Chambers of the heart and the great vessels
DISSECTION 4.11 Right atrium Objective
I. To study the internal surface of the right atrium.
Instructions
1. Identify the right auricle.
2. Identify the groove—the sulcus terminalis—on the right side. This separates the right atrium from the right auricle.
3. Follow the incisions shown in Fig. 4.32 to open into the right atrium.
4. Turn the flap to the left and remove the clotted blood.
5. Identify the crista terminalis separating the smooth-walled and rough-smooth-walled parts [Fig. 4.33].
6. Identify the smooth-walled part of the right atrium and the openings of the superior vena cava, inferior vena cava, and coronary sinus in it. Identify the valve of the coronary sinus and the valve of the inferior vena cava [Fig. 4.33].
7. On the posterior wall of the right atrium, identify the fossa ovalis and the thickened superior margin—the limbus fossa ovalis.
8. Note the continuity of the crista terminalis, the valve of the inferior vena cava, the valve of the coronary sinus, and the limbus fossa ovalis [Fig. 4.34].
9. Identify the right atrioventricular orifice.
B
C A
Fig. 4.32 Incision to open (A) the right atrium, (B) the right ventricle, and (C) the left ventricle.
Ascending aorta Superior vena cava
Margin of fossa ovalis
Opening of the superior vena cava
Fossa ovalis
Valve of coronary sinus
Opening of coronary sinus Right atrioventricular
orifice Crista terminalis
Fig. 4.33 The interior of the right atrium exposed by turning its right and anterior walls towards the left.
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The cavity of the thorax
and is covered by the pericardium. It opens into the superior part of the right atrium [Fig. 4.34]. The azygos vein arches over the superior surface of the right lung root [Fig. 4.13A] and enters the posterior surface of the superior vena cava at its midpoint, just before it enters the pericardium.
The superior vena cava is anterolateral to the trachea, anterior to the lung root, and lateral to the ascending aorta. % The phrenic nerve and the right mediastinal pleura lie on its right surface. The close association of the superior vena cava with the right lung root and its contained pulmonary veins [Fig. 4.13A] makes it possible for an abnormal pul-monary vein to drain into the superior vena cava.
Inferior vena cava
The intrathoracic part of the inferior vena cava is short [Fig. 4.13A]. It pierces the central tendon of the diaphragm and pericardium approximately at the level of the eighth thoracic vertebra. It imme-diately enters the inferior part of the right atrium between the fossa ovalis and the upper margin of
the limbus.
The opening of the inferior vena cava into the right atrium is directed towards the fossa ovalis on the interatrial septum. The valve of the infe-rior vena cava directs blood towards the fossa. The opening of the superior vena cava into the right atrium lies on a more anterior plane, and faces the right atrioventricular orifice. This arrangement produces an almost complete separation of the two venous streams before birth.
Veins entering the right atrium Superior vena cava
The superior vena cava lies on the right side and drains blood from the head and neck, upper limbs, and walls of the thorax and the upper abdomen.
The upper part lies in the superior mediastinum and will be described later. The lower part of the superior vena cava lies in the middle mediastinum
Aorta outside
pericardium Intervenous tubercle of right atrium
Right coronary A.
Pulmonary valve
Infundibulum
Ant. interventricular br. of L. coronary A.
Great cardiac V.
Septal cups of R.
atrioventricular valve Ant. papillary M. of L. ventricle
Right ventricle Right coronary A.
Fossa ovalis Crista terminalis
Posterior papillary M.
Parietal pleura Opening of superior vena cava
Opening of inferior vena cava
Fig. 4.34 The heart in situ. A coronal section through the right atrium, right ventricle, infundibulum of the right ventricle, pulmonary valve, and left ventricle.
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The middle mediastinum
Right atrioventricular orifice and valve
The right atrioventricular orifice lies between the antero-inferior part of the right atrium and the postero-inferior part of the right ventricle [Fig. 4.35]. It is approximately 2.5 cm in diam-eter and is guarded by the right atrioventricular, or tricuspid valve. The atrioventricular orifice is surrounded by a fibrous ring which gives at-tachment to the cusps of the atrioventricular valve, the atrial and ventricular muscle. This fibrous ring is part of the fibrous skeleton of the heart [Fig. 4.36]. The three cusps of the tricuspid at the level of the sixth right chondrosternal joint.
The right pleura and lung are wrapped round the right and posterior surfaces of the inferior vena cava and separate it from the vertebral column posteriorly. The right phrenic nerve descends on its right surface.
Coronary sinus
The coronary sinus is the main vein of the heart.
It is formed by the union of the great cardiac vein and the oblique vein of the left atrium [Fig. 4.26].
It runs from left to right in the posterior part of the coronary sulcus and empties into the right atrium. The main tributaries of the coronary si-nus are the great cardiac vein at its left extrem-ity, and the middle and small cardiac veins at its right extremity [Fig. 4.31]. It also drains the veins of the left ventricle and the oblique vein of the left atrium. It enters the right atrium im-mediately to the left of the valve of the inferior vena cava.
The oblique vein of the left atrium corres-ponds developmentally to the inferior half of the superior vena cava. Thus a persistent left superior vena cava may replace the oblique vein and enter the coronary sinus. Both the oblique vein and the coronary sinus are partly covered by cardiac muscle on their posterior aspects.
The right atrium also receives (1) the anterior cardiac veins which pierce the anterior wall of the right atrium and (2) small venae cordis minimae which enter the right atrium through irregularly scattered openings which are difficult to identify.
Arch of aorta Serous pericardium
Pulmonary trunk Anterior sinus
Pulmonary valve
Anterior papillary M.
Trabeculae carneae
Inferior wall Posterior cusp
Inferior vena cava Superior vena cava Ascending aorta
Right auricle
Anterior cups of right atrioventricular valve
Interventricular septum Infundibulum
Fig. 4.35 The interior of the right ventricle exposed from in front.
Pulmonary valve Fibrous ring—
pulmonary valve
Fibrous ring—
aortic valve Right aortic sinus Right coronary artery
Fibrous ring—left atrioventricular valve
Fibrous ring—right atrioventricular valve Fibrous trigone
Posterior cusp
Fibrous trigone Anterior cusp Lunule Infundibulum of right ventricle
Nodule
Left coronary artery Anterior inter-ventricular branch
Fig. 4.36 A diagram of the ventricular part of the heart with the atria and great vessels removed. The atrioventricular bundle is shown piercing the fibrous skeleton of the heart.
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The cavity of the thorax
and sends chordae tendineae to the posterior and septal cusps of the valve. A larger anterior papillary muscle is attached to the anterior wall and sends chordae tendinae into the anterior and posterior cusps. Several small septal papillary muscles arise from the septum and send chordae tendinae into the anterior and septal cusps. The anterior and posterior muscles are occasionally divided into a number of smaller projections.
When the papillary muscles contract, they draw the margins of adjacent cusps together and close the atrioventricular valve. They also prevent the valve from being forced back into the atrium as the intraventricular pressure rises. This mechanism in-creases the efficiency of the ventricular contraction and permits the atrium to fill completely while the ventricle is contracting.
The septomarginal trabecula is one of the tra-beculae carneae which passes from the septum to the anterior papillary muscle. It carries part of the right crus of the atrioventricular bundle (a part of the conducting system of the heart) [Fig. 4.35]. This and similar parts of the conducting system pass to the other papillary muscles and ensure early contraction of these muscles. Thus the chordae tendineae are al-ready taut when ventricular contraction begins.
In a transverse section, the right ventricle is crescentic in shape because the interventricular septum bulges to the right. The wall of the right ventricle is thinner than that of the left. The differ-ence in thickness of the muscle of the two ventri-cles is directly related to the work done by each in overcoming the vascular resistance against which it pumps. (The pulmonary systolic arterial pressure is low at 25–35 mmHg, compared with the much higher pressure of 120 mmHg systolic in the sys-temic circulation.) % Any condition which increas-es the pulmonary arterial rincreas-esistance raisincreas-es the right ventricular pressure and produces a compensatory hypertrophy of the right ventricular wall.
Right atrioventricular (tricuspid) valve
The three cusps of the right atrioventricular valve are named from their position. The anterior cusp separates the atrioventricular foramen from the infundibulum. The posterior cusp lies on the infe-rior wall of the right ventricle. The septal cusp lies on the interventricular septum. The anterior and posterior cusps are more nearly horizontal. The chordae tendineae are attached to the margins and ventricular surfaces of the cusps. The atrial surfaces valve are the anterior, posterior, and septal cusps.
Further details of the valve are described after the right ventricle is studied.
Right ventricle
Dissection 4.12 gives instructions on the dissection of the right ventricle.
The cavity of the right ventricle is approxi-mately triangular in shape when seen from in front [Fig. 4.35]. Blood enters the right ventricle through the right atrioventricular orifice at the right inferior angle and leaves through the pulmonary orifice at the superior angle. The smooth-walled anterosuperior part of the right ventricle is the infundibulum (= a funnel). It leads to the pulmonary orifice. The remainder of the wall of the right ventricle has irregular muscle ridges called trabeculae carneae. Three coni-cal papillary muscles arise from the wall of the ventricle and project into the cavity of the ven-tricle [Fig. 4.35]. They are inserted by tendinous strands called chordae tendineae into the mar-gins and ventricular surfaces of the cusps of the right atrioventricular valve. The large posterior papillary muscle arises from the inferior wall
DISSECTION 4.12 Right ventricle