Overview

Two overlapping partitions. The septum forms from the thin septum primum, which becomes the floor of the fossa ovalis, and the thicker septum secundum, whose free edge forms the raised limbus fossae ovalis.

Closure at birth. At birth, left atrial pressure rises above right atrial pressure. This pushes the flap of septum primum against septum secundum, and the two usually fuse over the following months. In about 25% of adults they never fuse completely, leaving a probe-patent foramen ovale (PFO). A PFO normally stays shut because left atrial pressure is higher than right.

AV node and AV septum. The AV node lies in the lower part of the septum near the tricuspid valve, at the apex of the triangle of Koch. A small atrioventricular septum separates the right atrium from the left ventricle. It exists because the septal tricuspid leaflet attaches lower (more apically) than the anterior mitral leaflet.

Location: Between the right and left atria. It lies obliquely, so the right atrium is in front of and to the right of the left atrium.

Parts & subdivisions

Fossa ovalis
Oval depression whose thin floor is the septum primum.
Limbus fossae ovalis
Raised crescentic rim formed by the septum secundum.
Atrioventricular septum
Separates the right atrium from the left ventricle. It has muscular and membranous parts.

Anatomy

Development: The septum primum grows down toward the endocardial cushions and closes the ostium primum. Programmed cell death in its upper part then opens the ostium secundum. The septum secundum grows to the right of the septum primum but leaves an oval gap, the foramen ovale.

Function

After birth, separates oxygenated blood (left atrium) from deoxygenated blood (right atrium).
In the fetus, the foramen ovale lets oxygen-rich IVC blood shunt from right to left.
Provides the route for catheter access to the left atrium (transseptal puncture) in AF ablation, mitral procedures and appendage occlusion.

Clinical significance

Atrial septal defect (ASD)

The secundum type, in the region of the fossa ovalis, is the commonest (about 70%). A primum defect is a partial atrioventricular septal defect and is associated with Down syndrome. A sinus venosus defect lies near the SVC and often comes with anomalous drainage of the right pulmonary veins.

The left-to-right shunt overloads the right heart with volume. Typical signs are fixed splitting of S2 and a pulmonary flow murmur. If unrepaired, pulmonary hypertension and Eisenmenger syndrome can follow.

Patent foramen ovale and paradoxical embolism

A patent foramen ovale is present in about a quarter of adults. It allows a venous clot to cross into the systemic circulation (paradoxical embolism). It is associated with unexplained (cryptogenic) stroke in younger patients and with decompression illness in divers. Device closure is considered in selected patients.

Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.