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Anatomy of the Heart: Difference between revisions
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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 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. | 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 | 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. | ||
[[Image:Figure 4.jpg|300px| | [[Image:Figure 4.jpg|300px|left|thumb|Figure 4.<br> | ||
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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. | |||
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. | 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. | ||
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==The morphologically right atrium== | ==The morphologically right atrium== | ||
[[Image:Figure 5.jpg|300px| | [[Image:Figure 5.jpg|300px|right|thumb|Figure 5.<br> | ||
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==The morphologically left atrium== | ==The morphologically left atrium== | ||
[[Image:Figure 6.jpg|300px| | [[Image:Figure 6.jpg|300px|left|thumb|Figure 6.<br> | ||
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==The morphologically left ventricle== | ==The morphologically left ventricle== | ||
[[Image:Figure 8.jpg|300px| | [[Image:Figure 8.jpg|300px|left|thumb|Figure 8.<br> | ||
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==The cardiac conduction system== | ==The cardiac conduction system== | ||
[[Image:Figure 10.jpg|300px| | [[Image:Figure 10.jpg|300px|left|thumb|Figure 10.<br> | ||
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==The sinus node== | ==The sinus node== | ||
[[Image:Figure | [[Image:Figure 11.jpg|300px|right|thumb|Figure 11.<br> | ||
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|Description= '''A.''' | |Description= '''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.''' | <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. | ||
|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 'ultimum moriens', the last part of the heart to stop beating when the organ is isolated from the body, first prompted Wenckebach to believe that this may also be the seat of the heart beat.<cite>Wenckebach</cite> The discovery of the sinus node in the heart of a mole culminated in a paper in 1907 in which Keith and Flack described 'a remarkable remnant of primitive fibres persisting at the sino•auricular junction in all mammalian hearts. These fibres are in close connection with the vagus and sympathetic nerves, and have a special arterial blood supply; in them the dominating rhythm of the heart is believed to normally arise'.<cite>Keith</cite> The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.<cite>Lewis</cite> | The 'ultimum moriens', the last part of the heart to stop beating when the organ is isolated from the body, first prompted Wenckebach to believe that this may also be the seat of the heart beat.<cite>Wenckebach</cite> The discovery of the sinus node in the heart of a mole culminated in a paper in 1907 in which Keith and Flack described 'a remarkable remnant of primitive fibres persisting at the sino•auricular junction in all mammalian hearts. These fibres are in close connection with the vagus and sympathetic nerves, and have a special arterial blood supply; in them the dominating rhythm of the heart is believed to normally arise'.<cite>Keith</cite> The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.<cite>Lewis</cite> | ||
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==The atrioventricular conduction system== | ==The atrioventricular conduction system== | ||
[[Image:Figure 12.jpg|300px|left|thumb|Figure 12.<br> | |||
[[Image:Figure 12.jpg|300px| | |||
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|Description= '''A.''' | |Description= '''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 | <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. | ||
|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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[[Image:Figure 13.jpg|300px|right|thumb|Figure 13.<br> | [[Image:Figure 13.jpg|300px|right|thumb|Figure 13.<br> | ||
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|Description= | |Description= 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.) | |||
|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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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. | |||
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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). | |||
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 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). | ||