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Interatrial Septum

What is it, Location, Function and More

Author:Anna Hernández, MD

Editors:Alyssa Haag,Ian Mannarino, MD, MBA

Illustrator:Jillian Dunbar

Copyeditor:Sadia Zaman, MBBS, BSc


What is the interatrial septum?

The interatrial septum is a thin wall of tissue that separates the two upper chambers of the heart, called the right and left atria

Where is the interatrial septum located?

The interatrial septum is located between the right and left atria. In the fully formed heart, the interatrial septum has a central depression called the oval fossa (i.e., fossa ovalis), which is a remnant of the foramen ovale, a structure that connected the atria during fetal development. The oval fossa is surrounded by a muscular rim, which makes up most of the interatrial septum and connects to the atrial walls. 

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What is the function of the interatrial septum?

In adults, the main function of the interatrial septum is to separate the two atrial chambers so that there is no shunting (movement) of blood between them. During fetal development, however, its function is just the opposite. 

The interatrial septum forms during the first weeks of fetal development, when a strip of tissue called the septum primum begins to grow downward between the left and right atria. Initially, the septum primum does not fully occlude the space between the two cavities, leaving an opening called the ostium primum (i.e., first opening). As the septum primum continues to grow downward, the ostium primum progressively narrows. Before the ostium primum is completely closed, another gap called the ostium secundum (i.e., second opening) appears in the upper area of the septum. Next, a septum secundum begins to form adjacent to the septum primum, covering the ostium secundum and leaving a small opening called the foramen ovale. Since the lungs aren’t fully developed during fetal development, the foramen ovale allows oxygenated blood from the placenta to be shunted directly from the right atria into the left atria, bypassing the pulmonary circulation. The oxygenated blood then goes to the left ventricle to be pumped through the aorta to the rest of the body. After birth, the septum secundum and septum primum slap shut due to the elevated pressures in the left heart, and then fuse to close the foramen ovale, so that the individual can rely on their own lungs for oxygen. If the foramen ovale fails to fuse correctly, this is known as a patent foramen ovale (PFO).

What disorders affect the interatrial septum?

An atrial septal defect, or ASD, describes a condition where the interatrial septum doesn’t completely close and remains open even after birth. The most common atrial septal defect arises due to a problem with the formation of the septum secundum, and is called an ostium secundum defect. This defect accounts for about 10-15% of all congenital heart defects and is the most common congenital heart defect in adults. Fewer cases of atrial septal defect are due to the malformation of the ostium primum. This congenital heart defect is most often seen in individuals with Down syndrome, and is commonly found in association with other congenital cardiac anomalies.

What are the most important facts to know about the interatrial septum?

The interatrial septum is a thin wall of tissue that separates the right and left atria of the heart. In adult life, its main function is to separate the two atrial chambers so that there is no shunting of blood between them. During fetal development, however, the interatrial septum allows blood to flow from the right atrium to the left atrium, in order for oxygenated blood from the placenta to directly reach the left atria and ventricle; this is achieved through bypassing the pulmonary circulation

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Related links

Anatomy of the heart
Fetal circulation
Atrial septal defect
Acyanotic congenital heart defect: Pathology review

Resources for research and reference

Drake, R., Vogl, A. W., & Mitchell, A. (2019). Gray’s anatomy for students: With student consult online access (4th ed.). Elsevier - Health Sciences Division.

Hansen, J. T., Netter, F. H. 1., & Machado, C. A. G. (2019). Netter's clinical anatomy (4th ed.). Philadelphia, PA: Elsevier.

Naqvi, N., McCarthy, K. P., & Ho, S. Y. (2018). Anatomy of the atrial septum and interatrial communications. Journal of Thoracic Disease, 10(S24), S2837–S2847. DOI:10.21037/jtd.2018.02.18