GROSS ANATOMY



The human heart is located in the middle mediastinum just above the diaphragm between the left and right lungs. About two thirds of the heart is left of the midline, resulting in an apical im-pulse normally palpated in the fourth to fifth left intercostal space at the midclavicular line. Two relatively thin-walled upper chambers termed the right and left atria and two thickerwalled lower chambers, the right and left ventricles, compose the heart (Fig. 1-1). The interatrial septum, the thinnest portion of which is the foramen ovale, separates the left and right atria. The ventricles are muscular stmctures that pump blood through the pulmonary arteries to the lungs (right ventricle) and through the aorta and the entire svsıemıc circulation to the body (left ventricle). The left ventricle has walls significantlv thickeı -_h=n those of the right ventricle because of the consid-erably higher systemic arterial pressure into which it pumps blood. The interventricular septum separates the two ventricles. The lower and much larger part of the interventricular syîtem is termed the muscular interventricular septum and is composed of muscle the same thickness as that of the left ventricular free wall. The uppermost portion of the septum, termed the membranous interventricular septum, also forms a portion of the right atrial wall. The thin inner layer of the heart wall that is in contact with the blood is the endocardium. Between the endocardium and the epicardium, or outer layer, is the heart muscle, or myocardium.

The tricuspid valve separates the right atrium and right ventricle. it is a three-leaflet structure that allows blood to flow from the right atrium to the right ventricle when right atrial pressure ex-ceeds right ventricular pressure (diastole). When the right ventricle contracts (systole), the valve closes, preventing retrograde flow of blood into the right atrium. The mitral valve separates the left atrium from the left ventricle and has only two leaflets, a large anteromedial and a small posterolateral leaflet. A fibrous ring called the annulus supports each valve and forms a portion of the fibrous structural skeleton of the heart. Chords of fibrous tissue, the chordae tendineae, extend from the ventricular surfaces of both atrioventricular (AV) valves and attach to the papillary muscles. Papillary muscles are bundles of cardiac muscle (myocardium) arising from the interior of the ven­tricular walls. As the ventricles contract, the pap­illary muscles also contract, pulling taut the chor­dae tendineae and preventing the AV valves from prolapsing back into the atria and leaking. There are two papillary muscles in the left ventricle (an­teromedial and posterolateral) and three in the right ventricle, connected via the chordae tendi­neae to each valve leaflet.
A somewhat different type of valve, the semi-lunar valve (meaning half moon, an indication of its shape), separates the ventricles from their re-spective outflow tracts. The pulmonic valve is the arterial valve of the right ventricle and is composed of three fibrous leaflets or cusps that are forced öpen against the walls of the pulmonary artery during ventricular ejection of blood but fail back into the pulmonary outflow tract during diastole, their free edges coapting to prevent blood from returning into the right ventricle. The aortic valve is a thicker but similar three-valved structure and lies between the left ventricle and the aorta. The aortic wall behind each aortic valve cusp bulges outward, forming three structures known as sinuses of Valsalva. The left and right coronary arteries emerge from the aortic wall of two of these sinuses of Valsalva. The two most anterior aortic cusps are known as the left and right coronary cusps because of the respective origins of the left and right coronary arteries while the remaining posterior cusp is known as the noncoronary cusp.

The pericardium, a double-layered fibrous structure, encloses the heart. The visceral layer is immediately adjacent to the heart and forms part of the epicardium (outer layer) of the heart. The parietal layer is exterior to the heart and is separated from the visceral layer by a thin film of lubricating fluid (10 to 20 ml total) that allows the heart to move freely within the pericardial sac.

Venous blood returning from the body enters the right atrium through the inferior vena cava from below and the superior vena cava from above (Fig. 1-2). Most venous blood returning from the coronary circulation enters the right atrium via the coronary sinus. Blood from these three sources mixes and enters the right ventricle during diastole, when the triscupid valve is open. The right ventricle subsequently contracts (systole), closing the tricuspid valve to prevent retrograde blood flow, and ejects blood through the pulmonic valve into the pulmonary artery. The right ventricle is anterior to the left ventricle, and the pulmonary artery is anterior to the aorta. The pulmonary artery bifurcates into left and right branches that travel to the left and right lungs. The pulmonary artery has thinner walls than the aorta, and pulmonary arterial pressure is normally much less than aortic pressure. The pulmonary artery progressively divides into smaller and smaller arteries, arterioles, and eventually capillaries, where carbon dioxide is exchanged for oxygen via the pulmonary alveoli. The capillaries lead to pulmonary veins that coalesce to form the four larger pulmonary veins entering the left atrium posteriorly. Oxygenated blood from the pulmonary veins passes from the left atrium through the mitral valve to the left ventricle, which ejects blood during systole across the aortic valve into the aorta. The aorta divides into branches that deliver blood to the entire body (Fig. 1-3). The division continues to form smaller arteries, arterioles, and eventually capillaries that deliver oxygen and metabolic substrates to the tissues in exchange for C02 and other waste products. Blood collected from the peripheral capillaries is returned to the right atrium via the venous system.

The right and left coronary arteries course over the epicardial surface of the heart to distribute blood to the myocardium (Fig. 1-4). The left main coronary artery bifurcates within a few centime­ters of its origin into two major vessels. The left anterior descending coronary artery proceeds an­teriorly in-the anterior interventricular groove (between both ventricles) toward the apex of the heart, supplying the anterior free wall of the left ventricle and the anterior two thirds of the septum. The circumflex coronary artery travels posteriorly in the ‘atrioventricular groove (between left atrium and ventricle) and usually supplies a portion of the posterolateral surface of the heart. The right coronary artery courses in the right atrioventricular groove (between right atrium and ventricle) and distributes several branches to the right ventricle before reaching the left ventricle, where the atrioventricular grooves meet the posterior interventricular groove (the “crux” of the heart). In 90 per cent of patients the right coronary artery reaches the crux of the heart and supplies the branches to the AV node and the inferobasal third of the septum (posterior descending artery). This pattern is termed “right dominant distribution” (even though the left coronary artery supplies the majority of the coronary circulation). In approximately 10 per cent of patients, a relatively large circumflex coronary artery reaches the crux of the heart and gives rise to the posterior descending coronary artery and the branch to the AV node. This situation is termed “left dominant,” and the diminutive right coronary artery supplies only the right ventricle. Blood is supplied to the sinus node via a branch of the right coronary artery (55 per cent of cases) or the circumflex coronary artery (45 per cent). Most of the venous network of the heart coalesces to form the coronary sinus. Some of the right ventricular and atrial venous drainage occurs via much smaller diac veins and tiny thebesian veins, most of which drain directly into the right atrium.





2007 April



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