Anatomy of the Heart: Difference between revisions

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'''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.]]  

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.  

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.  

==The morphologically left atrium==

'''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.]]

==The morphologically right ventricle==

'''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.]]

==The morphologically left ventricle==

'''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.]]

==The coronary circulation==

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.  

==The cardiac conduction system==

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==

<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.]]

==The atrioventricular conduction system==

<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.]]  

(Tawara S 1906 Das Reizleitungssystem des Säugetierherzens. Eine Anatomisch-Histologische Studie Über das Atrioventrikularbündel und die Purkinjeschen Fäden. Gustav Fischer, Jena.)]]