Interatrial septum

The interatrial septum is the cardiac septum that separates the right atrium from the left atrium of the heart.

It is composed of two main embryological components: the septum primum, which forms the thin, membranous floor of the fossa ovalis, and the septum secundum, a muscular rim that surrounds the fossa ovalis and is actually an infolding of the atrial wall containing epicardial adipose tissue.

Interatrial septum in dogs:

The interatrial septum is extending from the level of the coronary sinus to the opening into the right auricle.

On the right side of the interatrial septum is a transverse ridge of tissue placed between the two caval openings called the intervenous tubercle that diverts the converging inflowing blood from the two caval veins into the right ventricle.

The fossa ovalis is located just caudal to the intervenous tubercle on the medial wall of the atrium and is a slitlike depression, which varies from one to several millimeters in depth. The fossa ovalis is a thin, oval-shaped depression in the right atrial aspect of the interatrial septum, representing the remnant of the fetal foramen ovale after its postnatal closure. It is formed by the fusion of the septum primum and septum secundum and is bordered by a muscular rim called the limbus, which is most pronounced superiorly and laterally

The crescent-shaped ridge of muscle that projects from the caudal side of the intervenous tubercle and deepens the fossa is the limbus fossae ovalis.

The sinus of venae cavae (sinus venarum cavarum) is the smooth walled portion of the right atrium between the caval openings and the right atrioventricuar opening. It is bounded by the interatrial septum and the crista terminalis.