Knowledge of normal cardiac structure and function is crucial to understanding diseases that afflict the heart. This chapter reviews basic cardiac anatomy and electrophysiology as well as the events that lead to cardiac contraction.
Cardiac Anatomy and Histology
Although the study of cardiac anatomy dates back to ancient times, interest in this field has recently gained momentum. The development of sophisticated cardiac imaging procedures such as coronary angiography, echocardiography, computed tomography, and magnetic resonance imaging has made essential an intimate knowledge of the spatial relationships of cardiac structures. Such information also proves helpful in understanding the pathophysiology of heart disease. This section emphasizes the aspects of cardiac anatomy that are important to the clinician—that is, the “functional” anatomy.
The heart and roots of the great vessels are enclosed by a fibroserous sac called the pericardium ( Fig. 1.1). This structure consists of two layers: a strong outer fibrous layer and an inner serosal layer. The inner serosal layer adheres to the external wall of the heart and is called the visceral pericardium. The visceral pericardium reflects back on itself and lines the outer fibrous layer, forming the parietal pericardium. The space between the visceral and parietal layers contains a thin film of pericardial fluid that allows the heart to beat in a minimal-friction environment.
Figure 1.1. The position of the heart in the chest.
The superior vena cava, aorta, and pulmonary artery exit superiorly, whereas the inferior vena cava projects inferiorly.
The pericardium is attached to the sternum and the mediastinal portions of the right and left pleurae. Its many connections to the surrounding structures keep the pericardial sac firmly anchored within the thorax and therefore help to maintain the heart in its normal position.
Emanating from the pericardium in a superior direction are the aorta, the pulmonary artery, and the superior vena cava (see Fig. 1.1). The inferior vena cava projects through the pericardium inferiorly.
Surface Anatomy of the Heart
The heart is shaped roughly like a cone and consists of four muscular chambers. The right and left ventricles are the main pumping chambers. The less muscular right and left atria deliver blood to their respective ventricles.
Several terms are used to describe the heart’s surfaces and borders ( Fig. 1.2). The apex is formed by the tip of the left ventricle, which points inferiorly, anteriorly, and to the left. The base or posterior surface of the heart is formed by the atria, mainly the left, and lies between the lung hila. The anterior surface of the heart is shaped by the right atrium and ventricle. Because the left atrium and ventricle lie more posteriorly, they form only a small strip of this anterior surface. The inferior surface of the heart is formed by both ventricles, primarily the left. This surface of the heart lies along the diaphragm; hence, it is also referred to as the diaphragmatic surface.
Figure 1.2. The heart and great vessels.
A. The anterior view. B. The posterior aspect (or base), as viewed from the back. a, artery; lig, ligamentum; vv, veins.
Observing the chest from an anteroposterior view (as on a chest radiograph; see Chapter 3), four recognized borders of the heart are apparent. The right border is established by the right atrium and is almost in line with the superior and inferior venae cavae. The inferior border is nearly horizontal and is formed mainly by the right ventricle, with a slight contribution from the left ventricle near the apex.
The left ventricle and a portion of the left atrium make up the left border of the heart, whereas the superior border is shaped by both atria. From this description of the surface of the heart emerge two basic “rules” of normal cardiac anatomy: (1) right-sided structures lie mostly anterior to their left-sided counterparts, and (2) atrial chambers are located mostly to the right of their corresponding ventricles.
Internal Structure of the Heart
Four major valves in the normal heart direct blood flow in a forward direction and prevent backward leakage. The atrioventricular valves (tricuspid and mitral) separate the atria and ventricles, whereas the semilunar valves (pulmonic and aortic) separate the ventricles from the great arteries ( Fig. 1.3). All four heart valves are attached to the fibrous cardiac skeleton, which is composed of dense connective tissue. The cardiac skeleton also serves as a site of attachment for the ventricular and atrial muscles.
Figure 1.3. The four heart valves viewed from above with atria removed.
The figure depicts the period of ventricular filling (diastole) during which the tricuspid and mitral valves are open and the semilunar valves (pulmonic and aortic) are closed. Each annulus fibrosus surrounding the mitral and tricuspid valves is thicker than those surrounding the pulmonic and aortic valves; all four contribute to the heart’s fibrous skeleton, which is composed of dense connective tissue.
The surface of the heart valves and the interior surface of the chambers are lined by a single layer of endothelial cells, termed the endocardium. The subendocardial tissue contains fibroblasts, elastic and collagenous fibers, veins, nerves, and branches of the conducting system and is continuous with the connective tissue of the heart muscle layer, the myocardium. The myocardium is the thickest layer of the heart and consists of bundles of cardiac muscle cells, the histology of which is described later in the chapter.
External to the myocardium is a layer of connective tissue and adipose tissue through which pass the larger blood vessels and nerves that supply the heart muscle. The epicardium is the outermost layer of the heart and is identical to, and just another term for, the visceral pericardium previously described.
Right Atrium and Ventricle
Opening into the right atrium are the superior and inferior venae cavae and the coronary sinus ( Fig. 1.4). The venae cavae return deoxygenated blood from the systemic veins into the right atrium, whereas the coronary sinus carries venous return from the coronary arteries. The interatrial septum forms the posteromedial wall of the right atrium and separates it from the left atrium. The tricuspid valve is located in the floor of the atrium and opens into the right ventricle.
Figure 1.4. Interior structures of the right atrium and right ventricle.
(Modified from Goss CM. Gray’s Anatomy. 29th ed. Philadelphia, PA: Lea & Febiger; 1973:547.)
The right ventricle (see Fig. 1.4) is roughly triangular in shape, and its superior aspect forms a cone-shaped outflow tract, which leads to the pulmonary artery. Although the inner wall of the outflow tract is smooth, the rest of the ventricle is covered by a number of irregular bridges (termed trabeculae carneae) that give the right ventricular wall a spongelike appearance. A large trabecula that crosses the ventricular cavity is called the moderator band.
It carries a component of the right bundle branch of the conducting system to the ventricular muscle.
The right ventricle contains three papillary muscles, which project into the chamber and via their thin, stringlike chordae tendineae attach to the edges of the tricuspid valve leaflets. The leaflets, in turn, are attached to the fibrous ring that supports the valve between the right atrium and ventricle. Contraction of the papillary muscles prior to other regions of the ventricle tightens the chordae tendineae, helping to align and restrain the leaflets of the tricuspid valve as they are forced closed. This action prevents blood from regurgitating into the right atrium during ventricular contraction.
At the apex of the right ventricular outflow tract is the pulmonic valve, which leads to the pulmonary artery. This valve consists of three cusps attached to a fibrous ring. During relaxation of the ventricle, elastic recoil of the pulmonary arteries forces blood back toward the heart, distending the valve cusps toward one another. This action closes the pulmonic valve and prevents regurgitation of blood back into the right ventricle.
Left Atrium and Ventricle
Entering the posterior half of the left atrium are the four pulmonary veins ( Fig. 1.5). The wall of the left atrium is about 2 mm thick, being slightly greater than that of the right atrium. The mitral valve opens into the left ventricle through the inferior wall of the left atrium.
Figure 1.5. Interior structures of the left atrium and left ventricle.
A. The left atrium and left ventricular (LV) inflow and outflow regions. B. Interior structures of the LV cavity.
(Modified from Agur AMR, Lee MJ. Grant’s Atlas of Anatomy. 9th ed. Baltimore, MD: Williams & Wilkins; 1991:59.)
The cavity of the left ventricle is approximately cone shaped and longer than that of the right ventricle. In a healthy adult heart, the wall thickness is 9 to 11 mm, roughly three times that of the right ventricle. The aortic vestibule is a smooth-walled part of the left ventricular cavity located just inferior to the aortic valve. Inferior to this region, most of the ventricle is covered by trabeculae carneae, which are finer and more numerous than those in the right ventricle.
The left ventricular chamber (see Fig. 1.5B) contains two large papillary muscles. These are larger than their counterparts in the right ventricle, and their chordae tendineae are thicker but less numerous. The chordae tendineae of each papillary muscle distribute to both leaflets of the mitral valve. Similar to the case in the right ventricle, tensing of the chordae tendineae during left ventricular contraction helps restrain and align the mitral leaflets, enabling them to close properly and preventing the backward leakage of blood.
The aortic valve separates the left ventricle from the aorta. Surrounding the aortic valve opening is a fibrous ring to which is attached the three cusps of the valve. Just above the right and left aortic valve cusps in the aortic wall are the origins of the right and left coronary arteries (see Fig. 1.5B).
The interventricular septum is the thick wall between the left and right ventricles. It is composed of a muscular and a membranous part (see Fig. 1.5B). The margins of this septum can be traced on the surface of the heart by following the anterior and posterior interventricular grooves. Owing to the greater hydrostatic pressure within the left ventricle, the large muscular portion of the septum bulges toward the right ventricle. The small, oval-shaped membranous part of the septum is thin and located just inferior to the cusps of the aortic valve.
To summarize the functional anatomic points presented in this section, the following is a review of the path of blood flow through the heart: Deoxygenated blood is delivered to the heart through the inferior and superior venae cavae, which enters into the right atrium. Flow continues through the tricuspid valve orifice into the right ventricle. Contraction of the right ventricle propels the blood across the pulmonic valve to the pulmonary artery and lungs, where carbon dioxide is released and oxygen is absorbed.
The oxygen-rich blood returns to the heart through the pulmonary veins to the left atrium and then passes across the mitral valve into the left ventricle. Contraction of the left ventricle pumps the oxygenated blood across the aortic valve into the aorta, from which it is distributed to all other tissues of the body.
The impulse-conducting system ( Fig. 1.6) consists of specialized cells that initiate the heartbeat and electrically coordinate contractions of the heart chambers. The sinoatrial (SA) node is a small mass of specialized cardiac muscle fibers in the wall of the right atrium. It is located to the right of the superior vena cava entrance and normally initiates the electrical impulse for contraction. The atrioventricular (AV) node lies beneath the endocardium in the inferoposterior part of the interatrial septum.
Figure 1.6. Main components of the cardiac conduction system.
This system includes the sinoatrial node, atrioventricular node, bundle of His, right and left bundle branches, and the Purkinje fibers. The moderator band carries a large portion of the right bundle. IV, interventricular.
Distal to the AV node is the bundle of His, which perforates the interventricular septum posteriorly. Within the septum, the bundle of His bifurcates into a broad sheet of fibers that continues over the left side of the septum, known as the left bundle branch, and a compact, cablelike structure on the right side, the right bundle branch.
The right bundle branch is thick and deeply buried in the muscle of the interventricular septum and continues toward the apex. Near the junction of the interventricular septum and the anterior wall of the right ventricle, the right bundle branch becomes subendocardial and bifurcates. One branch travels across the right ventricular cavity in the moderator band, whereas the other continues toward the tip of the ventricle. These branches eventually arborize into a finely divided anastomosing plexus that travels throughout the right ventricle.
Functionally, the left bundle branch is divided into an anterior and a posterior fascicle and a small branch to the septum. The anterior fascicle runs anteriorly toward the apex, forming a subendocardial plexus in the area of the anterior papillary muscle. The posterior fascicle travels to the area of the posterior papillary muscle; it then divides into a subendocardial plexus and spreads to the rest of the left ventricle.
The subendocardial plexuses of both ventricles send distributing Purkinje fibers to the ventricular muscle. Impulses within the His–Purkinje system are transmitted first to the papillary muscles and then throughout the walls of the ventricles, allowing papillary muscle contraction to precede that of the ventricles. This coordination prevents regurgitation of blood flow through the AV valves, as discussed earlier.
The heart is innervated by both parasympathetic and sympathetic afferent and efferent nerves. Preganglionic sympathetic neurons located within the upper five to six thoracic levels of the spinal cord synapse with second-order neurons in the cervical sympathetic ganglia. Traveling within the cardiac nerves, these fibers terminate in the heart and great vessels. Preganglionic parasympathetic fibers originate in the dorsal motor nucleus of the medulla and pass as branches of the vagus nerve to the heart and great vessels.
Here the fibers synapse with second-order neurons located in ganglia within these structures. A rich supply of vagal afferents from the inferior and posterior aspects of the ventricles mediates important cardiac reflexes, whereas the abundant vagal efferent fibers to the SA and AV nodes are active in modulating electrical impulse initiation and conduction.
The cardiac vessels consist of the coronary arteries and veins and the lymphatics. The largest components of these structures lie within the loose connective tissue in the epicardial fat.
The heart muscle is supplied with oxygen and nutrients by the right and left coronary arteries, which arise from the root of the aorta just above the aortic valve cusps ( Fig. 1.7; see also Fig. 1.5B). After their origin, these vessels pass anteriorly, one on each side of the pulmonary artery (see Fig. 1.7).
Figure 1.7.Coronary artery anatomy.
A. Schematic representation of the right and left coronary arteries demonstrates their orientation to one another. The left main artery bifurcates into the circumflex artery, which perfuses the lateral and posterior regions of the left ventricle (LV), and the anterior descending artery, which perfuses the LV anterior wall, the anterior portion of the intraventricular septum, and a portion of the anterior right ventricular (RV) wall. The right coronary artery (RCA) perfuses the right ventricle and variable portions of the posterior left ventricle through its terminal branches. The posterior descending artery most often arises from the RCA. B. Anterior view of the heart demonstrating the coronary arteries and their major branches. C. Posterior view of the heart demonstrating the terminal portions of the right and circumflex coronary arteries and their branches.
The large left main coronary artery passes between the left atrium and the pulmonary trunk to reach the AV groove. There it divides into the left anterior descending (LAD) coronary artery and the circumflex artery. The LAD travels within the anterior interventricular groove toward the cardiac apex. During its descent on the anterior surface, the LAD gives off septal branches that supply the anterior two thirds of the interventricular septum and the apical portion of the anterior papillary muscle.
The LAD also gives off diagonal branches that supply the anterior surface of the left ventricle. The circumflex artery continues within the left AV groove and passes around the left border of the heart to reach the posterior surface. It gives off large obtuse marginal branches that supply the lateral and posterior wall of the left ventricle.
The right coronary artery (RCA) travels in the right AV groove, passing posteriorly between the right atrium and ventricle. It supplies blood to the right ventricle via acute marginal branches. In most people, the distal RCA gives rise to a large branch, the posterior descending artery (see Fig. 1.7C). This vessel travels from the inferoposterior aspect of the heart to the apex and supplies blood to the inferior and posterior walls of the ventricles and the posterior one third of the interventricular septum. Just before giving off the posterior descending branch, the RCA usually gives off the AV nodal artery.
The posterior descending and AV nodal arteries arise from the RCA in 85% of the population, and in such people, the coronary circulation is termed right dominant. In approximately 8%, the posterior descending artery arises from the circumflex artery instead, resulting in a left dominant circulation. In the remaining population, the heart’s posterior blood supply is contributed to from branches of both the RCA and the circumflex, forming a codominant circulation.
The blood supply to the SA node is also most often (70% of the time) derived from the RCA. However, in 25% of normal hearts, the SA nodal artery arises from the circumflex artery, and in 5% of cases, both the RCA and the circumflex artery contribute to this vessel.
From their epicardial locations, the coronary arteries send perforating branches into the ventricular muscle, which form a richly branching and anastomosing vasculature in the walls of all the cardiac chambers. From this plexus arise a massive number of capillaries that form an elaborate network surrounding each cardiac muscle fiber. The muscle fibers located just beneath the endocardium, particularly those of the papillary muscles and the thick left ventricle, are supplied either by the terminal branches of the coronary arteries or directly from the ventricular cavity through tiny vascular channels, known as thebesian veins.
Collateral connections, usually <200 µm in diameter, exist at the subarteriolar level between the coronary arteries. In the normal heart, few of these collateral vessels are visible. However, they may become larger and functional when atherosclerotic disease obstructs a coronary artery, thereby providing blood flow to distal portions of the vessel from a nonobstructed neighbor.
The coronary veins follow a distribution similar to that of the major coronary arteries. These vessels return blood from the myocardial capillaries to the right atrium predominantly via the coronary sinus. The major veins lie in the epicardial fat, usually superficial to their arterial counterparts. The thebesian veins, described earlier, provide an additional potential route for a small amount of direct blood return to the cardiac chambers.
The heart lymph is drained by an extensive plexus of valved vessels located in the subendocardial connective tissue of all four chambers. This lymph drains into an epicardial plexus from which are derived several larger lymphatic vessels that follow the distribution of the coronary arteries and veins. Each of these larger vessels then combines in the AV groove to form a single lymphatic conduit, which exits the heart to reach the mediastinal lymphatic plexus and ultimately the thoracic duct.