Anatomy of the Heart: Difference between revisions

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{{Information
{{Information
|Description= The long axis of the heart is at an angle to the long axis of the body. Approximately a third of the heart is to the right of the midline of the sternum and the remainder is to the left of the midline.
|Description= The long axis of the heart is at an angle to the long axis of the body. Approximately a third of the heart is to the right of the midline of the sternum and the remainder is to the left of the midline.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]
[[Image:Figure 3.jpg|300px|right|thumb|Figure 3.<br>
== {{int:filedesc}} ==
{{Information
|Description= '''A.''' Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
<br>'''B.''' The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Date= 2012
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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 (Figure 2). 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. In the infant, the upper part of the cardiac shadow is broad owing to the prominence of the overlying thymus gland.  
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 (Figure 2). 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. In the infant, the upper part of the cardiac shadow is broad owing to the prominence of the overlying thymus gland.  


[[Image:Figure 3.jpg|thumb|300px|center|'''A.''' Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
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 the double-layered serous pericardium. The parietal pericardium is adherent to the fibrous pericardium while the visceral layer is densely adherent 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 occupies the inner heart curvature and 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 four pulmonary veins and the inferior caval vein forms the oblique sinus.  
<br>'''B.''' The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.]]


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 the double-layered serous pericardium. The parietal pericardium is adherent to the fibrous pericardium while the visceral layer is densely adherent 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 occupies the inner heart curvature and 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 four pulmonary veins and the inferior caval vein forms the oblique sinus.  
When the pericardium is removed, the major part of the heart visible from the front is the ventricular mass. Here, the morphologically right ventricle occupies the greater part (Figure 3). The left ventricle appears only as a narrow slip along the left cardiac border. The shape of the heart is generally likened to a pyramid. The apex points downwards, forwards and to the left while the base faces posteriorly and to the right. While the cardiac apex is usually represented by the vortex of the left ventricle, the cardiac base is less well defined owing to differences in definition. The anatomical base is formed mainly by the left atrium receiving the pulmonary veins and to a small extent by the posterior part of the right atrium. 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 towards the right shoulder. This orientation deviates considerably from the long axis of the body. Furthermore, the position of the cardiac septum at about 45º to the median brings the ‘right heart’ structures anterior to the ‘left heart’ structures (Figure 3A). The ventricles are situated inferior and leftward relative to their corresponding atria.
 
[[Image:Figure 4.jpg|300px|right|thumb|Figure 4.<br>
== {{int:filedesc}} ==
{{Information
|Description= '''A.''' This frontal view shows the right and left surfaces of the heart. The left anterior descending coronary artery buried in epicardial fat marks the plane of the ventricular septum.
<br>'''B.''' The obtuse and acute margins of the ventricles are demonstrated in this apical view.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


When the pericardium is removed, the major part of the heart visible from the front is the ventricular mass. Here, the morphologically right ventricle occupies the greater part (Figure 3). The left ventricle appears only as a narrow slip along the left cardiac border. The shape of the heart is generally likened to a pyramid. The apex points downwards, forwards and to the left while the base faces posteriorly and to the right. While the cardiac apex is usually represented by the vortex of the left ventricle, the cardiac base is less well defined owing to differences in definition. The anatomical base is formed mainly by the left atrium receiving the pulmonary veins and to a small extent by the posterior part of the right atrium. 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 towards the right shoulder. This orientation deviates considerably from the long axis of the body. Furthermore, the position of the cardiac septum at about 45º to the median brings the ‘right heart’ structures anterior to the ‘left heart’ structures (Figure 3A). The ventricles are situated inferior and leftward relative to their corresponding atria. [[Image:Figure 4.jpg|thumb|300px|center|'''A.''' This frontal view shows the right and left surfaces of the heart. The left anterior descending coronary artery buried in epicardial fat marks the plane of the ventricular septum.
This results in the right atrioventricular junction being in a nearly vertical plane. The left atrium is the most posterior cardiac chamber being directly anterior to the oesophagus at the bifurcation of the trachea. In frontal projection, only its appendage is visible. The aorta has a deep-seated origin and only becomes part of the cardiac silhouette as it arches upwards and backwards, forming a spiral with the pulmonary trunk. 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 horizontally situated behind the third costal cartilage. The aortic valve lies posterior and to the right, above the nearly vertically orientated tricuspid valve (Figure 3B). 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.  
<br>'''B.''' The obtuse and acute margins of the ventricles are demonstrated in this apical view.]]This results in the right atrioventricular junction being in a nearly vertical plane. The left atrium is the most posterior cardiac chamber being directly anterior to the oesophagus at the bifurcation of the trachea. In frontal projection, only its appendage is visible. The aorta has a deep-seated origin and only becomes part of the cardiac silhouette as it arches upwards and backwards, forming a spiral with the pulmonary trunk. 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 horizontally situated behind the third costal cartilage. The aortic valve lies posterior and to the right, above the nearly vertically orientated tricuspid valve (Figure 3B). 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.  


The cardiac surfaces are described as the sternocostal, diaphragmatic, left and right (Figure 4). The sternocostal surface is covered anteriorly by the sternum and pleurae. The diaphragmatic surface is horizontally orientated. The sharp angle formed mainly by the right ventricle and occupying the lower heart border is the acute margin of the heart. The rounded obtuse margin of the heart is formed mainly by the left ventricle to the left of the sternocostal surface.
The cardiac surfaces are described as the sternocostal, diaphragmatic, left and right (Figure 4). The sternocostal surface is covered anteriorly by the sternum and pleurae. The diaphragmatic surface is horizontally orientated. The sharp angle formed mainly by the right ventricle and occupying the lower heart border is the acute margin of the heart. The rounded obtuse margin of the heart is formed mainly by the left ventricle to the left of the sternocostal surface.


==The morphologically right atrium==
==The morphologically right atrium==
[[Image:Figure 5.jpg|thumb|300px|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.
[[Image:Figure 5.jpg|300px|left|thumb|Figure 5.<br>
<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.]]
== {{int:filedesc}} ==
{{Information
|Description= '''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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]
 
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:Figure 6.jpg|thumb|300px|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.  
[[Image:Figure 6.jpg|300px|right|thumb|Figure 6.<br>
<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.]]
== {{int:filedesc}} ==
{{Information
|Description= '''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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


The left atrium also has a venous component, a characteristic appendage, a septal component and a vestibule that leads to the mitral orifice. Other than the appendage, the main chamber of the left atrium is relatively smooth-walled. The appendage is hook-shaped with a crenelated external appearance and a narrow junction with the venous component (Figure 6). The junction is not marked by any structure comparable to the terminal crest although in many hearts there is a prominent infolding of the atrial wall between the orifice of the atrial appendage and the orifices of the left pulmonary veins. The venous portion is anchored by the pulmonary veins which drain directly into its superior and posterior parts. There are usually four pulmonary venous orifices but variations are not uncommon. The coronary sinus runs inferiorly behind the posterior wall to open into the right atrium. The flap valve of the oval fossa on the septal aspect has a small crescent marking the free edge of the valve at the fossa opening (the site of the PFO) whereas the rest of the valve blends into the atrial wall (Figure 6).  
The left atrium also has a venous component, a characteristic appendage, a septal component and a vestibule that leads to the mitral orifice. Other than the appendage, the main chamber of the left atrium is relatively smooth-walled. The appendage is hook-shaped with a crenelated external appearance and a narrow junction with the venous component (Figure 6). The junction is not marked by any structure comparable to the terminal crest although in many hearts there is a prominent infolding of the atrial wall between the orifice of the atrial appendage and the orifices of the left pulmonary veins. The venous portion is anchored by the pulmonary veins which drain directly into its superior and posterior parts. There are usually four pulmonary venous orifices but variations are not uncommon. The coronary sinus runs inferiorly behind the posterior wall to open into the right atrium. The flap valve of the oval fossa on the septal aspect has a small crescent marking the free edge of the valve at the fossa opening (the site of the PFO) whereas the rest of the valve blends into the atrial wall (Figure 6).  


==The morphologically right ventricle==
==The morphologically right ventricle==
[[Image:Figure 7.jpg|thumb|300px|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.
[[Image:Figure 7.jpg|300px|right|thumb|Figure 7.<br>
<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.]]
== {{int:filedesc}} ==
{{Information
|Description= '''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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


Description of the ventricular chambers is facilitated by considering them in terms of three components - inlet, apical trabecular and outlet. The inlet contains the atrioventricular valve and its tension apparatus; the outlet supports the arterial valve. The apical trabecular portion is the most distinctive in each ventricle being characteristically coarse in the right ventricle (Figure 7A) and fine in the left ventricle. In a similar way, the muscular ventricular septum can be considered in terms of inlet, apical trabecular and outlet portions. A small fibrous area, the membranous septum, is located at this tripartite junction. The attachment of the septal tricuspid valve leaflet divides the membranous septum into atrioventricular and interventricular components (Figure 7B). It is important to appreciate that the entire ventricular septum is not on one plane. Owing to the 'wrap-around' relationship of the right ventricle to the left ventricle, the various portions are arranged at angles to each other. The inlet septum (between the ventricular inlet portions) is more or less at the sagittal plane of the body. Extending out apically and curving between the inlet and outlet components is the trabecular septum. In lateral projection, the right ventricle is seen to sweep from beneath to above the left ventricle. When viewed in frontal projection the right ventricle passes in front of the left ventricle (Figure 1). A prominent Y-shaped muscle band, the septomarginal trabeculation, is adherent onto the septal surface. Clasped in between the limbs of the septomarginal trabeculation is the supraventricular crest, a distinctive feature of the right ventricle. The tricuspid valve is separated from the pulmonary valve by this crest (Figure 7A). Much of the crest is simply the infolded inner heart curvature with fatty tissue containing the right coronary artery on its epicardial aspect. The body of the septomarginaI trabeculation gives origin to the moderator band that crosses the ventricular cavity to insert to the anterior wall. The right ventricular inlet component extends from the tricuspid valve orifice to the attachment of the papillary muscle but a discrete demarcation is not seen. The tricuspid valve lacks a well-formed fibrous annulus. Its three leaflets are not always easy to identify owing to clefts within its major leaflets. The commissural chords will identify the divisions between the three leafets - the antero-superior, the septal and the postero-inferior. The direct attachment of the septal leaflet to the septum is a distinguishing feature of the tricuspid valve.
Description of the ventricular chambers is facilitated by considering them in terms of three components - inlet, apical trabecular and outlet. The inlet contains the atrioventricular valve and its tension apparatus; the outlet supports the arterial valve. The apical trabecular portion is the most distinctive in each ventricle being characteristically coarse in the right ventricle (Figure 7A) and fine in the left ventricle. In a similar way, the muscular ventricular septum can be considered in terms of inlet, apical trabecular and outlet portions. A small fibrous area, the membranous septum, is located at this tripartite junction. The attachment of the septal tricuspid valve leaflet divides the membranous septum into atrioventricular and interventricular components (Figure 7B). It is important to appreciate that the entire ventricular septum is not on one plane. Owing to the 'wrap-around' relationship of the right ventricle to the left ventricle, the various portions are arranged at angles to each other. The inlet septum (between the ventricular inlet portions) is more or less at the sagittal plane of the body. Extending out apically and curving between the inlet and outlet components is the trabecular septum. In lateral projection, the right ventricle is seen to sweep from beneath to above the left ventricle. When viewed in frontal projection the right ventricle passes in front of the left ventricle (Figure 1). A prominent Y-shaped muscle band, the septomarginal trabeculation, is adherent onto the septal surface. Clasped in between the limbs of the septomarginal trabeculation is the supraventricular crest, a distinctive feature of the right ventricle. The tricuspid valve is separated from the pulmonary valve by this crest (Figure 7A). Much of the crest is simply the infolded inner heart curvature with fatty tissue containing the right coronary artery on its epicardial aspect. The body of the septomarginaI trabeculation gives origin to the moderator band that crosses the ventricular cavity to insert to the anterior wall. The right ventricular inlet component extends from the tricuspid valve orifice to the attachment of the papillary muscle but a discrete demarcation is not seen. The tricuspid valve lacks a well-formed fibrous annulus. Its three leaflets are not always easy to identify owing to clefts within its major leaflets. The commissural chords will identify the divisions between the three leafets - the antero-superior, the septal and the postero-inferior. The direct attachment of the septal leaflet to the septum is a distinguishing feature of the tricuspid valve.


==The morphologically left ventricle==
==The morphologically left ventricle==
[[Image:Figure 8.jpg|thumb|300px|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).
[[Image:Figure 8.jpg|300px|right|thumb|Figure 8.<br>
<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.]]
== {{int:filedesc}} ==
{{Information
|Description= '''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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


In contrast to the right ventricle, the left ventricle is a conical structure with thick tubular walls tapering to a rounded apex (Figure 6A) where the apical wall becomes as thin as 1-2 mm. Very little of the left ventricle is visible from the front of the heart (Figures 1 and 3A) although in the infant a relatively greater portion may be seen. As with the right ventricle, the left ventricle comprises inlet, trabecular and outlet portions. The acute angle between inlet and outlet portions brings the aortic valve in adjacency and in fibrous continuity with the mitral valve. There is no structure comparable to the supraventricular crest in the left ventricle. There is also no structure corresponding to the septomarginal trabeculation on the smooth septal surface (Figure 8A).
In contrast to the right ventricle, the left ventricle is a conical structure with thick tubular walls tapering to a rounded apex (Figure 6A) where the apical wall becomes as thin as 1-2 mm. Very little of the left ventricle is visible from the front of the heart (Figures 1 and 3A) although in the infant a relatively greater portion may be seen. As with the right ventricle, the left ventricle comprises inlet, trabecular and outlet portions. The acute angle between inlet and outlet portions brings the aortic valve in adjacency and in fibrous continuity with the mitral valve. There is no structure comparable to the supraventricular crest in the left ventricle. There is also no structure corresponding to the septomarginal trabeculation on the smooth septal surface (Figure 8A).
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==The coronary circulation==
==The coronary circulation==
[[Image:Figure 9.jpg|thumb|300px|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.]]
[[Image:Figure 9.jpg|300px|right|thumb|Figure 9.<br>
== {{int:filedesc}} ==
{{Information
|Description= 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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


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:Figure 10.jpg|thumb|300px|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.]]
[[Image:Figure 10.jpg|300px|right|thumb|Figure 10.<br>
== {{int:filedesc}} ==
{{Information
|Description= 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.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


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:Figure 3.jpg|300px|right|thumb|Figure 3.<br>
== {{int:filedesc}} ==
{{Information
|Description= '''A.''' Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
<br>'''B.''' The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]
[[Image:Figure 11.jpg|thumb|center|300px|'''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.
[[Image:Figure 11.jpg|thumb|center|300px|'''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:Figure 12.jpg|thumb|300px|center|'''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.
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|300px|right|thumb|Figure 12.<br>
== {{int:filedesc}} ==
{{Information
|Description= '''A.''' Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
<br>'''B.''' The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]
 
[[Image:Figure 12.jpg|thumb|300px|center|'''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).  
<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|300px|right|thumb|Figure 13.<br>
== {{int:filedesc}} ==
{{Information
|Description= '''A.''' Viewed from the front, the right atrium and right ventricle overlaps the left atrium and left ventricle. The atrial chambers are to the right of their respective ventricular chambers.
<br>'''B.''' The four cardiac valves are at different levels and different planes with the pulmonary(P) valve situated the most cephalad. The aortic(A) valve is wedged between the tricuspid(T) and mitral(M) valves.
|Source= provided by S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Date= 2012
|Author= S. Yen Ho, PhD FRCPath FESC FHEA, Royal Brompton Hospital, UK
|Permission=
|other_versions=
}}
]]


[[Image:Figure 13.jpg|thumb|center|This picture from Tawara’s monograph (1906) shows the tree-fascicular arrangement of the left bundle branch in man.  
[[Image:Figure 13.jpg|thumb|center|This picture from Tawara’s monograph (1906) shows the tree-fascicular arrangement of the left bundle branch in man.  
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