The ventricles are separated from each other by the interventricular septum and the atria are separated from each other by the interatrial septum. The interior of the right atrium has a rough anterior part, the atrium proper and a smooth part called the sinus venarum. Also it has an appendage called the auricle. All the large veins open into the smooth part. The opening of the superior vena cava is situated in its upper and posterior part and that of inferior vena cava into its lower part, close to the interatrial septum. The opening of the inferior vena cava is bounded by a fold called the valve of the inferior vena cava. Just to the left of this is the opening of the coronary sinus. Besides these there are small openings called venae cordis minimae for small veins of the heart.

The sinus venarum and the atrium proper meet at a muscular ridge called the  crista terminalis which corresponds to a groove externally called the  sulcus terminalis.

Musculi pectinati are the ridges found in the atrium proper. The interatrial septum shows an oval depression called the  fossa ovalis in its lower part, the upper margin of which forms a curved ridge called the  limbus fossa ovalis. The wall of the fossa ovalis is thin and represents the embryonic septum primum. The limbus fossa ovalis represents the lower curved edge of the septum secundum.  The right atrium opens into the right ventricle through the right atrioventricular orifice, which is guarded by the tricuspid valve.