Atrial septation: septum primum, septum secundum and foramen ovale !

Process

Two overlapping walls divide the atrium and leave a door, the foramen ovale, so blood can cross to the left before birth.

Updated 2026-11-01

See it in 3D: Interatrial septum · Right atrium · Left atrium · Pulmonary veins · Coronary sinus · Superior vena cava · Inferior vena cava

What it is

Between weeks 4 and 6 the primitive atrium is divided by two septa: the thin septum primum and the thick muscular septum secundum to its right. Between them lies the foramen ovale, a one-way valve that lets blood pass right to left during fetal life. At birth left atrial pressure rises and closes the valve; errors in this process give atrial septal defects.

Key facts

End of week 4: the septum primum grows down from the atrial roof.
Week 5: the ostium secundum opens; the ostium primum closes as the septum primum fuses with the cushions.
Weeks 5–6: the septum secundum grows but never completes: its free edge rims the foramen ovale.
At birth: pulmonary resistance falls, left atrial pressure rises and the septum primum is pressed against the secundum.
Anatomic fusion happens over the first months to years; if it does not, a patent foramen ovale remains.

Structure

Septum primum: thin membrane that forms the floor of the fossa ovalis.
Septum secundum: thick muscular fold that forms the limbus (rim) of the fossa ovalis.
Foramen ovale: the oblique passage between the two septa.
The sinus venosus is absorbed into the right atrium and the pulmonary veins into the left atrial wall.

Step by step

The septum primum grows down from the atrial roof toward the endocardial cushions.
The space between its edge and the cushions is the ostium primum.
Perforations in the upper septum primum merge into the ostium secundum.
The septum primum fuses with the cushions and closes the ostium primum.
The septum secundum grows to its right, partly covering the ostium secundum and leaving the foramen ovale.
In the fetus, blood from the inferior vena cava pushes the valve open and enters the left atrium.
At birth, higher left atrial pressure shuts the valve; fusion follows later.

Markers

TBX5
NKX2-5
GATA4

Clinical notes

Secundum ASD: the most common; pulmonary flow murmur and a fixed split second heart sound.
Primum ASD: part of the atrioventricular canal (cushion) defect spectrum.
Sinus venosus ASD: near the superior vena cava, often with anomalous drainage of right pulmonary veins.
A patent foramen ovale allows paradoxical embolism: a venous clot crosses to the left side and causes a stroke.

Test yourself

Think of the answer, then tap the question to see it.

How does the ostium secundum form?

By programmed cell death in the upper septum primum, before the ostium primum closes.

Which structure acts as the valve of the foramen ovale?

The septum primum, which closes against the septum secundum when left atrial pressure rises at birth.

What is the most common type of atrial septal defect and its cause?

Secundum ASD: excessive resorption of the septum primum or underdevelopment of the septum secundum.

Why can a patent foramen ovale cause a stroke?

It lets a venous clot pass right to left (paradoxical embolism), especially when right atrial pressure rises.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015); Atrial Septal Defect (ASD) | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.