|Year : 2020 | Volume
| Issue : 3 | Page : 148-154
Cardiac anatomy: The essentials
Siew Yen Ho
Cardiac Morphology, Royal Brompton Hospital; National Heart & Lung Institute, Imperial College, London, UK
|Date of Submission||10-Aug-2020|
|Date of Acceptance||02-Sep-2020|
|Date of Web Publication||30-Sep-2020|
Siew Yen Ho
Royal Brompton and Harefield NHS Foundation Trust, Sydney Street, London SW3 6NP
Source of Support: None, Conflict of Interest: None
In presenting the essentials of cardiac anatomy for healthcare workers, this article aims to demystify the arrangement of cardiac chambers and valves one to the other. In so doing, it provides better understanding of imaging previously seen in 2 dimensions and in 3 dimensions in the current era. Fundamental to this is the central location of the aorta in relation to all four cardiac chambers. Furthermore, recognition of cardiac chambers is based on distinguishing morphological components irrespective of their location in the heart. Morphological criteria are especially useful in understanding of congenitally malformed hearts.
Keywords: Aortic valve; Atrial appendage; Atrial septum; Pericardium
|How to cite this article:|
Ho SY. Cardiac anatomy: The essentials. Cardiol Plus 2020;5:148-54
| Introduction|| |
The heart remains anatomically enigmatic to many in part due to the portrayal of an almond shape standing on its apex, with a straight septum down the middle and its four chambers arranged in two-up and two-down fashion. Although in early embryonic life the interventricular septum and interatrial septum approximate to the median sagittal plane, allowing the designation of left and right cardiac chambers, the definitive arrangement is more convoluted. A simple transverse section through the ventricular mass, or short axis plane, exemplifies the contorted anatomy. The ventricular septum will be seen as a curved structure separating a slim right ventricle (RV) to one side and a circular left ventricle (LV) to the other. Indeed, in his outstanding monograph, McAlpine  stressed the importance of the attitudinal approach that underscores understanding of cardiac anatomy in the clinical setting.
In this era of widespread use of imaging tools producing cross-sectional imaging techniques such as multislice computed tomography, magnetic resonance imaging, and most commonly echocardiography, together with three-dimensional reconstructions, has allowed a better understanding of the true spatial relationships of the chambers and valves. Indeed, multimodality imaging has enabled some of the more subtle details of heart structures to be viewed in the living patient. Knowledge of the intrinsic morphology of each chamber is as relevant as their relative dispositions. As alluded to the above, right-heart chambers are not strictly to the right nor are left-heart chambers strictly to the left; the terms “right” or “left” are used only in the context of morphology, that is, morphologically right or left based on structural features and without presumption of their location. This convention is most important in describing congenitally malformed hearts. In this article, the first of a series on cardiac anatomy, the relative locations of cardiac chambers one to another, the distinctive features of each chamber, and structure of the cardiac septum are presented to provide some essentials for any health-care worker in cardiology.
| Position of the Heart|| |
The cardiac silhouette is generally taken to be trapezoidal in shape. The rib cage provides good markers for charting the cardiac silhouette. The normal position of the cardiac apex is generally taken to be in the fifth intercostal space in the mid-clavicular line. The lower border is a nearly horizontal line in the area of the left sixth rib to the right sixth costal cartilage. The upper border is hidden behind the sternum at the level of the second and third cartilages. The right margin of the heart peeps out behind the right border of the sternum between the right third and sixth cartilages [Figure 1]a. In infants, the upper part of the cardiac shadow is broad, owing to the prominence of the overlying thymus gland.
|Figure 1: (a) Anterior perspective of the heart overlapped by the sternum, ribcage, and lungs. (b) Anterior view of an endocast prepared from a normal heart to show the spatial relationship between right-heart (blue) and left-heart (red) chambers. The aortic valve (yellow dotted line) is centrally located in the heart and tilts at an angle inferior to the pulmonary valve (white dotted line). The plane of the tricuspid valve is indicated by the blue line. LAA: Left atrial appendage, LV: Left ventricle, PT: Pulmonary trunk, RA: Right atrium, RV: Right ventricle|
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Inferior to the thymus, a fibrous pericardial sac encloses the mass of the heart. The sac has cuff-like attachments around the adventitia of the great arteries and veins as they enter or emerge from the heart. The pericardial cavity is contained between a double-layered serous pericardium: the parietal layer is adherent to the fibrous pericardium, whereas the visceral layer adheres to the cardiac surface, forming the epicardium. Due to the contours of the heart and great arteries, there exist two recesses within the pericardial cavity. These are the transverse and oblique sinuses.
The transverse sinus lies between the posterior surface of the great arteries and the anterior surface of the atrial chambers. The reflection of the serous pericardium around the pulmonary veins (PVs) and the inferior caval vein (ICV) forms the oblique sinus. When the fibrous pericardium is removed, the major part of the heart visible from the front is the ventricular mass because the atria are relatively posterior. The morphologically RV occupies the greater part [Figure 1]b. The LV appears only as a narrow slip along the left cardiac border. Pyramidal in shape, the cardiac apex points downward, forward, and to the left, whereas the base faces posteriorly and to the right. While the cardiac apex is usually represented by the vortex of the LV, the cardiac base is less well defined owing to differences in definition. The anatomical base is formed mainly by the left atrium receiving the PVs and to a small extent by the posterior part of the right atrium (RA). The base in clinical practice, however, refers to the portion of the heart near the parasternal parts of the second intercostal spaces. The cardiac long axis, therefore, lies in a line drawn from the left hypochondrium toward the right shoulder. This orientation deviates considerably from the long axis of the body. Furthermore, the position of the cardiac septum at about 45° to the median brings right-heart structures anterior to the left-heart structures. The ventricles are situated inferior and leftward relative to their corresponding atria [Figure 1]b. This results in the right atrioventricular junction being in a nearly vertical plane. The RA is in right anterior position relative to the left atrium. Being the most posteriorly situated cardiac chamber, the left atrium is directly anterior to the esophagus at the bifurcation of the trachea. Only its appendage is visible when viewing the heart from the front [Figure 1]b.
The aorta has a deep-seated origin and only becomes part of the cardiac silhouette as it arches upward and backward, forming a spiral with the pulmonary trunk (PT). The cardiac valves are offset from one another, in keeping with the disposition of the cardiac chambers and great arteries. When viewed in frontal projection, the pulmonary valve, being the most superior valve, is nearly horizontally situated behind the third costal cartilage. The aortic valve lies posterior and to the right, above the nearly vertically orientated tricuspid valve [Figure 1]b. The mitral valve is further posterior, overlapped by the more anterior but inferior tricuspid valve. The aortic valve therefore occupies a central position in the heart, wedged between the two atrioventricular valves and surrounded by the four cardiac chambers.
The cardiac surfaces are described as the sternocostal, diaphragmatic, left, and right. The sternocostal surface is covered anteriorly by the sternum and pleurae. The diaphragmatic surface is horizontally orientated. The sharp angle formed mainly by the RV and occupying the lower heart border is the acute margin of the heart [Figure 2]. The rounded obtuse margin of the heart is formed mainly by the LV to the left of the sternocostal surface.
|Figure 2: (a) Anterior (sternocostal) surface and (b) inferior (diaphragmatic) surface of the heart with the acute and obtuse (left) margins. Arrows indicate the level of the atrioventricular junction. ICV: Inferior caval vein. Other abbreviations are as in Figure 1|
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| The Morphologically Right Atrium|| |
The RA is dominated by its distinctively large triangular-shaped appendage that forms much of the anterior and lateral walls [Figure 3]a. The venous sinus receiving the superior and ICVs and coronary sinus (CS) lies posterior to the appendage. The junction between the two parts is marked on the epicardial aspect by an atrial groove, termed the terminal groove, which, on the endocardial aspect, is represented by a muscle bundle, named the terminal crest, from which pectinate muscles radiate to line the wall of the appendage [Figure 3]b. In between the muscle bundles, the appendage wall is exceptionally thin. The rough wall of the appendage is in stark contrast to the smooth wall of the venous sinus. The wall of the atrial vestibule leading to the orifice of the tricuspid valve is also smooth. The septal aspect is mainly smooth walled and bears an oval-shaped depression, marking the site of the oval fossa which comprises a thin valve that acts like a flap separating the atrial chambers.
|Figure 3: (a) Anterior view of the RA is dominated by its broad triangular-shaped appendage (RAA), with its tip pointing toward the aorta. (b) A right lateral view into the RA after making a cut into the RAA free wall and deflecting it posteriorly. The terminal crest passes anterior to the orifice of the superior caval vein and gives rise to the sagittal bundle and an array of pectinate muscles. The oval fossa is a depression surrounded by a muscular rim. (c) Left lateral view showing the finger-like left atrial appendage and its narrow junction (between arrows) with the left atrium. (d) Double-headed arrow indicates the left atrial appendage orifice. Pectinate muscles line the inside of the left atrial appendage, leaving the endocardial surface of the left atrium relatively smooth. A crescent marks (blue arrow) the site of postnatal closure of the oval foramen. CS: Coronary sinus, LPV: Common left pulmonary vein, RA: Right atrium, RAA: Right atrial appendage, PV: Pulmonary vein, RIPV and RSPV: Right inferior and superior PVs, respectively. Other abbreviations are as in previous figures|
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| The Morphologically Left Atrium|| |
The left atrium has a venous component situated posteriorly, a body, and a finger-like appendage that projects anteriorly from the left. It is distinctly smaller than its right counterpart and is usually hook shaped with a crenelated external appearance [Figure 3]c. It has a narrow junction, or orifice, with the atrial body that is not marked by any muscle bundle comparable to the terminal crest. Pectinate muscles mainly line the lumen of the appendage, leaving the remaining walls that form the larger part of the atrium with a smooth endocardial surface. Often, the thin flap valve on the septal aspect is slightly less smooth and a crescentic crevice marks the site of any persistent patency of the oval fossa (PFO) [Figure 3]d. The venous portion is anchored by the PVs which drain directly into its posterior part. Like in the RA, the left atrial vestibule leads to the mitral orifice. The CS runs inferiorly on the epicardial aspect of the vestibule to open into the RA.
| The Morphologically Right Ventricle|| |
Description of the ventricular chambers is facilitated by considering them in terms of three components – inlet, apical trabecular, and outlet – although there are no lines demarcating each. The inlet contains the atrioventricular valve and its tension apparatus; the outlet part leads to and supports the arterial valve. The apical portion is lined with bundles of muscle termed trabeculations that criss-cross each other. The relative thickness of the trabeculations allows the ventricles to be distinguished morphologically. The bundles are distinctly fine in the LV and coarse in the RV. A small fibrous area, the membranous septum, is located at this tripartite junction (see below – the cardiac septum).
Owing to the curvature of the ventricular septum, the RV has a “wrap-around” relationship with the LV. When viewed in frontal projection, the RV sweeps from the right to lie leftward and anterior to the LV [Figure 1]b.
In the RV, there is a wide muscular separation between its inlet valve and outlet valve. Viewing the heart from the right lateral perspective, the aorta can be seen passing between these two right-heart valves [Figure 4]a, whereas the left lateral view reveals the RV outlet passing antero-cephalad to the LV outlet [Figure 4]b.
|Figure 4: (a) Right lateral and (b) left lateral perspectives of an endocast showing the spatial relationships of the chambers, great arteries, and cross-over arrangement between the RV and LV outflow tracts. LV: Left ventricle, RV: Right ventricle, LI: Left inferior, LS: Left superior, RI: Right inferior, RS: Right superior, PV: Pulmonary vein|
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On the septal aspect, adherent to the septum, the RV bears a prominent Y-shaped muscle band, the septomarginal trabeculation. This muscle band clasps the ventriculo-infundibular fold between its arms to form the supraventricular crest, a structure that distances the tricuspid valve from the pulmonary valve [Figure 5]a. On its epicardial side, the fold contains fatty tissue bearing the proximal course of the right coronary artery. The body of the septomarginaI trabeculations branches into smaller muscle bundles as it approaches the apex to contribute to the characteristic coarse trabeculations. One of these is distinctive of the RV. Named the moderator band, it connects the septomarginal trabeculation to the ventricular free wall. Another distinctive feature of the RV is the septal leaflet of the tricuspid valve that has direct chordal attachments to the septum. The tricuspid valve is supported by small papillary muscles arising from the posterior arm of the septomarginal trabeculation, a large anterior papillary muscle, and several smaller posterior papillary muscles. The outlet portion is shaped like a muscular tube leading to the pulmonary valve.
|Figure 5: (a) RV opened displaying the septal aspect. The ventriculo-infundibular fold inserts between the arms of the septomarginal trabeculation (blue dashed outline) to form the supraventricular crest, giving muscular separation between tricuspid and pulmonary valves. A prominent muscle bundle, the moderator band, crossing the cavity is distinctive of the RV. (b) The LV opened to show the aortic and mitral valves not separated by muscle. The criss-crossing muscle bundles (trabeculations) in the apical region are distinctly finer than those in theRV. Big arrows point to the thin muscle at the apex of both ventricles. LV: Left ventricle, RV: Right ventricle|
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| The Morphologically Left Ventricle|| |
Very little of the LV is visible from the front of the heart [Figure 1]b, although in the infant a relatively greater portion may be seen. The inlet component surrounds and contains the mitral valve and its tension apparatus. Owing to its nearly conical shape, the LV has a sharp angle between its inlet and outlet portions. Consequently, the outflow tract is sandwiched between the septum and the anterior mitral leaflet which is suspended like a curtain between the inflow and outflow tracts [Figure 5]b. In stark contrast to the tricuspid valve in the RV, the mitral valve does not have attachments to the septum. It is supported by two distinct groups of papillary muscles. The deeply wedged position of the aortic valve puts it immediately adjacent and in fibrous continuity with the mitral valve. There is no structure comparable to the supraventricular crest in the LV although very rarely a narrow strip of muscle may be seen between the left-heart valves. The upper part of the septal surface is smooth. The outlet component supports the aortic valve. Unlike the pulmonary outlet, it is an incomplete muscular tube. The semilunar leaflets are attached within the expanded aortic sinuses. The sinuses are not strictly in right and left positions although they are so designated in consideration of the origin of the coronary arteries. The central location of the aorta places it in close relationship to each of the cardiac chambers and other valves [Figure 1]b.
| The Cardiac Septum|| |
The locations of the cardiac chambers and valves relate to the complex form and structure of the cardiac septum. This section highlights some of the features of the atrial septum, the ventricular septum, the membranous septum, and the so-called muscular atrioventricular septum, which are important to note when carrying out intracardiac procedures.
The atrial septum is obliquely orientated extending from the right posterior to left anterior position. When viewed from the RA aspect, the atrial septum is characterized by a muscular rim – the limbus – which surrounds the thin flap valve of the oval fossa [Figure 3]b. In fetal life, the flap valve of the oval fossa allows blood mostly from the ICV to enter the left atrium. After birth the valve is normally large enough to close the interatrial communication as long as left atrial pressure remains higher than the right. Persistent patency of the oval foramen (PFO) allows passage from the RA to the left atrium through an unsealed antero-superior part that exists in about a quarter of the normal population.
The atrial septum, however, is not as extensive as it appears. Its true extent is limited to the flap valve and its immediate surrounding rim which is an infolding of the atrial wall forming the interatrial groove that is filled with variable amounts of epicardial fat [Figure 6]a., For transseptal puncture, the safest site is within the flap valve. If puncture is made in the rim too anteriorly, the needle may pass through the anterior atrial wall, which is directly related to the transverse pericardial sinus and the aortic root [Figure 6]b.
|Figure 6: (a) This cut through the four heart chambers displaying the atrial septum in profile reveals the thin valve of the oval fossa (open arrow). The muscular rim around the fossa is a muscular fold filled with epicardial fat tissue (*). (b) This view is into the RA and RV from the right anterior perspective. This cut through the RA free wall and the ventriculo-infundibular fold reveals the adjacency of the non- and right coronary (N and R) aortic sinuses to the atrial wall (blue arrows) forming the anterior margin of the oval fossa. Occasionally, small depressions (green triangle) are found in this region. L: Left coronary aortic sinus, RV: Right ventricle, RA: Right atrium|
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The antero-inferior rim comprises of an atrial wall overlying epicardial fat and muscular ventricular septum that interposes between the RA and LV owing to the more apical attachment of the tricuspid valve [Figure 7]a. Previously termed the muscular atrioventricular septum, its sandwich-like configuration should be recognized. The so-called “septum” comprises of an atrial wall separated from a ventricular wall by epicardial fatty tissues. Transgression through it can potentially lead outside the heart.
|Figure 7: (a) This close up of a four-chamber cut shows the composition of the atrioventricular junction at the septum. Owing to the different levels of the hingelines of the tricuspid and mitral valves (blue broken lines), the RA wall overlaps epicardial fat (**) that lie on the crest of the ventricular septum. (b) The septal aspect of the right heart is displayed. The red line marks the plane of cut used to produce (a). The hingeline (broken line) of the tricuspid valve crosses the membranous septum, dividing it into an atrioventricular portion and an interventricular portion. RA: Right atrium|
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At the cephalad margin of this “septum” is a knob of dense fibrous tissue, the central fibrous, body that is the junction between the tricuspid, mitral, and aortic valves. Adjoining the fibrous body is a thin fibrous septum, the membranous septum, which is the smallest and thinnest part of the cardiac septum [Figure 7]b. The hingeline of the septal leaflet of the tricuspid valve divides it into two portions: an atrioventricular portion separating the RA from the LV and an interventricular portion.
As discussed previously, the muscular ventricular septum is curved. While the septum can be described as having an inlet component and an apical component separating the ventricles, there is no outlet component. The tube-shaped RV outlet elevates the pulmonary valve above the level of the septum and cephalad to the aortic valve [Figure 1]b. This arrangement puts the right and posterior walls of the muscular pulmonary outlet immediately anterior to the aortic sinuses, giving origin to the coronary arteries [Figure 6]b.
| The Aorta|| |
The ascending aorta arises in the right posterior position relative to the PT [Figure 1]b. It ascends superiorly, obliquely to the right, and slightly anterior toward the sternum. On the right is the medial wall of the RA. Anteriorly are the RA appendage, the RV outflow tract, and the PT. The transverse pericardial sinus separates the back of the aorta from the left atrium and right pulmonary artery. The arch of the aorta begins just above the cuff of pericardial reflection, proximal to the origin of the brachiocephalic artery. The arch rises superiorly for a short distance before passing posteriorly to the left and finally terminating on the lateral aspect of the vertebral column. Along its course, the arch gives origin to the neck and arm arteries. The arterial duct connects the left pulmonary artery to the aorta just distal to the origin of the left subclavian artery. In adults, the duct is represented by a fibrous ligament.
| The Pulmonary Arteries|| |
The PT (pulmonary trunk/common pulmonary artery) is also covered with a cuff of serous pericardium at its origin. It arises from the anterior aspect of the heart, just behind the left lateral edge of the sternum. It swings diagonally to the left side of the ascending aorta [Figure 1]b. Being a short vessel, it soon bifurcates into the left and right pulmonary arteries. The left pulmonary artery passes in front of the descending aorta and superior to the left main bronchus before branching in the lung hilum. The longer right pulmonary artery traverses the mediastinum under the aortic arch before passing behind the superior caval vein to reach the right lung hilum.
| The Coronary Circulation|| |
As mentioned previously, the left and right coronary arteries emerge from the left and right coronary aortic sinuses, respectively. Usually, the arteries arise from within the sinus just beneath the aortic bar that marks the junction between the sinus and tubular portions. In the left sinus, there is usually a single orifice, but in the right, it is usual in older individuals to find two or more orifices where the early branches of the right coronary artery take direct origin. The main coronary arteries run in the atrioventricular and interventricular grooves. The right coronary artery runs in the right atrioventricular groove, giving rise to atrial and ventricular branches along the way to the cardiac crux. The left coronary has a short main stem that branches soon after its origin into the anterior descending and circumflex arteries. The circumflex runs in the left atrioventricular groove, giving branches to supply the atrial and ventricular walls, whereas the anterior descending runs in the anterior interventricular groove and provides branches to the ventricles and the septum.
In the majority of hearts, there is “right dominance,” with the posterior descending artery in the posterior interventricular groove being a branch or the continuation of the right coronary artery. In a little under 10% of hearts, the posterior descending artery is a branch of the circumflex, giving “left dominance.” A “balanced” circulation is seen when both right and left coronary arteries give rise to parallel posterior descending branches. The artery to the atrioventricular node of the conduction system arises from the dominant artery at the cardiac crux.
After passing through the capillary network, coronary arterial blood is collected by venules which drain to the cardiac veins. The major veins drain to the CS and smaller veins drain directly to the cardiac chambers. The great cardiac vein ascends along the anterior descending coronary artery and turns into the left atrioventricular groove. In the posterior atrioventricular groove, it becomes the CS. It is joined near its entrance to the RA by the middle cardiac vein which ascends in the posterior interventricular groove, and the small cardiac vein. The latter ascends along the acute marginal coronary artery before entering the posterior atrioventricular groove. Atrial veins also empty into the CS. A further group of veins, the anterior cardiac veins, run across the anterior aspect of the heart to drain directly into the RA.
In addition to the coronary arteries and veins, the heart also has an extensive lymphatic network. These are divided into the deep, middle, and superficial plexuses, which drain into the collecting channels accompanying the major arterial stems and finally into primary lymph nodes situated in the anterior mediastinum.
| Cardiac Nerves|| |
Cardiac innervation includes both extrinsic innervation and influence from the intrinsic cardiac nervous system. The intrinsic cardiac nervous system is largely an atrial network of neural ganglia. Generally, the innervation of the atria is predominantly parasympathetic, whereas the innervation of the ventricles is predominantly sympathetic. Between six and ten ganglionated plexuses have been described in the human heart. Approximately half are located on the atria and the other half on the ventricles. These are generally associated with islands of adipose tissue referred to as fat pads.
| Conclusions|| |
The convoluted arrangement between the right and left hearts makes it challenging to understand and interpret spatial relationships of the various cardiac chambers and their valves. Imaging tools with the facility to reconstruct the heart and its surrounding structures in three dimensions are helpful especially for the beginner.
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Conflicts of interest
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]