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Anatomy of the Heart: Difference between revisions
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==The morphologically right atrium== | ==The morphologically right atrium== | ||
[[Image: | [[Image:Figure 5.jpg|thumb|center|'''A.''' This right lateral view shows the right atrium dominated by its large, triangluar shaped appendage. The dots mark the terminal groove. The arrow indicates the crest of the appendage. | ||
<br>'''B.''' The lateral wall of the appendage incised and flipped backward to show the pectinate muscles and the thin, membrane-like atrial wall between the muscle bundles. The terminal crest (dots) marks the border between the pectinated appendage and the smooth-walled venous sinus. The oval fossa is surrounded by its muscular rim. The smooth-walled vestibule leads to the tricuspid valve orifice.]] | <br>'''B.''' The lateral wall of the appendage incised and flipped backward to show the pectinate muscles and the thin, membrane-like atrial wall between the muscle bundles. The terminal crest (dots) marks the border between the pectinated appendage and the smooth-walled venous sinus. The oval fossa is surrounded by its muscular rim. The smooth-walled vestibule leads to the tricuspid valve orifice.]] | ||
The right atrium is composed of an anterior appendage, a posterior venous sinus, a septal portion and a vestibule. The junction between the appendage and the venous sinus is marked epicardially by an atrial groove the terminal groove, in which lies the sinus node. Inside the chamber, the terminal groove is represented by a muscle bundle, the terminal crest (crista terminalis), from which pectinate muscles radiate into the appendage (Figure 5). The appendage has a characteristic triangular shape and a wide communication with the venous sinus. The smooth-walled venous sinus receives the superior and inferior caval veins in its cephalic and caudal extremities respectively. The coronary sinus opens close to the septal portion and near the opening of the inferior caval vein. The outlet portion of the atrium, the vestibule leading to the tricuspid valve orifice, is also smooth walled. The obliquely orientated atrial septum extends from right posterior to left anterior position. When viewed from the right atrial aspect, the atrial septum is characterised by a muscular rim – the limbus - which surrounds the flap valve of the oval fossa (Figure 5). The extent of the true septum, however, is limited to the flap valve and the immediate part of its surrounding muscular rim. On the epicardial side much of the rim is filled by the interatrial groove which separates the right atrium from the right pulmonary veins posteriorly and superiorly. In its anterior part, the infolded rim contains the continuation of the interatrial groove and its musculature extends to the anterior wall of the right atrium, directly related to the transverse pericardial sinus. Only a small portion of the inferior rim is part of the true atrial septum. Its major portion is the continuation of the right atrial wall, the vestibule, overlying the crest of the ventricular septum (Figure 5). In fetal life, the flap valve of the oval fossa allows venous return mostly from the inferior caval vein to enter the left atrium. After birth the valve is normally large enough to close the interatrial communication as higher left atrial pressure pushes the valve against the muscular rim forming a complete seal. A probe patency (a probe could be passed from right to left atrium through an unsealed antero-superior part of the rim) exists in about a quarter of the normal population and is generally referred to as a PFO. | The right atrium is composed of an anterior appendage, a posterior venous sinus, a septal portion and a vestibule. The junction between the appendage and the venous sinus is marked epicardially by an atrial groove the terminal groove, in which lies the sinus node. Inside the chamber, the terminal groove is represented by a muscle bundle, the terminal crest (crista terminalis), from which pectinate muscles radiate into the appendage (Figure 5). The appendage has a characteristic triangular shape and a wide communication with the venous sinus. The smooth-walled venous sinus receives the superior and inferior caval veins in its cephalic and caudal extremities respectively. The coronary sinus opens close to the septal portion and near the opening of the inferior caval vein. The outlet portion of the atrium, the vestibule leading to the tricuspid valve orifice, is also smooth walled. The obliquely orientated atrial septum extends from right posterior to left anterior position. When viewed from the right atrial aspect, the atrial septum is characterised by a muscular rim – the limbus - which surrounds the flap valve of the oval fossa (Figure 5). The extent of the true septum, however, is limited to the flap valve and the immediate part of its surrounding muscular rim. On the epicardial side much of the rim is filled by the interatrial groove which separates the right atrium from the right pulmonary veins posteriorly and superiorly. In its anterior part, the infolded rim contains the continuation of the interatrial groove and its musculature extends to the anterior wall of the right atrium, directly related to the transverse pericardial sinus. Only a small portion of the inferior rim is part of the true atrial septum. Its major portion is the continuation of the right atrial wall, the vestibule, overlying the crest of the ventricular septum (Figure 5). In fetal life, the flap valve of the oval fossa allows venous return mostly from the inferior caval vein to enter the left atrium. After birth the valve is normally large enough to close the interatrial communication as higher left atrial pressure pushes the valve against the muscular rim forming a complete seal. A probe patency (a probe could be passed from right to left atrium through an unsealed antero-superior part of the rim) exists in about a quarter of the normal population and is generally referred to as a PFO. | ||
==The morphologically left atrium== | ==The morphologically left atrium== | ||
[[Image: | [[Image:Figure 6.jpg|thumb|center|'''A.''' This view from the left-lateral aspect shows the finger-like left atrial appendage with the left atrium situated posteriorly. The left ventricle tapers to a rounded apex. | ||
<br>'''B.''' This section through the aortic root and mitral valve displays the left atrial aspect of the septum enface. The crescentic edge (arrow) of the fossa valve has not sealed completely resulting in a PFO. The asterisk marks the location of the transverse pericardial sinus.]] | <br>'''B.''' This section through the aortic root and mitral valve displays the left atrial aspect of the septum enface. The crescentic edge (arrow) of the fossa valve has not sealed completely resulting in a PFO. The asterisk marks the location of the transverse pericardial sinus.]] | ||
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==The morphologically right ventricle== | ==The morphologically right ventricle== | ||
[[Image: | [[Image:Figure 7.jpg|thumb|right|'''A.''' The right ventricle is opened to show the septum and the muscular crest separating tricuspid from pulmonary valves. The moderator band (open arrow) extends from the foot of the septomarginal trabeculation to the free wall of the right ventricle. Coarse trabeculations fill the apical component. | ||
<br>'''B.''' This close-up view of the tricuspid valve at the commissure between septal and antero-septal leaflets shows the annulus (broken line) crossing the membranous septum (dots) dividing it into atrioventricular(av) and interventricular(iv) components.]] | <br>'''B.''' This close-up view of the tricuspid valve at the commissure between septal and antero-septal leaflets shows the annulus (broken line) crossing the membranous septum (dots) dividing it into atrioventricular(av) and interventricular(iv) components.]] | ||
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==The morphologically left ventricle== | ==The morphologically left ventricle== | ||
[[Image: | [[Image:Figure 8.jpg|thumb|right|'''A.''' The left ventricle is opened through its outflow tract into the aortic valve. The aortic valve leaflets are in fibrous continuity with the anterior leaflet of the mitral valve. The fibrous continuity is expanded at the right and left fibrous trigones. The right trigone(asterisk) is the landmark for the atrioventricular conduction bundle. Note how the thickness of the left ventricular wall diminishes remarkably at the apex (open arrow). | ||
<br>'''B.''' This dissection shows the central location of the aortic valve. L, N and R are the left-coronary, non-coronary and right-coronary aortic sinuses respectively.]] | <br>'''B.''' This dissection shows the central location of the aortic valve. L, N and R are the left-coronary, non-coronary and right-coronary aortic sinuses respectively.]] | ||
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==The coronary circulation== | ==The coronary circulation== | ||
[[Image: | [[Image:Figure 9.jpg|thumb|right|Diagram showing the right (RCA) and left (LCA) coronary arteries and their main ventricular branches. The left anterior descending (LAD) and posterior descending (PDA) coronary arteries mark the anterior and posterior margins of the ventricular septum.]] | ||
As mentioned previously, the left and right coronary arteries emerge from the left and right coronary sinuses respectively. Usually the arteries arise from within the sinus just beneath or at the level of the aortic bar (sinutubular junction). In the left sinus there is usually a single orifice but in the right sinus it is usual to find multiple orifices where the early branches of the right coronary artery take direct origin. The main coronary arteries pass within the fatty tissues of the atrioventricular and interventricular grooves. The left coronary has a short main stem that branches into the anterior descending and circumflex arteries (Figure 9). The circumflex runs in the left atrioventricular groove and the right coronary artery runs in the right atrioventricular groove to variable lengths. From the atrioventricular groove, the encircling arteries give origin to ventricular and atrial branches. An early atrial branch is the sinus node artery which arises slightly more frequently from the right than the left coronary artery. It usually ascends the interatrial musculature to reach the terminal groove but recent evidence has shown a more variable course. In the majority of hearts the posterior descending artery, which runs in the posterior interventricular groove, is a branch from the right coronary artery and this is termed 'right dominance'. In a little under 10% of hearts the posterior descending 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 arises from the dominant artery at the cardiac crux. | As mentioned previously, the left and right coronary arteries emerge from the left and right coronary sinuses respectively. Usually the arteries arise from within the sinus just beneath or at the level of the aortic bar (sinutubular junction). In the left sinus there is usually a single orifice but in the right sinus it is usual to find multiple orifices where the early branches of the right coronary artery take direct origin. The main coronary arteries pass within the fatty tissues of the atrioventricular and interventricular grooves. The left coronary has a short main stem that branches into the anterior descending and circumflex arteries (Figure 9). The circumflex runs in the left atrioventricular groove and the right coronary artery runs in the right atrioventricular groove to variable lengths. From the atrioventricular groove, the encircling arteries give origin to ventricular and atrial branches. An early atrial branch is the sinus node artery which arises slightly more frequently from the right than the left coronary artery. It usually ascends the interatrial musculature to reach the terminal groove but recent evidence has shown a more variable course. In the majority of hearts the posterior descending artery, which runs in the posterior interventricular groove, is a branch from the right coronary artery and this is termed 'right dominance'. In a little under 10% of hearts the posterior descending 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 arises from the dominant artery at the cardiac crux. | ||
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==The cardiac conduction system== | ==The cardiac conduction system== | ||
[[Image: | [[Image:Figure 10.jpg|thumb|center|The cardiac conduction system. Normally, the insulating fibro-fatty tissue plane at the atrioventricular junction prevents atrial myocardium from contacting ventricular myocardium. The penetrating bundle is the only muscular bridge.]] | ||
The full complement of the histologically specialised tissues making the conduction system of the heart comprises the sinus node and the atrioventricular system (Figure 10). The latter is made up of the atrioventricular node, the penetrating atrioventricular bundle and the ventricular bundle branches. The geometry of the right atrium is such that it is made up of bands of muscle which separate the orifices of the great veins and the oval fossa. The spread of excitation from the sinus to the atrioventricular node has been shown to spread preferentially along these broad bands of ordinary atrial myocardium. | The full complement of the histologically specialised tissues making the conduction system of the heart comprises the sinus node and the atrioventricular system (Figure 10). The latter is made up of the atrioventricular node, the penetrating atrioventricular bundle and the ventricular bundle branches. The geometry of the right atrium is such that it is made up of bands of muscle which separate the orifices of the great veins and the oval fossa. The spread of excitation from the sinus to the atrioventricular node has been shown to spread preferentially along these broad bands of ordinary atrial myocardium. | ||
==The sinus node== | ==The sinus node== | ||
[[Image: | [[Image:Figure 11.jpg|thumb|right|'''A.''' The sinus node (dotted shape) is superimposed onto the terminal groove in this picture of the right atrium viewed from the right side. The arrows indicate the sectioning plane of the histological section shown in B. | ||
<br>'''B.''' This section from an infant heart is stained in Masson’s trichrome stain that colours myocardium red and fibrous tissue blue. The sinus node is readily identifiable by its composition of small myocytes in a fibrous matrix.]] | <br>'''B.''' This section from an infant heart is stained in Masson’s trichrome stain that colours myocardium red and fibrous tissue blue. The sinus node is readily identifiable by its composition of small myocytes in a fibrous matrix.]] | ||
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==The atrioventricular conduction system== | ==The atrioventricular conduction system== | ||
Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.<cite>HisW</cite> Tawara's monograph<cite>Tawara</cite> accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.<cite>Purkinje</cite> This firmly estabIished the presence of an atrioventricular conduction system (Figure 10) and was subsequently confirmed by Keith and Flack in the same year.<cite>Flack</cite> Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications. [[Image: | Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.<cite>HisW</cite> Tawara's monograph<cite>Tawara</cite> accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.<cite>Purkinje</cite> This firmly estabIished the presence of an atrioventricular conduction system (Figure 10) and was subsequently confirmed by Keith and Flack in the same year.<cite>Flack</cite> Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications. [[Image:Figure 12.jpg|thumb|left|'''A.''' This view of the right atrium and right ventricle shows the anterior and posterior borders of the triangle of Koch (broken lines) that mark location of the atrioventricular node and bundle (orange shapes). The arrows B, C, D indicate the cuts made through the conduction system as shown on the histologic sections. | ||
<br>'''B''', '''C''' and '''D''' are step sections stained with Masson’s trichrome technique and displayed in similar orientation tracing the atrioventicular conduction system from the AV node (AVN) that adjoins the central fibrous body (cfb), to the penetrating His bundle (H), and the branching bundle (BB) dividing into the left (LBB) and right (RBB) bundle branches.]] The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 12). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.<cite>Koch</cite> The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 12D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 12C). [[Image: | <br>'''B''', '''C''' and '''D''' are step sections stained with Masson’s trichrome technique and displayed in similar orientation tracing the atrioventicular conduction system from the AV node (AVN) that adjoins the central fibrous body (cfb), to the penetrating His bundle (H), and the branching bundle (BB) dividing into the left (LBB) and right (RBB) bundle branches.]] The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 12). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.<cite>Koch</cite> The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 12D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 12C). [[Image:Figure 13.jpg|thumb|right|This picture from Tawara’s monograph (1906) shows the tree-fascicular arrangement of the left bundle branch in man. | ||
(Tawara S 1906 Das Reizleitungssystem des Säugetierherzens. Eine Anatomisch-Histologische Studie Über das Atrioventrikularbündel und die Purkinjeschen Fäden. Gustav Fischer, Jena.)]] The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 12B). | (Tawara S 1906 Das Reizleitungssystem des Säugetierherzens. Eine Anatomisch-Histologische Studie Über das Atrioventrikularbündel und die Purkinjeschen Fäden. Gustav Fischer, Jena.)]] The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 12B). | ||
The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 13).<cite>Tawara</cite> Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.<cite>Rosenbaum</cite> | The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 13).<cite>Tawara</cite> Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.<cite>Rosenbaum</cite> | ||