Chambers of the heart and the great vessels
DISSECTION 4.13 Left ventricle Objective
I. To study the interior of the left ventricle.
Instructions
1. Identify the three cusps of the aortic valve—anterior, left posterior, and right posterior cusps—by examining the valve from above [Fig. 4.36].
2. Make the incisions shown in Fig. 4.32. The cut should pass through the aorta, aortic vestibule, and left ven-tricle.
3. Turn the flap to the left and remove the clotted blood.
4. Identify the anterior and posterior cusps of the left atrioventricular valve.
5. Identify the anterior and posterior papillary muscles.
Follow the chordae tendinae arising from them to both cusps of the left atrioventricular valve.
6. Examine the trabeculae carneae on the inner surface of the left ventricle.
7. Examine the interventricular septum by holding it be-tween your finger and thumb.
8. Note the position of the muscular part of the interven-tricular septum, its relation to the membranous part, and the septal cusp of the right atrioventricular valve.
9. Examine the anterior cusp of the left atrioventricular valve by placing a finger in the mitral valve and another in the aortic orifice. Appreciate how this cusp sepa-rates the inflow and outflow tracts of the left ventricle.
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The cavity of the thorax
surfaces. Note also that the aortic orifice is supe-rior to the interventricular septum.
Aortic valve
The aortic valve is similar to the pulmonary valve in having three cusps. The cusps of the aortic valve are thicker and differently placed (anterior, left posterior, and right posterior), and the aortic si-nuses are larger [Fig. 4.36].
Interventricular septum
The interventricular septum is mostly thick and muscular. Posterosuperiorly, it is thin and membra-nous [Figs. 4.37, 4.38] and connected to the fibrous skeleton. The muscular interventricular sep-tum is placed obliquely between the anterior and posterior interventricular sulci. The right surface of the septum faces forwards and to the right and bulges into the right ventricle [Fig. 4.37]. Towards the aortic orifice, the septum becomes thin and collagenous to form the membranous part of muscle is attached to the anterior part of the left
wall. The posterior papillary muscle arises from the inferior wall further posteriorly. Each papillary muscle sends chordae tendineae to both cusps of the left atrioventricular (mitral) valve [Figs. 4.38, 4.39].
The aortic vestibule is the outflow tract of the left ventricle. Its smooth walls are mainly fibrous and not muscular [Fig. 4.39].
Aortic orifice and aortic valve
The aortic orifice lies in the right posterosuper-ior part of the left ventricle. It is surrounded by a fibrous ring to which the cusps of the aortic valve are attached [Fig. 4.36]. It is separated from the left atrioventricular orifice only by the anterior cusp of the mitral valve [Figs. 4.36, 4.37, 4.39].
The anterior cusp of the mitral valve separates two blood streams. Blood entering the ventricle from the atrium, and that leaving the ventricle through the aorta. As a result, the cusp is smooth on both
Pulmonary trunk Left auricle
Circumflex br. of L. coronary A.
Posterior cusp of left atrioventricular valve Anterior cusp
Fibrous ring Valve of inferior
vena cava Opening of inferior
vena cava Opening of coronary
sinus Membranous atrioventricular septum Aortic valve
Origin of left coronary A.
Ascending aorta Superior vena cava
Orifice of azygos V.
Fig. 4.37 The heart in situ. A coronal section through the right atrium, right ventricle, left auricle, left ventricle, aortic vestibule, aortic valve, and aorta. The arrow lies in the transverse sinus of the pericardium.
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The middle mediastinum
abdomen and ends on the anterior surface of the fourth lumbar vertebra by dividing into the right and left common iliac arteries. For descriptive pur-poses, the thoracic part of the aorta is divided into the ascending aorta, the arch of the aorta, and the descending aorta. The ascending aorta lies in the middle mediastinum, the arch in the superior me-diastinum, and the descending aorta in the poste-rior mediastinum [Fig. 4.14B].
Ascending aorta
The ascending aorta lies in the middle mediasti-num. It is the first part of the aorta. It runs up-wards, forup-wards, and to the right [Fig. 4.39]. Behind the right half of the sternal angle, it continues as the arch of the aorta. It is enclosed with the pul-monary trunk in a sheath of serous pericardium.
The ascending aorta has four dilatations of its wall, three aortic sinuses at its root corresponding to the cusps of the aortic valve [Figs. 4.29, 4.39]. These are the right, left anterior, and left posterior aortic the interventricular septum. Superiorly, the
posterior part of the membranous interventricular septum narrows and continues with a septum be-tween the right atrium and the left ventricle. This is the membranous atrioventricular septum.
The membranous interventricular septum is devel-oped from the same tissue that forms the valves of the heart. % Since the membranous interventricu-lar septum develops separately from the muscuinterventricu-lar part of the septum, it may be deficient, leaving an interventricular foramen immediately inferior to the aortic orifice. The presence of such an inter-ventricular foramen inevitably means that the pressures in both ventricles are the same—a situa-tion which leads to hypertrophy of the right ven-tricular wall.
The aorta
The aorta arises from the aortic orifice behind the third left intercostal space at the margin of the ster-num [Fig. 4.25]. It runs through the thorax and
Interatrial septum
Fibrous trigone
Fibrous trigone Fibrous ring of R. atrioventricular orifice
Right atrium
Left atrium
Posterior cusp
Fibrous ring of L. atrioventricular orifice
Post. cusp of L.
atrioventricular valve
Anterior cusp
Wall of left ventricle
Interventricular septum Membranous
atrioventricular septum Septal cusp of R. atrioventricular valve
Aortic vestibule
Fig. 4.38 A section through the heart to show the interatrial, atrioventricular, and interventricular septae, and the fibrous rings that surround the atrioventricular orifices.
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The cavity of the thorax
sinuses. The right and left coronary arteries arise from the right and left posterior aortic sinuses. At the right border of the aorta is a fourth swelling where the full thrust of the blood discharged from the left ventricle is felt. This is the bulb of the aorta. % The bulb is a common site for the forma-tion of an abnormal dilataforma-tion or aneurysm.
Initially, the ascending aorta lies between the left atrium and the infundibulum of the right ventri-cle. The right atrium is on its right side. It then ascends anterior to the right pulmonary artery and right bronchus, with the superior vena cava on the right and the pulmonary trunk running posteriorly on its left [Figs. 4.19, 4.33, 4.35]. The ascending aorta is separated from the sternum by the pericardium, pleura, lung, and the remains of
Trachea Right brachiocephalic V.
Left brachiocephalic V.
Superior vena cava
Ascending aorta
Right aortic sinus
Right auricle
Right coronary A.
Septomarginal trabecula (cut end)
Septomarginal trabecula (cut end)
Anterior papillary M.
Anterior papillary M.
Posterior papillary M.
Right atrioventricular orifice
Left atrioventricular orifice Anterior cusp of left atrioventricular valve Aortic vestibule Left coronary A.
Left auricle Pulmonary trunk turned upwards Arch of aorta
Interventricular septum
Fig. 4.39 A diagrammatic representation of the dissected ventricles of the heart. The root of the aorta has been exposed by separating the pulmonary trunk from the right ventricle and turning the trunk upwards.
the thymus. The vasa vasorum of the ascending aorta arises principally from a branch of the left coronary artery.
Left atrium
Dissection 4.14 provides instructions on the dissec-tion of the left atrium.
The left atrium lies posteriorly and forms the base of the heart [Fig. 4.27]. The long, narrow left auricle projects forwards on the left side and partly overlaps the beginning of the pulmonary trunk [Fig. 4.24]. The right and left superior and inferior pulmonary veins enter the upper half of the left atrium, close to the lateral margins of the posterior surface. The superior veins are on a plane anterior to that of the inferior veins [Fig. 4.41].
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The middle mediastinum
The interior of the left atrium is almost entire-ly smooth. Musculi pectinati are seen onentire-ly in the auricle. A prominent muscular ridge projects into the left atrium anterior to the left pulmonary veins so that the orifices of these veins are not visible from in front. The interatrial septum [Fig. 4.41]
slopes posteriorly and to the right so that a consid-erable part of the left atrium lies posterior to the right atrium.
Veins entering the left atrium
The four pulmonary veins enter the left atrium [Figs. 4.41, 4.42, 4.43]. Their openings are not guarded by valves.
Left atrioventricular valve
The left atrioventricular orifice lies at the an-tero-inferior part of the left atrium [Figs. 4.38, 4.41, 4.43]. The orifice is smaller (2 cm) than the right atrioventricular orifice and is guarded by the left atrioventricular (mitral) valve. The left atrio-ventricular valve has two cusps. The larger anterior cusp lies anterior to, and to the right of, the poste-DISSECTION 4.14 Left atrium
Objective
I. To study the interior of the left atrium.
Instructions
1. Examine the posterior surface of the heart and identify the four pulmonary veins.
2. Make the incision shown in Fig. 4.40.
3. Turn the flap inferiorly and remove the clotted blood.
4. Note the smooth wall of the left atrium and the rough wall of the small left auricle.
5. Examine the left atrioventricular opening and valve.
Fig. 4.40 Position of incision to open the left right atrium.
Left pulmonary Vv.
Left atrium
Coronary sinus
Inferior vena cava
Right pulmonary Vv.
The upper part of the anterior surface of the left atrium is covered by the ascending aorta and the pulmonary trunk. The pulmonary arteries course along its superior margin. The coronary sinus in the coronary sulcus runs along the inferior margin [Fig. 4.27].
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The cavity of the thorax
rior cusp. The cusps are attached to the fibrous ring of the left atrioventricular orifice [Figs. 4.36, 4.41]
and project into the ventricle. The anterior papil-lary muscle sends chordae tendinae to the left or anterior halves of both cusps. The posterior papil-lary muscle is similarly attached to the posterior or right halves of both cusps. The anterior cusp forms
the anterior wall of the atrioventricular orifice and the posterior wall of the aortic vestibule. It has blood flowing over both surfaces and, as a result, both surfaces are smooth, with only a few chordae tendinae attached to its ventricular surface.
Fibrous skeleton of heart
The fibrous skeleton of the heart is made up of dense fibrous tissue. It surrounds the atrioventri-cular, pulmonary, and aortic orifices. The cusps of all four cardiac valves are attached to the fibrous skeleton. At the junction of the atrioventricular valves and the aortic valve are fibrous tissue trian-gles called fibrous trigones [Fig. 4.36]. The atrioven-tricular part of the skeleton prevents conduction of impulses from atrial to ventricular muscle in all areas, except through the atrioventricular bundle.
This special conducting bundle of muscle fibre pierces the fibrous tissue at the postero-inferior part of the interventricular septum. This arrange-ment ensures a time delay between atrial and ven-tricular contractions.
Apical br. of R.
pulmonary A.
Apical ramus R.
sup. pulmonary V.
Fossa ovalis Right coronary A. Middle cardiac V. Post. interventricular br. of R. coronary A.
Posterior cusp of left atrioventricular valve
Left phrenic N.
Interatrial septum Circumflex branch of left coronary A.
Left atrium Transverse sinus of pericardium Branch of left pulmonary A.
Bifurcation of pulmonary trunk Arch of aorta
Trachea
Fig. 4.41 The heart in situ. A section through the heart to show the left atrium, interatrial septum, right atrium, left atrioventricular valve, and left ventricle. The arrows emerge from the pulmonary veins.
Fig. 4.42 Posterior view of 3D volume-rendered image of the left atrium and pulmonary veins.