium (En). Purkinje fibers (PF) of the cardiac conduction system are seen in the atrial wall between the relatively thin subendocardial connective tissue (CT) and the underlying modified cardiac muscle cells (CM) of the atrioventricular node (AVN). Some of the adipose tissue (AT) that forms a major component of the septum and serves to cushion the heart is seen near the lower left corner of the image. Dense fibrous connective tissue (DCT) that is continuous with that of the septum and the subendocardial layers of the atrium and ventricle extends from the root of the valve into the leaflet. Thin cardiac muscle fibers can also be seen extending from the wall of the atrium into the upper portion of the valve. Inset. This higher-magnification view of the field outlined by the rectangle (turned -90°) shows more clearly the endothelial layer of the endocardium (En) and the dense fibrous connective tissue of the endocardium (DCT) and subendocardial layer. A thin layer of smooth muscle (SM) appears between the more densely packed fibrous tissue immediately subjacent to the endothelium and the more loosely packed dense fibrous tissue of the suben-docardium. Particularly evident are the longitudinally sectioned Purkinje fibers (PF) of the cardiac conduction system. These modified cardiac muscle cells contain the same fibrillar contractile system as their smaller counterparts in the myocardium, but the fibrils are fewer, are more loosely packed, and often surround what appear to be vacuolated areas. Intercalated disks (ID), typical of cardiac muscle cell organization, are evident in some areas.

Figure 2, Heart, atrioventricular septum, coronary artery and cardiac vein, human, H&E x30.

This micrograph shows cross sections of a coronary artery and cardiac vein in the atrioventricular septum. The coronary artery (CA) in the lower left of this micrograph is surrounded by small bundles of small cardiac muscle cells (CM) that are part of the atrioventricular node (AVN). A loop of the conduction bundle (CB) containing Purkinje fibers is evident to the right of the artery. The darkly stained tunica intima (77) is delimited by an internal elastic membrane (1EM) that is easily distinguished even at this relatively low magnification. The thick muscular tunica media

(TM) is also easily distinguished from the thinner, fibrous tunica adventitia (TA). A smaller arterial vessel (A ') is seen in the adipose tissue near the upper left edge of the figure. The cardiac vein (CV) is much larger than the corresponding artery and occupies most of the field. In this vessel, the tunica intima (Tl) appears denser than either of the other two layers, which are, themselves, impossible to distinguish at this magnification. The fibrous nature of the media and adventitia of the vein is evident, however. In this particular preparation, remnants of fixed blood (B) are evident in all three vessels. A small lymph node (LN) is seen adjacent to the coronary artery.

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