Right atrium (Fig. 1)

 The right atrium receives the superior vena cava in its upper and posterior part, the inferior vena cava and coronary sinus in its lower part, and the anterior cardiac vein (draining much of the front of the heart) anteriorly. Running more or less vertically downwards between the venae cavae is a distinct muscular ridge, the crista terminalis (indicated on the outer surface of the atrium by a shallow groove – the sulcus terminalis). This ridge separates the smooth-walled posterior part of the atrium, derived from the sinus venosus, from the rough-walled anterior portion which is prolonged into the auricular appendage and which is derived from the true fetal atrium.

The trabeculations in the rough-walled part of the atrium are produced by parallel columns of muscle termed musculi pectinati (pectinate muscles); ‘pectinate’ comes from the Latin for ‘like a comb’. The same term applies to the columns of muscle seen in the auricular appendage of the left atrium.

The openings of the inferior vena cava and the coronary sinus are guarded by rudimentary valves; that of the inferior vena cava being continuous with the annulus ovalis around the shallow depression on the atrial septum, the fossa ovalis, which marks the site of the fetal foramen ovale.

Fig1. The interior of the right atrium and ventricle.

Right ventricle

 The right ventricle is joined to the right atrium by way of the vertically disposed tricuspid valve, and with the pulmonary trunk through the pulmonary valve. A muscular ridge, the infundibuloventricular crest, between the atrioventricular and pulmonary orifices, separates the ‘inflow’ and ‘outflow’ tracts of the ventricle. The inner aspect of the inflow tract path is marked in the presence of a number of irregular muscular elevations (trabeculae carneae), from some of which the papillary muscles project into the lumen of the ventricle and find attachment to the free borders of the cusps of the tricuspid valve by way of the chordae tendineae.

When the ventricle contracts in systole, the papillary muscles shorten, the chordae tendineae are pulled upon and the tricuspid valve is pre vented from prolapsing into the right atrium. (The same mechanism, of course, takes place with the mitral valve of the left ventricle.) Rupture of a papillary muscle, following an adjacent myocardial infarction, will allow prolapse of the affected cusp to occur into the atrium at each systole, with consequent acute cardiac failure.

The moderator band is a muscular bundle crossing the ventricular cavity from the interventricular septum to the anterior wall and is of some importance since it conveys the right branch of the atrioventricular bundle to the ventricular muscle.

The outflow tract of the ventricle or infundibulum is smooth-walled and is directed upwards and to the right towards the pulmonary trunk. The pulmonary orifice is guarded by the pulmonary valves, comprising three semilunar cusps.

Left atrium

 The left atrium is rather smaller than the right but has somewhat thicker walls. On the upper part of its posterior wall it presents the openings of the four pulmonary veins, and on its septal surface there is a shallow depression corresponding to the fossa ovalis of the right atrium. As on the right side, the main part of the cavity is smooth-walled but the surface of the auricle is marked by a number of ridges due to the underlying pectinate muscles.

Left ventricle (Fig. 2)

 The left ventricle communicates with the left atrium by way of the mitral valve (so-called because it vaguely resembles a bishop’s mitre), which possesses a large anterior and a smaller posterior cusp attached to papillary muscles by chordae tendineae. With the exception of the fibrous vestibule immediately below the aortic orifice, the wall of the left ventricle is marked by thick trabeculae carneae.

The term ‘cusps’ applied to the tricuspid and mitral valves is somewhat misleading. It suggests that, as in the aortic and pulmonary valves, these are individual structures and separate from each other (Fig. 2). In fact, the tricuspid and mitral valves can be compared to circular curtains that hang down from each atrioventricular orifice, and that descend into the lumen of the ventricle with three drapes on the right side and two drapes on the left, held in place by chordae tendineae.

The aortic orifice is guarded by the three semilunar cusps of the aortic valve, immediately above which are the dilated aortic sinuses. The mouths of the right and left coronary arteries are seen in the anterior and left posterior sinus, respectively. In diastole, blood refluxing into the aortic sinuses sets up turbulent flow, which helps to close the valve. The same mechanism applies to the pulmonary valve. In addition, on the left, blood refluxes in diastole into the two coronary artery ostia, placed within the sinuses, so that cardiac perfusion takes place in the ventricular diastolic phase of the cardiac cycle.

Fig2. The interior of the left ventricle. AO, aorta; RV, right ventricle.