Robert E. Hobbs
The heart is a muscular organ, pyramidal in shape, consisting of two parallel-valved pumps, located within the middle mediastinum, two-thirds to the left of the centerline. The base of the heart is oriented superiorly, whereas the apex points leftward, anteriorly, and slightly inferiorly. The cardiac apex is located at the fifth intercostal space near the midclavicular line. The heart is enclosed by the fibrous pericardium, which is bordered by the diaphragm inferiorly; the sternum and ribs anteriorly; the pleurae laterally; and the esophagus, descending aorta, and vertebrae posteriorly.
The average adult heart measures 12 cm from base to apex, 8 to 9 cm in width, and 6 cm in depth, approximating the size of a clenched fist. The heart weighs approximately 325 ± 75 g in men and 275 ± 75 g in women, accounting for 0.45% of body weight in males and 0.40% in females.
Viewed from the front, visible structures include the superior and the inferior vena cavae draining into the right atrium, the right ventricle (the most anterior chamber of the heart), the main pulmonary artery that courses superiorly and posteriorly before bifurcating, the left atrial appendage, a small portion of the left ventricle visible to the left of the left anterior descending coronary artery, and the ascending aorta (Fig. 3.1). The right heart forms the largest part of the anterior surface, whereas the left heart is largely posterior. The epicardial surface of the heart usually is covered with fat, proportional to age and the amount of body fat. Beneath the epicardial fat, the interventricular grooves separate the right and left ventricles and contain arteries, veins, nerves, and lymphatics. The atrioventricular (AV) grooves, which separate the atria from the ventricles, are located at the base of the heart. The interatrial grooves mark the borders between the atria. Posteriorly, the crux (“cross”) is the intersection of the AV, interatrial, and interventricular grooves. The acute cardiac margin is the junction of the inferior and anterior walls of the right ventricle. The obtuse margin is the rounded lateral wall of the left ventricle.
FIGURE 3.1 Frontal view of the heart and great vessels.
The heart is composed of four chambers. The right and the left atria are weakly contractile reservoirs that receive blood from the body and the lungs. They are positioned above the ventricles and are separated by the AV (tricuspid and mitral) valves. The right and left ventricles are muscular pumping chambers separated from each other by the interventricular septum. The ventricles eject blood through the semilunar (pulmonic and aortic) valves to the pulmonary artery and aorta, respectively.
The shape of the heart and the position of the valves are maintained by an internal fibrous skeleton. The cardiac skeleton consists of four valve annuli (or rings), the membranous septum, and the right and left fibrous trigones. The right fibrous trigone, also known as the central fibrous body, is located between the aortic, mitral, and tricuspid valves. It houses the His bundle and is the strongest component of the cardiac skeleton.
THE PERICARDIUM
The pericardium is a fibrous sac surrounding the heart and great vessels and containing 10 to 50 mL of pericardial fluid (Fig. 3.2). It maintains the position of the heart within the mediastinum, lubricates the cardiac surfaces, and provides a barrier against infection. Inferiorly, the pericardium is anchored to the central tendon of the diaphragm. Anteriorly, it is attached by ligamentous connections to the posterior sternum. Superiorly, the pericardium extends to the level of the second intercostal space, and laterally it is attached to the pleurae. The pericardium encloses the heart, portions of the vena cavae, most of the ascending aorta, the main pulmonary artery, and the four pulmonary veins. The pericardium consists of two layers. The inelastic fibrous pericardium is the outermost layer. The serous pericardium forms a thin mesothelial layer on the cardiac surface (the visceral pericardium or the epicardium) and lines the inferior surface of the fibrous pericardium (the parietal layer). The visceral pericardium covers the heart and contains the coronary arteries and veins, autonomic nerves, lymphatics, and fat. Posteriorly, the pericardium folds upon itself to create several distinct sinuses. The oblique sinus is a pericardial reflection along the vena cavae and the pulmonary veins. The transverse sinus is a pericardial reflection located between the aorta, pulmonary artery, and atria. The ligament of Marshall is a pericardial fold containing the remnant of the embryonic left superior vena cava.
FIGURE 3.2 Frontal view of the pericardium.
CARDIAC CHAMBERS
Right Atrium
The right atrium is a low-pressure capacitance chamber that receives blood from the superior vena cava, inferior vena cava, and coronary sinus (Fig. 3.3). The right atrial volume is approximately 75 to 80 mL, and its free-wall thickness is 1 to 3 mm. The superior vena cava enters the superior aspect of the right atrium and directs its blood flow toward the tricuspid valve. The inferior vena cava returns blood from the lower body, and its eustachian valve directs blood flow toward the foramen ovale or the fossa ovalis. The coronary sinus returns most of the blood from the heart itself through an orifice partially guarded by the thebesian valve (valve of the coronary sinus). When the eustachian or thebesian valves are large and fenestrated, it is described as a Chiari net. Thebesian veins drain cardiac blood into the right atrium via multiple small orifices. The fossa ovalis, representing a closed foramen ovale, forms a 1.5- to 2.0-cm depression on the interatrial septum. A patent foramen ovale is found in up to one-third of adults. The crista terminalis is a C-shaped muscular ridge on the right atrial free wall that separates the smooth posterior portion of the right atrium from the muscular anterior portion. The pectinate (“comb”) muscles arise from the crista terminalis and course as bands anteriorly on the right atrial free wall. The right atrial appendage is a large triangular structure that overlies the right coronary artery and contains pectinate muscles. In the lower medial portion of the right atrium, Koch triangle overlies the AV node and the proximal His bundle. The tendon of Todaro is a fibrous band located between the valves of the inferior vena cava and the coronary sinus.
FIGURE 3.3 Frontal cutaway view of the right atrium and right ventricle.
Right Ventricle
The right ventricle is the most anterior chamber of the heart (Fig. 3.4). It is the smaller of the two ventricular chambers, separated from the left ventricle by the interventricular septum, which bulges into the right ventricle. It is triangular shaped when viewed from the right, and crescent shaped when viewed in cross section from the left. The right ventricular free wall is approximately 3 to 4 mm thick, or about one-third the thickness of the left ventricle. The right ventricle consists of an inlet portion, a trabeculated apical portion, and a smooth right ventricular outflow tract (infundibulum or conus portion). The walls of the right ventricle contain a latticework of muscle fibers called trabeculae carneae. The right ventricular apex is heavily trabeculated, more so than the left ventricle. The infundibulum (outflow tract or conus) portion of the right ventricle is smooth walled to the pulmonic valve. The right ventricle contains three papillary muscles, although the septal papillary muscle occasionally may be absent. Chordae tendineae (fibrous cords) extend upward from the papillary muscles and attach to the leaflet edges and to the ventricular side of the tricuspid valve. Chordae from one papillary muscle often attach to more than one tricuspid leaflet, and some chordae arise from the septum. The crista supraventricularis (supraventricular crest) separates the inflow and outflow portions of the right ventricle. It consists of a septal band on the ventricular septum and a parietal band on the right ventricular free wall. The moderator band is an intracavity muscular bridge that connects the distal septum with the right ventricular free wall at the anterior papillary muscle. Blood enters the right ventricle via the tricuspid valve, turns upward at a 45- to 60-degree angle, and passes through the pulmonic valve into the main pulmonary artery.
FIGURE 3.4 Coronal section of the heart and great vessels.
Left Atrium
The left atrium is the left upper posterior chamber of the heart (Fig. 3.5). It is cuboidal shaped, smaller than the right atrium (volume, 55 to 65 mL), but with thicker walls (3 mm) and higher pressure. It receives oxygenated blood from the lungs via four pulmonary veins (two from each lung). Unlike the right atrium, the left atrium has smooth interior walls and does not have bands of pectinate muscles except in the left atrial appendage. The left atrial muscle extends a variable distance within the pulmonary veins to prevent reflux during atrial contraction. The left atrial appendage, overlying the left circumflex coronary artery, is smaller, longer, more tortuous, and less triangular than the right atrial appendage, often containing two or more lobes. Left atrial contraction generates a stroke volume of 20 to 30 mL.
FIGURE 3.5 Posterior view of the heart and great vessels.
Left Ventricle
The left ventricle is a high-pressure, muscular chamber, 2.5 to 3 times thicker than the right ventricle (see Fig. 3.4). It is ellipsoid, or cone shaped when viewed from the right, and ring or doughnut shaped when viewed in cross section from the left. It is longer and narrower than the right ventricle, measuring approximately 7.5 cm in length and 4.5 cm in width. Structurally, the left ventricle consists of the inflow tract, the apical zone, and the left ventricular outflow tract. The anterior mitral leaflet separates the left ventricle into the posterior inflow tract and the anterior outflow tract. The septum consists of a large inferior muscular portion and a small superior membranous portion. The septum is thickest at the midportion and thinnest at the membranous portion near the aortic valve. The membranous septum has AV and intraventricular portions divided by the septal tricuspid leaflet. The His bundle is located within the interventricular portion of the membranous septum.
The left ventricular free wall measures approximately 8 to 12 mm and is thicker at the base than at the apex. It is composed of three layers: the endocardium, the myocardium, and the epicardium (or visceral pericardium). The outer two-thirds of the myocardium contains compact layers of muscle that twist and spiral inward from apex to base during contraction. The inner third of the myocardium consists of a latticework of trabeculae carneae that are more intricate than right ventricular trabeculations. The septal surface of the left ventricle is smooth.
Two papillary muscles, the larger anterolateral and the smaller posteromedial, arise from the free wall and have a variable number of heads. They anchor the chordae tendineae of the mitral valve, which are thicker than tricuspid valve chordae. Chordae tendineae restrict valve excursion during ventricular systole, thereby preventing the mitral valve leaflets from prolapsing into the left atrium. Most chordae arise from the heads of the papillary muscles, but some arise from the free wall. Chordae from one papillary muscle may diverge and attach to both mitral leaflets. False chordae occur in half of normal hearts, and may connect walls, papillary muscles, and the septum, but are not attached to the mitral leaflets. They often cross the left ventricular outflow tract and can be identified by echocardiography. Many false chordae contain extensions of left ventricular conducting fibers.
Blood enters the left ventricle via the mitral valve and is ejected at a 90- to 120-degree angle through the aortic valve. The ejection phase is shorter in the left ventricle, but the pressure is greater compared with right ventricular contraction.
CARDIAC VALVES
Tricuspid Valve
The tricuspid valve is the largest of the heart valves and maintains forward flow of blood through the right heart (Fig. 3.6). The functional components of the tricuspid valve include the three leaflets, commissures, annulus, chordae tendineae, papillary muscles, and the right ventricle. The leaflets are named for their anatomic position: anterior, posterior, and septal. The anterior leaflet, which is the largest and the most mobile, partially separates the right ventricular inflow and outflow tracts. The posterior leaflet is the smallest, whereas the septal leaflet is the least mobile and is occasionally absent. The valve leaflets are attached to a discontinuous fibrous annulus that has a “D” shape. Chordae tendineae (tendinous cords) are attached to the edges and undersurface of each leaflet and are anchored by the papillary muscles and the interventricular septum.
FIGURE 3.6 Cross-sectional view of the cardiac valves.
Pulmonic Valve
The pulmonic valve is the most anterior valve of the heart, located between the right ventricular outflow tract and the main pulmonary artery (see Fig. 3.6). It is the mirror image of the aortic valve, containing right, left, and anterior cusps (or leaflets) that are thinner than those of the aortic valve. The pulmonary sinuses are partially embedded within the right ventricular infundibulum. During systole, the valve opens to form a rounded, triangular-shaped central orifice.
Mitral Valve
The mitral valve is named after the miter, a tall ornamental hat worn by bishops and abbots (Figs. 3.6 and 3.7). The valve, located between the left atrium and the left ventricle, maintains the forward flow of blood in the left heart. The mitral valve has six components: leaflets, commissures, annulus, chordae tendineae, papillary muscles, and left ventricle. When viewed from the side, the valve is funnel shaped, with the leaflets forming an apex protruding into the left ventricle. There are two mitral leaflets, the anterior and the posterior, which have similar surface areas but different shapes. The anterior leaflet is semicircular or oval shaped, broader but narrow transversely. It partially separates the left ventricular inflow tract from the left ventricular outflow tract. The posterior leaflet is crescent shaped, longer and narrower, half the height but twice the length of the anterior leaflet. It attaches over two-thirds of the posterior valve circumference. The posterior leaflet has two or more indentations forming three scallops (the middle usually is the largest). During atrial contraction, the valve forms an ellipsoid orifice. During ventricular contraction, the atrial side of the leaflets coapt, preventing regurgitation of blood into the atrium. Two commissures, the anterolateral and the posteromedial, separate the two leaflets. The chordae tendineae prevent the mitral valve from prolapsing into the left atrium during ventricular systole. Approximately 100 primary, secondary, and tertiary chordae attach to the free edge and underside of the valve and are anchored by two papillary muscles in the left ventricle. Some of the posterior leaflet chordae arise from the left ventricular free wall. Unlike the tricuspid chordae, mitral chordae do not have insertions into the septum. The mitral valve is surrounded by a saddle-shaped fibrous ring, the mitral annulus, which anchors the valve. It is connected to the tricuspid annulus by the right fibrous trigone (central fibrous body), forming part of the fibrous skeleton of the heart.
FIGURE 3.7 Surgical view of the atrial surface of the mitral valve.
Aortic Valve
The aortic valve, located between the left ventricle and the aorta, is thicker and stronger than the pulmonic valve (Figs. 3.6 and 3.8). It consists of three semilunar (half-moon) cusps located within the sinuses of Valsalva, three commissures, and an annulus. The three semilunar cusps, left, right, and noncoronary (or posterior), are pocket-like structures. The valve has a triangular-shaped central orifice when fully opened during systole. In diastole, blood fills the pocket-like cusps, causing the valve to close by coapting on the ventricular surfaces of the cusps. The nodules of Arantius are small fibrous mounds at the center of the free edge of each cusp. Three commissures radiate from the center of the valve, giving the appearance of a “peace sign.” Approximately 1% to 2% of aortic valves are bicuspid.
FIGURE 3.8 Side and top views of the aortic valve.
GREAT VESSELS
Vena Cavae
These large veins return blood from the body to the right atrium (see Fig. 3.1). The superior vena cava is formed by the juncture of the left and right innominate veins. The azygos vein enters the superior vena cava in the midthorax. Half of the superior vena cava is contained within the pericardium. The superior vena cava enters the upper portion of the right atrium, where its blood flow is directed toward the tricuspid valve. The inferior vena cava is larger than the superior vena cava. It receives blood from the lower body and from the abdominal viscera via the hepatic veins. Only 1 to 2 cm of the inferior vena cava is enclosed by the pericardium. The inferior vena cava enters the right atrium on the lower lateral free wall, where its blood flow is directed by the eustachian valve toward the fossa ovalis.
Pulmonary Arteries
The main pulmonary artery is the most anterior cardiac vessel, located entirely within the pericardium (see Fig. 3.1). It arises from the base of the right ventricle, courses superiorly and posteriorly below the aortic arch, where it bifurcates into the left and right pulmonary arteries. The right pulmonary artery is slightly larger and longer than the left, dividing into a superior ascending branch and an inferior descending branch. The left pulmonary artery passes over the left mainstem bronchus and subdivides into a variable number of branches that parallel bronchial bifurcations. The left pulmonary artery is connected to the descending thoracic aorta by the ligamentum arteriosum (ductal artery ligament), a remnant of the ductus arteriosus.
Pulmonary Veins
Four pulmonary veins, the right and left, superior and inferior, return oxygenated blood from the lungs to the left atrium (see Fig. 3.5). Occasionally, five or six pulmonary veins may be found. Atrial muscle extends for 1 to 3 cm within the pulmonary veins and functions as a sphincter to prevent reflux of blood during atrial systole.
The Aorta
The aorta arises from the aortic fibrous ring and passes superiorly and to the right as the ascending aorta (Fig. 3.9). The proximal aorta (aortic root) is dilated and contains the aortic valve and the sinuses of Valsalva. The coronary arteries, the first two branches of the aorta, arise from the left and the right sinus of Valsalva, respectively. The aortic root measures approximately 3 cm at the annulus. The sinotubular junction, at the top of the sinuses of Valsalva, separates the aortic root (sinus portion) from the tubular ascending aorta. The proximal two-thirds of the ascending aorta is located within the fibrous pericardium. In the upper thorax, the aorta courses to the left and posteriorly, forming the transverse aortic arch. Three large vessels arise from the transverse aortic arch: the right innominate (brachiocephalic) artery, the left carotid artery, and the left subclavian artery. The aorta passes over the left pulmonary artery and then descends through the posterior mediastinum to the left of the midline. The ligamentum arteriosum (ductal artery ligament) is the remnant of the ductus arteriosus that is connected to the left pulmonary artery. In the thorax, the aorta gives rise to 12 pairs of intercostal arteries, the anterior spinal artery, and several bronchial arteries. It passes through the diaphragm, where it narrows to approximately 1.75 cm, and bifurcates into the iliac arteries at the level of the fourth lumbar vertebra.
FIGURE 3.9 Aortic valve, thoracic aorta, and arch vessels.
Coronary Arteries
The coronary arteries are the first branches of the aorta, located on the surface of the heart in the AV and interventricular grooves between the cardiac chambers. They are often covered with fat, which is proportional to body fat and aging. The coronary arteries deliver oxygenated blood to the underlying heart muscle.
The right coronary artery is located in the right AV groove. It forms a C-shaped structure when viewed from the left, and an L-shaped structure when viewed from the right (Figs. 3.10 and 3.11). The right coronary artery bifurcates at the crux of the heart into a posterior descending branch and an AV branch. The posterior descending artery (PDA) follows the posterior interventricular groove and provides blood supply to the inferior (diaphragmatic) portion of interventricular septum. The AV branch passes beyond the crux of the heart, where it gives off branches that perfuse the posterolateral left ventricular myocardium. A dominant right coronary artery provides a posterior descending branch, an AV branch, and posterior ventricular or posterolateral branches. A codominant (balanced) right coronary artery provides a posterior descending branch but does not perfuse the posterior left ventricular myocardium. A nondominant right coronary artery is a small vessel that does not reach the crux of the heart and does not have posterior descending, AV, or posterior ventricular branches. The size of the right coronary artery is inversely proportional to the size of the circumflex branch. The first branch of the right coronary artery is the conus branch, which perfuses the right ventricular outflow tract. It has a “hook” or “question mark” shape when viewed from the right. Fifty percent of hearts have a separate origin of the conus branch from within the right sinus of Valsalva. The second branch of the right coronary artery is the branch to the sinoatrial (SA) node. This thin vessel courses superiorly and posteriorly, supplying blood to the right atrium and the SA node. It has the appearance of a “tree branch” or an “antler” when viewed angiographically. The right coronary artery has a variable number of right atrial marginal branches and right ventricular marginal branches that arise perpendicularly from the main vessel. The AV nodal artery arises from the AV branch at the crux of the heart and courses superiorly.
FIGURE 3.10 Right coronary artery, left anterior oblique view.
FIGURE 3.11 Right coronary artery, right anterior oblique view.
The left main trunk varies from 3 to 10 mm in diameter and from 1 to 4 cm in length (Figs. 3.12 and 3.13). Occasionally the left main trunk is absent, whereby the left anterior descending branch and the left circumflex branch arise from separate but adjacent orifices within the left sinus of Valsalva. The left main trunk trifurcates in 30% of hearts into the left anterior descending, a ramus branch, and the left circumflex branch.
FIGURE 3.12 Left coronary artery, left anterior oblique view.
FIGURE 3.13 Left coronary artery, right anterior oblique view.
The left anterior descending branch, located in the anterior interventricular groove, supplies blood to the anterolateral wall of the left ventricle and most of the interventricular septum. It reaches the cardiac apex in 80% of hearts. The left anterior descending provides four to seven septal perforators, which supply blood to the anterior interventricular septum, and two to three diagonal branches, which perfuse the anterolateral wall of the heart.
The left circumflex coronary artery arises from the left main trunk and supplies blood to the lateral wall of the heart and to the left atrium. The branches of the circumflex sometimes are referred to as obtuse marginal branches. A different classification system describes these branches in relation to their position on the left ventricle: high lateral, lateral, posterolateral, posterior ventricular, and posterior descending. The size of the left circumflex is inversely proportional to the size of the right coronary artery.
Cardiac Veins
The venous system of the heart consists of the coronary sinus, cardiac veins, and the thebesian venous system (Fig. 3.14). The cardiac veins generally follow the anatomic course of the coronary arteries, and return blood to the right atrium via the coronary sinus. The great cardiac vein parallels the left anterior descending coronary in the anterior interventricular groove. It drains upward from the apex toward the base and then passes leftward and posteriorly, paralleling the left circumflex artery, and entering the coronary sinus at its origin. The posterior vein of the left ventricle drains into the distal end of the coronary sinus. The middle cardiac vein, located in the posterior interventricular groove adjacent to the posterior descending coronary artery, drains into the distal coronary sinus. The small cardiac vein parallels the course of right coronary artery and drains into the distal coronary sinus. There are 3 to 12 smaller anterior cardiac veins, some of which drain directly into the right atrium. Many small thebesian veins drain directly into cardiac chambers, most commonly the right atrium and the right ventricle. Venous anatomy is extremely variable, with multiple venous anastomoses. The coronary sinus is the largest cardiac vein, measuring approximately 2 to 5 cm in length and 3 to 5 mm in diameter. It is located in the posterior AV groove and drains into the right atrium.
FIGURE 3.14 Frontal view of the cardiac veins.
Cardiac Lymphatic System
A plexus of lymphatic channels is located throughout the myocardium of all four cardiac chambers, draining outward from the endocardium toward the epicardium. Larger lymphatic channels follow the paths of the coronary arteries and veins and coalesce to forms a large lymphatic trunk that courses over the left main coronary artery and below the left pulmonary artery, where it drains into pretracheal lymph nodes and then into the thoracic duct.
THE CONDUCTING SYSTEM
The conducting system consists of the SA node, the AV node, the His bundle, the right and left bundles, and the Purkinje fibers (Fig. 3.15). The components of the conducting system consist of modified myocardial cells that either have spontaneous automaticity or conduct electrical impulses throughout the myocardium to initiate contraction. The SA and the AV nodes have spontaneous automaticity, whereas the other components conduct electrical impulses rapidly.
FIGURE 3.15 Cardiac conducting system.
The SA (sinus) node is a histologically distinct structure located in the roof of the right atrium between the superior vena cava and the right atrial appendage. It spontaneously depolarizes 60 to 90 times a minute, functioning as the heart’s intrinsic pacemaker. Sinus node depolarization is more rapid in children and during exercise. The blood supply to the SA node consists of a central artery arising from the right coronary artery in 60% of hearts and from the left circumflex branch in 40%. There are no histologically distinct pathways between the SA and the AV nodes. Conduction spreads via ordinary atrial myocardium rather than specialized bundles of conducting fibers.
The AV node is located inferomedially within the right atrium, beneath Koch triangle, above the septal leaflet of tricuspid valve, and near the orifice of the coronary sinus. The AV node regulates the number of impulses that pass to the ventricles. It also has spontaneous depolarization, usually occurring at a rate of 40 to 60 per minute. AV nodal function is modulated by the autonomic nervous system, parasympathetics via the vagus nerve, and sympathetics via the sympathetic trunk. The AV node has a dual blood supply arising from the AV branch of the dominant coronary artery and the first septal perforator of the left anterior descending. Impulses from the AV node pass to the His bundle located in the upper portion of the interventricular septum. Abnormal bypass tracks are strands of muscular tissue connecting the atrium and the ventricle, bypassing the AV node, originally described by Kent, Mahaim, and James.
The His bundle is a continuation of the AV node within the central fibrous body, which conducts electrical impulses rapidly. At the top of the muscular interventricular septum, the His bundle divides into the right and left bundles.
The right bundle courses down the septum, passes across the right ventricle within the moderator and septal bands toward the anterior papillary muscle, and extends upward to the right ventricular outflow tract. The left bundle courses down the septum and fans out into multiple conduction fibers to the papillary muscles and the rest of the left ventricle. False chordae may contain conduction fibers from the left bundle. The bundle branches have a dual blood supply arising from septal perforators of the left anterior descending and the posterior descending arteries.
Purkinje fibers are a terminal network of electrical conducting fibers, which initiate cardiac contraction in the myocytes. Muscular contraction starts in the papillary muscles and then spreads from the endocardial to epicardial segments of the apex upward to the outflow tract.
INNERVATION OF THE HEART
The heart receives parasympathetic and sympathetic afferent and efferent nerves. Preganglionic sympathetic nerves are located in upper thoracic spinal cord. Second-order neurons are found in cervical sympathetic ganglia. Postganglionic fibers terminate in the heart and great vessels. Parasympathetic fibers are located within the vagus nerves. They synapse with second-order neurons in ganglia located on the wall of the heart and great vessels. Sympathetic and parasympathetic fibers are contained within two cardiac nerve bundles. These nerve fibers course down the AV groove as the right coronary plexus and down the interventricular groove as the left coronary plexus.
SUGGESTED READINGS
Agur AM, Dalley AF. Grant’s Atlas of Anatomy. 11th ed. Philadelphia: Lippincott Williams & Wilkins; 2004.
Atlas of Human Anatomy. Springhouse, PA: Springhouse; 2001.
Chung KW. Gross Anatomy. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2004.
Gosling JA, Harris PF, Whitmore I, et al. Human Anatomy Color Atlas and Text. 4th ed. Edinborough: Mosby; 2002.
Malouf J, Edwards WD, Tajik AJ. Functional anatomy of the heart. In: Fuster V, Alexander RW, O’Rourke RA, Roberts R, King SB, Nash IS, Prystowsky EN, eds. Hurst’s The Heart. 11th ed. New York: McGraw-Hill; 2004:45–86.
Marieb EN, Mallatt J, Wilhelm PB. Human Anatomy. 4th ed. San Francisco: Benjamin Cummings; 2005.
Netter FH, Hansen JT. Atlas of Human Anatomy. 3rd ed. Carlstadt, NJ: ICON Learning Systems; 1997.
Rohen JW, Yokochi C, Lutjen-Drecoll E. Color Atlas of Anatomy: A Photographic Study of the Human Body. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2002.
Sauerland EK, Grant JCB. Grant’s Dissector. 12th ed. Philadelphia: Lippincott Williams & Wilkins; 1999.
Stranding S. Gray’s Anatomy. 39th ed. Philadelphia: Elsevier; 2004.
Zuidema GD. The Johns Hopkins Atlas of Human Functional Anatomy. 4th ed. Baltimore: The Johns Hopkins University Press; 1997.
QUESTIONS AND ANSWERS
Questions
1. Which of the following structures are not found in the right atrium?
a. Tendon of Todaro
b. Moderator band
c. Koch triangle
d. Pectinate muscle
2. Which of the following statements about the pericardium is false?
a. The pericardium contains 80 to 90 mL of pericardial fluid.
b. It is connected to the diaphragm, sternum, and pleurae.
c. It encloses the entire main pulmonary artery.
d. The ligament of Marshall is a pericardial fold that contains the remnant of the embryonic left superior vena cava.
3. Which of the following statements about coronary arteries is true?
a. The conus branch arises from a separate orifice in the aorta in 10% of patients.
b. The left main trunk trifurcates into a left anterior descending, ramus branch, and left circumflex in 75% of patients.
c. The sinoatrial (SA) nodal artery usually arises from an atrial branch of the left circumflex coronary artery.
d. A dominant right coronary artery gives rise to a posterior descending artery (PDA), an atrioventricular (AV) branch, an artery supplying the AV node, and a posterior ventricular branch.
4. Which of the following statements about false chordae is true?
a. False chordae are found in 1% of normal hearts.
b. False chordae connect the upper septum with the mitral valve leaflets.
c. False chordae often contain conducting fibers.
d. False chordae are identified only at autopsy.
5. Which of the following statements about the tricuspid valve is true?
a. It has three leaflets: anterior, lateral, and posterior.
b. It is the largest heart valve.
c. It is the most anterior heart valve.
d. The posterior leaflet is the largest and most mobile.
6. A knife wound to the anterior chest is most likely to lacerate which cardiac structure?
a. The left ventricle
b. The right ventricle
c. The left atrium
d. The right atrium
7. Which statement about cardiac valves is false?
a. Approximately 1% to 2% of aortic valves are bicuspid.
b. The aortic valve normally leaks a small amount.
c. The pulmonic valve is the most anterior heart valve.
d. Approximately 100 chordae are attached to the mitral valve.
8. Which statement about the cardiac chambers is false?
a. The right atrium is smaller than the left atrium.
b. The right ventricle is smaller than the left ventricle.
c. The right ventricle is the most anterior chamber of the heart.
d. The left ventricle is thicker at the base than at the apex.
9. Which statement about the great vessels is false?
a. The main pulmonary artery is located entirely within the pericardium.
b. The pulmonary veins contain atrial muscle.
c. The ligamentum arteriosum connects the aorta to the left pulmonary artery.
d. The inferior vena cava is smaller than the superior vena cava.
10. Which statement about the cardiac conducting system is false?
a. There are no histologically distinct conducting pathways between the SA and the AV nodes.
b. The AV node is located in the upper interventricular septum just below the tricuspid valve.
c. The first site of electrical activation in the left ventricle is in the papillary muscles.
d. False chordae may contain conduction fibers from the left bundle.
Answers
1. Answer B: The moderator band, located in the right ventricle, is a muscular bridge that connects the distal septum and the right ventricular free wall at the anterior papillary muscle. The tendon of Todaro is a fibrous band located between the valves of the inferior vena cava and the coronary sinus in the right atrium. Koch triangle is located in the lower medial portion of the right atrium, overlying the AV node and the proximal His bundle. Pectinate muscles arise from the crista terminalis and course as bands on the right atrial free wall.
2. Answer A: The pericardium contains 10 to 50 mL of pericardial fluid. It is attached to the central tendon of the diaphragm, posterior sternum, and laterally to the pleurae. It encloses the heart, portions of the vena cavae, most of the ascending aorta, the main pulmonary artery, and pulmonary veins. The ligament of Marshall is a pericardial fold containing the remnant of the embryonic left superior vena cava.
3. Answer D: A dominant right coronary artery usually is a large vessel that crosses the crux of the heart to perfuse the posterolateral aspect of the left ventricle. The conus branch arises from a separate orifice in the aorta in 50% of patients. The SA nodal artery arises from the right coronary artery in 60% of patients and from the left circumflex in 40%. The left main trunk gives rise to a ramus branch in 30% of patients.
4. Answer C: Many false chordae contain extensions of left ventricular conducting fibers. They are found in 50% of normal hearts. False chordae may connect walls, papillary muscles, and the septum, but are not attached to mitral valve leaflets. They often cross the left ventricular outflow tract and can be identified by echocardiography.
5. Answer B: The tricuspid valve is the largest heart, whereas the pulmonic valve is the most anterior. The tricuspid valve consists of three leaflets, anterior, septal, and posterior. The anterior leaflet is the largest and the most mobile. The posterior leaflet is the smallest, whereas the septal leaflet is the least mobile and occasionally absent.
6. Answer B: The right ventricle is the most anterior chamber of the heart and the most likely to be injured by trauma. A small portion of the left ventricle is located along the left cardiac border. The right atrium occupies the right border of the heart and the left atrium is a left upper posterior chamber.
7. Answer B: All of the other valves leak a small amount but any regurgitation at the aortic valve is considered pathologic.
8. Answer A: The left atrium is smaller than the right atrium but has thicker walls and higher pressure.
9. Answer D: The interior vena cava is larger than the superior vena cava. It receives blood from the lower body and from the abdominal viscera via the hepatic veins.
10. Answer B: The AV node is located inferomedially within the right atrium, beneath Koch triangle, above the septal leaflet of the tricuspid valve, near the orifice of the coronary sinus.