The heart, enclosed in pericardium, occupies the middle mediastinum. It is roughly cone-shaped and lies behind the sternum with its base facing posteriorly and its apex projecting inferiorly, anteriorly and to the left, producing the cardiac impression in the left lung.
|Fig. 2.31 Transverse CT image at the level of the eighth thoracic vertebra.|
The heart consists of four chambers, namely the right and left atria and the right and left ventricles (Fig. 2.31). A fat-filled groove, the coronary or atrioventricular sulcus, separates the surfaces of the atria from the ventricles and carries the right and left coronary arteries and the coronary sinus. The right atrium receives the superior and inferior venae cavae and the coronary sinus. The right and left pulmonary veins drain into the left atrium. The right ventricle is continuous with the pulmonary trunk while the left ventricle opens into the ascending aorta.
Fig. 2.32 Borders and valves of the heart and their relationships to the anterior chest wall.
It is useful to represent the outline of the heart as a projection onto the anterior chest wall. When represented in this way, the heart has right, inferior and left borders (Fig. 2.32). The right border is formed by the right atrium and runs between the third and sixth right costal cartilages approximately 3 cm from the midline. The inferior border is formed mainly by the right atrium and right ventricle. At its left extremity, the border is completed by that part of the left ventricle which forms the apex of the heart. The inferior border runs from the sixth right costal cartilage approximately 3 cm from the midline to the apex, which usually lies behind the fifth left intercostal space, 6 cm from the midline. In the living, the apex usually produces an impulse (apex beat) palpable on the anterior chest wall. The left ventricle together with the left auricle (left atrial appendage) form the left border of the heart, which slopes upwards and medially from the apex to the second left intercostal space, approximately 3 cm from the midline.
Most of the anterior surface of the heart consists of the right atrium and right ventricle (Fig. 2.33). The left ventricle contributes a narrow strip adjacent to the left border of the heart. The anterior surface is completed by the right and left auricles. The coronary sulcus descends more or less vertically on the anterior surface and contains the right coronary artery embedded in fat. The anterior surfaces of the right and left ventricles are separated by the anterior interventricular artery (left anterior descending artery).
Most of the inferior (diaphragmatic) surface of the heart (Fig. 2.34) consists of the two ventricles, the left usually contributing the greater area. The posterior interventricular vessels mark the boundary between these two chambers. The surface is completed by a small portion of the right atrium adjacent to the termination of the inferior vena cava.
The posterior surface or base of the heart (Fig. 2.35) consists mostly of the left atrium together with a small part of the right atrium.
Fig. 2.33 Anterior surface of the heart.
Fig. 2.34 Inferior surface of the heart. The inferior part of the fibrous pericardium has been removed with the diaphragm.
Fig. 2.35 The posterior surface of the heart showing the reflection of the serous pericardium and the site of the oblique pericardial sinus.
Chambers and valves
The cavities of the right and left atria are continuous with those of their respective ventricles through the atrioventricular orifices. Each orifice possesses an atrioventricular valve, which prevents backflow of blood from the ventricle into the atrium. The myocardium of the atria is separated from that of the ventricles by connective tissue, which forms a complete fibrous ring around each atrioventricular orifice. Interatrial and interventricular septa separate the cavities of the atria and ventricles. Valves, each with three semilunar cusps, guard the orifices between the right ventricle and pulmonary trunk (pulmonary valve) and the left ventricle and ascending aorta (aortic valve). All these valves close passively in response to differential pressure gradients.
The right atrium receives blood from the superior and inferior venae cavae and from the coronary sinus and cardiac veins, which drain the myocardium. The superior vena cava enters the upper part of the chamber. Adjacent to its termination is a broad triangular prolongation of the atrium, the auricle (atrial appendage), which overlaps the ascending aorta (Fig. 2.36).
Fig. 2.36 Interior of the right atrium and auricle, exposed by reflection and excision of part of the anterior atrial wall.
Internally, the anterior wall of the right atrium possesses a vertical ridge, the crista terminalis (Fig. 2.36). From the crista, muscular ridges (musculi pectinati) run to the left and extend into the auricle. The posterior (septal) wall is relatively smooth but possesses a well-defined ridge surrounding a shallow depression named the fossa ovalis. This fossa is the site of the foramen ovale, which, in the fetus, allows blood to pass directly from the right to the left atrium. The coronary sinus empties into the chamber close to the atrioventricular orifice. Inferiorly, the right atrium receives the inferior vena cava immediately after the vessel has pierced the central tendon of the diaphragm. A fold called the valve of the inferior vena cava (Fig. 2.36) projects into the chamber and is the remnant of a fetal structure that directed the flow of blood across the right atrium towards the foramen ovale.
From the right atrium, blood flows into the right ventricle through the right atrioventricular orifice, which is guarded by the tricuspid valve (Fig. 2.37). The valve possesses three cusps, the bases of which attach to the margins of the atrioventricular orifice, while their free borders project into the cavity of the right ventricle (Fig. 2.38), where they are anchored by fibrous strands (chordae tendineae) to the papillary muscles of the ventricle. During ventricular contraction (systole), the papillary muscles pull on the chordae, preventing eversion of the valve cusps and reflux of blood into the atrium. The valve lies in the midline behind the lower part of the body of the sternum (Fig. 2.32) and its sounds are heard best by auscultation over the xiphisternum.
Fig. 2.37 Tricuspid valve, revealed after removal of the lateral wall of the right atrium.
Fig. 2.38 Interior of the right ventricle seen after removal of its anterior wall.
The right ventricle has the right atrium on its right and the left ventricle both behind and to its left. The chamber forms parts of the anterior and inferior surfaces of the heart and narrows superiorly at the infundibulum, which leads into the pulmonary trunk (Fig. 2.38). The walls of the right ventricle are thicker than those of the right atrium and internally possess numerous muscular ridges called trabeculae carneae (Fig. 2.43). One of these, the moderator band (Fig. 2.54), often bridges the cavity of the chamber, connecting the interventricular septum to the anterior ventricular wall. When present, it carries the right branch of the atrioventricular bundle of conducting tissue (p. 56). Projecting from the ventricular walls into the interior of the chamber are processes of myocardium, the papillary muscles, each attached at its apex to several chordae tendineae. The right ventricle is separated from the left ventricle by the interventricular septum, which is muscular inferiorly and membranous superiorly (Figs 2.43 & 2.46).
The pulmonary orifice lies between the infundibulum and the pulmonary trunk and is guarded by the pulmonary valve (Figs 2.39 & 2.40), which consists of three semilunar cusps. The valve closes during ventricular relaxation (diastole), preventing backflow of blood from the pulmonary trunk into the right ventricle. The valve lies behind the left border of the sternum at the level of the third costal cartilage (Fig. 2.32). Sounds generated by this valve are loudest over the anterior end of the second left intercostal space.
Fig. 2.39 Ventricular surfaces of the cusps of the pulmonary valve seen after removal of part of the anterior wall of the right ventricle.
Fig. 2.40 Pulmonary and aortic valves seen from above.
The left atrium lies behind the right atrium and forms the base of the heart. It possesses a hook-like auricle (left atrial appendage), which projects forwards to the left of the pulmonary trunk and infundibulum. The chamber receives superior and inferior pulmonary veins from each lung (Fig. 2.35). The four pulmonary veins, together with the two venae cavae, are all enclosed in a sleeve of serous pericardium, forming the superior limit of the oblique pericardial sinus. The left atrium forms the anterior wall of this sinus, which separates the chamber from the fibrous pericardium and oesophagus. Most of the inner surface of the left atrium is smooth (Fig. 2.41), although musculi pectinati are present in the auricle.
Mitral (bicuspid) valve
The left atrium communicates anteroinferiorly with the left ventricle through the left atrioventricular orifice, which is guarded by the mitral valve. This valve possesses two cusps, whose bases attach to the margins of the atrioventricular orifice (Fig. 2.41), while their free borders and cusps are anchored by chordae tendineae to the papillary muscles within the left ventricle (Fig. 2.42). The valve prevents reflux during ventricular contraction. Although it lies in the midline at the level of the fourth costal cartilages (Fig. 2.32), the sounds of the mitral valve are best heard over the apex of the heart.
From the left atrioventricular orifice, the left ventricle extends forwards and to the left as far as the apex. The thickness of the wall of the chamber is normally three times that of the right ventricle (Fig. 2.43). Internally, there are prominent trabeculae carneae and papillary muscles (Fig. 2.46). The chamber narrows as it passes upwards and to the right behind the infundibulum to form the aortic vestibule (Fig. 2.44), the part of the ventricle that communicates with the ascending aorta through the aortic orifice.
The aortic valve consists of three semilunar cusps (Fig. 2.45), which prevent backflow of blood from the ascending aorta during ventricular diastole. The valve lies behind the sternum to the left of the midline at the level of the anterior end of the third left intercostal space (Fig. 2.32). However, its sounds are best heard over the medial ends of the first and second right intercostal spaces.
The pulmonary trunk and the ascending aorta lie within the fibrous pericardium, enclosed together in a sleeve of serous pericardium anterior to the transverse pericardial sinus (Fig. 2.40). The pulmonary trunk extends upwards and backwards, while the ascending aorta initially lies behind it and passes upwards and forwards, overlapped by the right auricle. At the origin of each vessel are three dilatations or sinuses (Figs 2.39 & 2.45), one immediately above each of the cusps of the pulmonary and aortic valves. When ventricular contraction ceases, blood flows into the sinuses, thus pushing against the cusps and closing the valves. Two of the aortic sinuses give rise to the right and left coronary arteries.
The pulmonary trunk emerges from the pericardium and divides into right and left pulmonary arteries in the concavity of the aortic arch, anterior to the bifurcation of the trachea at the level of the fourth thoracic vertebra. As the ascending aorta pierces the fibrous pericardium, it turns backwards and to the left, becoming the arch of the aorta.
Connecting the aortic arch to the pulmonary trunk (or to the commencement of the left pulmonary artery) is the ligamentum arteriosum (Fig. 2.46), the remnant of the fetal ductus arteriosus which conveyed blood from the pulmonary trunk to the aorta, bypassing the pulmonary circulation. Occasionally, the ductus remains patent after birth, giving rise to serious circulatory abnormalities.
The arterial supply to the heart is provided by the right and left coronary arteries, which arise from the ascending aorta just above the aortic valve (Fig. 2.47). They supply the myocardium, including the papillary muscles and conducting tissue. The principal venous return is via the coronary sinus and the cardiac veins.
Right coronary artery
This vessel arises from the anterior aspect of the root of the aorta and descends in the anterior coronary sulcus (Figs 2.47 & 2.48). At the inferior border, it gives off a marginal branch, which runs to the left towards the apex of the heart. The right coronary artery continues on the inferior surface in the coronary sulcus (Fig. 2.49) and terminates by anastomosing with the circumflex branch of the left coronary artery. On the inferior surface, the posterior (inferior) interventricular artery arises from the right coronary artery (occasionally the left coronary artery) and runs in the posterior interventricular groove towards the apex. When the posterior interventricular artery arises from the right coronary artery, the heart is described as right dominant. The right coronary artery and its branches supply the anterior surface of the right atrium, the lower part of the left atrium, most of the right ventricle and parts of the left ventricle and interventricular septum (Fig. 2.51B). In addition, branches from this artery usually supply most of the conducting tissue of the heart (p. 56).
This artery takes origin from the posterior aspect of the root of the ascending aorta and runs to the left behind the pulmonary trunk where its major branch, the anterior interventricular artery, arises (Figs 2.47 & 2.50). The latter vessel descends in the anterior interventricular groove towards the apex of the heart. The left coronary artery continues as the circumflex artery in the posterior part of the coronary sulcus and terminates by anastomosing with the right coronary artery. The vessel supplies the posterior wall of the left atrium and auricle, most of the left ventricle and parts of the right ventricle and interventricular septum (Fig. 2.51A).
Fig. 2.50 Left coronary artery and its branches, viewed from the left.
Coronary sinus and cardiac veins
Most of the venous return from the heart is carried by the coronary sinus, which runs along the posterior part of the coronary sulcus and terminates in the right atrium. The coronary sinus is formed near the left border of the heart by the union of the posterior vein of the left ventricle and the great cardiac vein (Fig. 2.52), which accompanies the anterior interventricular artery. Other veins enter the coronary sinus, including the middle cardiac vein (Fig. 2.53), which accompanies the posterior interventricular artery. Some cardiac veins enter the right atrium independently (Fig. 2.48).
Fig. 2.52 Oblique view of the coronary sinus lying in the coronary sulcus.
Coordinated contraction of the myocardium is controlled by specialized conducting tissues, consisting of the sinuatrial (SA) node, the atrioventricular (AV) node, the atrioventricular bundle (of His) and its right and left branches (Fig. 2.54).
Fig. 2.53 Posteroinferior view of the termination of the coronary sinus in the right atrium.
The SA node lies in the anterior wall of the right atrium close to the termination of the superior vena cava. It occupies part of the root of the auricle and the upper end of the sulcus terminalis. Numerous autonomic nerves supply the node and modify its rate of discharge. The SA node usually receives blood from an atrial branch of either the right or left coronary artery. From the SA node the cardiac excitation wave passes through the atrial myocardium to reach the AV node.
The AV node lies in the interatrial septum anterosuperior to the termination of the coronary sinus. It is continuous with the atrioventricular bundle, which passes through the fibrous ring separating the atria and ventricles. The bundle gains the upper part of the interventricular septum and promptly divides into right and left branches. The AV node and bundle are supplied by branches of the posterior interventricular artery. Interruption of the arterial supply to the conducting tissues may result in cardiac arrhythmias.
Lying beneath the endocardium, the right branch of the atrioventricular bundle descends in the interventricular septum and often passes in the moderator band (Fig. 2.55) to ramify within the anterior wall of the right ventricle. The left branch runs on the left side of the interventricular septum. Both branches divide repeatedly at the ventricular apices and spread out into the myocardium of the respective ventricles.
Fig. 2.54 Location of the conducting tissues.
Fig. 2.55 Moderator band, seen through a window cut in the anterior wall of the right ventricle.