The Right Atrium

Key points

• • It has a smooth-walled posterior part (the sinus venarum, of venous origin) and a rough anterior part with muscular ridges called musculi pectinati, separated by a ridge, the crista terminalis.
• • The superior vena cava opens into its upper part, the inferior vena cava into its lower part, and the coronary sinus opens between the inferior vena cava and the right atrioventricular opening.
• • The interatrial septum shows a shallow oval depression, the fossa ovalis, the remnant of the fetal foramen ovale; its floor can stay unsealed as a probe-patent foramen.
• • The sinuatrial node, the natural pacemaker of the heart, sits in the wall at the upper end of the crista terminalis near the opening of the superior vena cava.
• • The atrioventricular node lies in the lower interatrial septum near the opening of the coronary sinus, within a region called the triangle of Koch.
• • Blood leaves the right atrium through the tricuspid (right atrioventricular) valve into the right ventricle.