A The heart in situ, anterior view

a Simplified illustration. The chest has been widely opened, and the pleural cavities and fibrous pericardium have been cut open. The connective tissue has been removed from the anterior mediastinum to display the heart. Although the pleural cavities have been opened, the lungs are not shown in a collapsed state. b Projection of the heart on the bony thorax. The heart lies within the pericardium, which is firmly attached to the diaphragm (see p. 90) but is mobile in relation to the parietal pleura. A longitudinal axis drawn from the base to the apex of the heart demonstrates that this “long axis” is directed forward and downward from right to left. Thus the heart, when viewed from the front, has an oblique orientation and is tilted counterclockwise within the chest. Along this axis it appears slightly “rolled” in a posterior direction. Thus the right ventricle faces forward, as pictured here, while the left ventricle is only partly visible. As a result, all of the great vessels cannot be seen even when the base of the heart is viewed from the front. The short pulmonary veins are covered by the cardiac silhouette because they terminate at the left atrium, which is directed posteriorly. The right and left auricles (atrial appendages) are clearly visible. The cardiac apex points downward and to the left. Most of it is still covered by pericardium in this dissection. Its movement, called the apical beat, is palpable as a fine motion in the fifth intercostal space on the left midclavicular line (see p. 109). The thin serous membrane of the epicardium (see p. 98) gives the surface of the heart a shiny appearance. Under this membrane are clusters of fatty tissue in which the coronary vessels are embedded.

Transverse section through the thorax at the level of T8 vertebra. Viewing the transverse section demonstrates the asymmetrical position of the heart in the middle mediastinum and its slight degree of physiologic counterclockwise rotation: the left ventricle faces downward and to the left, while the right ventricle faces forward and to the right. The right ventricle thus lies almost directly behind the posterior wall of the sternum (with only the narrow anterior mediastinum intervening, see p. 79). The left atrium is in very close relationship to the esophagus. The costomediastinal recess is interposed between the heart and sternum on the right and left sides. A relatively small space remains between the heart and vertebral column for the passage of neurovascular structures and organs: thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, and portions of the autonomic nervous system. Each lung bears an indentation from the heart called the cardiac impression. This impression is larger in the left lung than in the right lung because of the heart’s asymmetric position. The potential spaces between the pleural layers and the serous portions of the pericardium are considerably smaller than pictured here.

C Cardiac dullness on percussion of the chest

Anterior view (a) and transverse section viewed from above (b). In contrast to the sonorous sound that is produced by the percussion of air-filled lung (see p. 136), the fluid-filled heart produces a flat sound on percussion known as cardiac dullness. The dullness may be absolute (at sites where there is no lung tissue to moderate cardiac dullness) or relative (at sites where lung tissue overlies the heart and adds resonance to the percussion sound). Accordingly, the area of absolute cardiac dullness is located between the chest wall and heart while the area of relative cardiac dullness is located over the right and left costomediastinal recesses, which contain small expansions of lung tissue (see B).

Note: Cardiac dullness gives way to hepatic dullness in the epigastrium and right hypochondriac region due to the anatomical extent of the liver (see a). The boundaries of the heart can be roughly estimated from the area of cardiac dullness because the sound characteristics at the cardiac borders contrast with the more resonant lung sounds.

A Location of the pericardium in the thorax, anterior view

The chest has been opened to display the pericardium, which is the dominant structure in the inferior mediastinum. It is attached inferiorly to the diaphragmatic fascia by connective tissue. Anteriorly, it is separated from the posterior surface of the sternum only by connective tissue of the anterior mediastinum (removed here, see p. 79). The pericardium is bounded laterally by the pleural cavities, from which it is separated by mediastinal pleura.

B Pericardial cavity and structure of the pericardium

Anterior view of the empty pericardial sac. The pericardium consists of two layers, one within the other, that enclose and protect the heart:

• Parietal layer. The parietal pericardium forms a sac with an outer surface, the fibrous pericardium, composed of tough and indistensible connective tissue which is partially attached to the diaphragm. Its inner surface, facing the heart, is lined with a serous membrane.

• Visceral layer (epicardium). This is a thin serous membrane which covers, and is firmly adherent to, the heart itself and the proximal parts of the great vessels.

The two serous membranes of the parietal and visceral layers are closely apposed, but move freely over one another, allowing a gliding motion during the heartbeat. These two serous membranes are referred to together as the serous pericardium.

In the locations where the parietal layer is folded back onto the visceral layer covering the vessels, two sinuses are formed (see arrows):

• The transverse pericardial sinus located between the arteries and veins

• The oblique pericardial sinus located between the left and right pulmonary veins.

Note: Because the pericardium cannot expand significantly, bleeding into the pericardial cavity (e.g., from a ruptured myocardial aneurysm) will place increasing pressure on the heart as the blood accumulates within the sac. This condition, called cardiac tamponade, seriously compromises the ability of the ventricles to fill and pump blood, creating a threat of cardiac arrest. Similar problems may arise from inflammation of the pericardium (pericarditis).

A Heart, sternocostal surface

Anterior view. The heart is a muscular hollow organ shaped approximately like a flattened cone. It consists topographically of a base, apex, and three surfaces:

• The base of the heart, which is occupied by entering and emerging vessels, is directed superiorly, posteriorly, and to the right.

• The apex is directed inferiorly, anteriorly, and to the left.

• The surfaces are described as anterior (sternocostal), posterior, and inferior (diaphragmatic) (see B).

The sternocostal surface of the heart is formed chiefly by the right ventricle, whose boundary with the left ventricle is marked by the anterior interventricular sulcus. The left ventricle (occupying the inferior and posterior cardiac surfaces) forms the left border and apex of the heart.

The anterior interventricular sulcus contains the anterior interventricular branch of the left coronary artery (see p. 120) and the anterior interventricular (great cardiac) vein. Both vessels are embedded in fat and almost completely occupy the groove, so that the anterior surface of the heart appears nearly smooth. The left and right atria are separated from the ventricles by the coronary sulcus, which also transmits coronary vessels (the intrinsic vessels of the heart, see pp. 120–123) The right auricle lies at the root of the ascending aorta, the left auricle at the root of the pulmonary trunk. The origin of the right pulmonary artery from the pulmonary trunk is hidden by the ascending aorta. For clarity, all three illustrations in this series (A, C, D) show sites where the visceral layer of the pericardium is reflected to form the parietal layer. The pericardium extends onto the roots of the great arteries.

A Myocardial architecture

a, b External musculature of the heart, simplified anteroinferior view. The muscular walls of the right and left ventricles have been windowed to display the deeper fibers.

Note: The epicardium has been removed in a and b along with the subepicardial fat. The coronary vessels are not shown in order to display more clearly the cardiac surface grooves (anterior and posterior interventricular sulci).

The musculature of the atria is arranged in two layers, superficial and deep. The superficial layer (shown here) extends over the atria and is common to both, whereas each atrium has its own deep layer. Looped and annular muscle fibers extend down to the atrioventricular boundary and also encircle the venous orifices. The ventricular musculature has a complex arrangement, consisting basically of a superficial (subepicardial), middle, and deep (subendocardial) layer. The superficial layer joins apically with the deeper layers to form a whorled arrangement of muscle fibers around the cardiac apex (vortex of the heart). The right ventricle, which is a low-pressure system (see c), is less muscular than the left and almost completely lacks a middle layer. The subendocardial layer forms the trabeculae carneae and papillary muscles (see d and p. 109).

The histological unit of the myocardium is the cardiac myocyte, a specialized form of cardiac muscle cell. Unlike their electronically isolated counterparts in skeletal muscle, cardiac myocytes form a syncytium in which membrane depolarization and contraction spread in a wave.

A Chambers of the right heart

a Right view of the atrium. b Anterior view of the ventricle. The ventricular and atrial walls have been opened widely, and the heart wall has been cut open to display the internal chambers.

The right atrium (see a) consists of:

• the anterior segment, which comprises the actual atrium with the auricle, and

• the posterior segment with the sinus of the vena cava (not visible here). It bears the orifices of the superior and inferior venae cavae.

The orifice of the superior and inferior venae cavae and the margin of the septal tricuspid valve leaflet define what is known as the triangle of Koch, an area on the wall of the right atrium. This is the location of the atrioventricular node. A small valve at the orifice of the inferior vena cava (valve of the inferior vena cava) directs blood in the prenatal circulation through the foramen ovale in the interatrial septum. The foramen ovale is sealed shut postnatally, becoming the fossa ovalis (surrounded by a rounded margin, the limbus). The orifice of the coronary sinus also bears a small crescent-shaped valve (valve of the coronary sinus). The anterior segment, which comprises the actual atrium with the auricle, is separated from the posterior segment by a ridge, the crista terminalis. Small muscular trabeculae, the pectinate muscles, arise from this ridge, giving this segment an irregular wall texture. In contrast, the wall of the posterior segment is smooth.

The right ventricle is characterized by two muscular ridges, the supraventricular crest and the septomarginal trabecula. It is also divided into two sections:

• the inflow tract posteroinferiorly (with the heart positioned in situ) and

• the outflow tract anterosuperiorly (see also p. 119). The muscular ridges of the trabeculae carneae are visible on the wall of the ventricular inflow tract. Specialized extensions of the trabeculae, the papillary muscles, are attached to the cusps of the right atrioventricular valve by collagenous cords, the chordae tendineae (see p. 109). The outflow tract is cone-shaped and consists mainly of the conus arteriosus, which has a smooth wall. The right ventricular outflow tract expels blood into the pulmonary trunk, whose orifice is guarded by the pulmonary valve. The wall of the right ventricle is relatively thin (low-pressure system). All of the cardiac chambers are lined with endocardium.

The left atrium is smaller than the right (see Aa). Its muscular wall is thin (low-pressure system) and is smooth in areas derived embryologically from the orifices of the pulmonary veins. The rest of the atrium is lined by pectinate muscles. The pulmonary veins, usually four in number, terminate in the left atrium. Occasionally a narrow tissue fold (valve of the foramen ovale) is found on the interatrial septum, formed by a protrusion of the fossa ovalis into the left atrium. It marks the site of fusion between the embryonic septum primum and septum secundum. The left ventricle has an inflow tract and outflow tract. The inflow tract begins at the left atrioventricular orifice, which is guarded by the left atrioventricular valve (see p. 107). As in the right ventricle, the wall of the left ventricular inflow tract is studded with trabeculae carneae, and papillary muscles are attached by chordae tendineae to the left atrioventricular valve. The outflow tract of the left ventricle has smooth inner walls and lies close to the interventricular septum. It leads to the aorta and is capped by the aortic valve at the root of the ascending aorta (see p. 107). The interventricular septum consists largely of muscle tissue (muscular part), and only a small portion near the aorta consists entirely of connective tissue (membranous part). The placement of the interventricular septum between the cardiac chambers is marked externally by the anterior and posterior interventricular sulci on the cardiac surface. The muscular wall of the left ventricle is thick (high-pressure system), having approximately three times the thickness of the right ventricular wall (see Ab). The chambers of the left (and right) heart are lined by endocardium.

A Overview of the cardiac valves

Plane of the cardiac valves viewed from above. The atria have been removed, and the great arteries have been transected at their roots. The cardiac valves are classified into two types—atrioventricular and semilunar. All heart valves lie in a plane, the valve plane. The cardiac valves function as one-way valves. They ensure the unidirectional flow of blood between the atria and ventricles (left and right atrioventricular valves), and out of the heart (aortic and pulmonary valves).

Atrioventricular valves. Located between the atria and ventricles, the left and right atrioventricular valves are composed of thin, avascular connective tissue covered by endocardium. They are classified mechanically as sail valves (see C) because the chordae tendineae (see C) constrain the movement of each cusp like the tethering ropes on a sail. The function of these valves is to prevent the reflux of blood from the ventricles into the atria.

• The left atrioventricular valve has two cusps (bicuspid valve): an anterior cusp (anteromedial) and a posterior cusp (posterolateral). The anterior cusp is continuous with the wall of the aorta. The alternate term mitral valve is derived from the two major cusps, which are similar in shape to a bishop’s miter. Subdivisions in the lateral margins of the otherwise smooth valve have led some anatomists to describe small accessory cusps called the commissural cusps (usually two). These are not true cusps, however, and are not connected to the fibrous anulus of the cardiac skeleton (see B). The cusps are tethered by papillary muscles (see C).

• The right atrioventricular valve has three cusps (tricuspid valve): anterior, posterior, and septal. One or two small accessory cusps may also be found; they do not extend to the fibrous anulus.

Semilunar valves. These valves have three crescent-shaped cusps of approximately equal size placed at the orifices of the pulmonary trunk (pulmonary valve) and aorta (aortic valve). Like the atrioventricular valves, they are composed of thin connective tissue covered by endocardium. The semilunar valves are classified mechanically as pocket valves because their cusps pouch into the ventricle like bulging pockets. The wall of the aorta and pulmonary trunk show slight dilations just above the valve (the pulmonary and aortic sinuses). The aortic sinuses expand the cross-section of the aorta, forming the aortic bulb. The right and left coronary arteries branch off the base of the aorta just past the aortic valve (see pp. 120–123 for details).

In a broad sense, the fibrous ring of the pulmonary valve also contributes to the cardiac skeleton. It is connected by a collagenous band (tendon of infundibulum) to the fibrous ring of the aortic valve. The atrioventricular valves are anchored to the fibrous anuli, while the semilunar valves are each attached by connective tissue to their valvular fibrous rings. Thus, the cardiac skeleton in the broad sense provides a mechanical framework for all the cardiac valves. Besides mechanically stabilizing the heart, the fibrous skeleton also functions as an electrical insulator between the atria and ventricles. The electrical impulses that stimulate cardiac contractions (see p. 116f) can pass from the atrium to the ventricles only through the bundle of His, and there is only one opening in the fibrous skeleton (in the right fibrous trigone) which transmits that bundle).

C Function of the heart valves during the cardiac cycle

a and b Ventricular diastole; c and d Ventricular systole. a and c Direction of blood flow in the left heart; b and d Valve plane viewed from above.

A Semilunar valves of the outflow tracts (aortic and pulmonary valve)

The aortic valve (a) and pulmonary valve (b) have been displayed by cutting open the ascending aorta and pulmonary trunk and opening them up like a book. The aortic valve and pulmonary valve close the ventricular outflow tracts during diastole:

• The aortic valve closes the left ventricular outflow tract.

• The pulmonary valve closes the right ventricular outflow tract.

These valves almost completely prevent the regurgitation of blood expelled by the ventricles. The origins of the left and right coronary arteries can be clearly identified in the aortic sinuses past the semilunar cusps (a), and the origin of the right pulmonary artery can be identified in the pulmonary trunk (b). The free margin of each semilunar cusp is thickened centrally to form a valvular nodule, and on each side of the nodule is a fine rim called the lunule. The nodule and lunule ensure that the margins of the cusps appose tightly and completely during valve closure. Both the atrioventricular valves (see p. 106) and the semilunar valves may undergo pathological changes, usually due to inflammation (endocarditis). Inflammation may result in secondary vascularization of the initially avascular valves, causing them to undergo fibrotic changes that stiffen the valves and compromise their function. There are two main abnormalities of valvular mechanics, which may coexist in the same valve:

• Valvular stenosis: Opening of the valve is impaired, causing a reduction of blood flow across the valve. Usually this creates a pressure overload on the chamber proximal to the obstruction.

• Valvular insufficiency: Closure of the valve is impaired, allowing blood to regurgitate into the chamber proximal to the valve. Such a pathological reflux creates a volume overload on the affected cardiac segments. When the load exceeds a certain magnitude, surgical replacement of the valve may be necessary to prevent further damage to the heart.

• Stenosis and insufficiency may coexist: A valve may become stuck in an intermediate position, unable to open or close completely.

Note: Like other myocardial regions, the myocardium of the papillary muscles may suffer necrosis due to a myocardial infarction, leaving the corresponding cusp prone to prolapse into the atrium. Conversely, pathological shortening of the chordae may also prevent the valve from closing completely. This also would allow blood to regurgitate into the atrium during ventricular systole, producing an audible heart murmur (see p. 118).

C Auscultation of the cardiac valves

The diagram shows the anatomical projection of the valves onto the thorax and the auscultation sites (the areas to which abnormal heart murmurs of the respective valves are transmitted). In the healthy heart, blood does not generate a perceptible sound as it flows across the cardiac valves (see p. 118 for the physiologic heart sounds). But if the valves are functionally impaired as a result of disease, the blood flow at the cardiac valves becomes turbulent. This type of flow produces audible sounds that are transmitted via the bloodstream. As the thick cardiac wall muffles these sounds, they are not heard best over the anatomical projections of the valves on the chest wall, but are heard more clearly at sites located downstream from the valves (see D).