---
title: "Atrial Septum | Anatomy and Physiology I"
description: "Atrial septum is the wall between the heart's atria, guiding fetal blood flow through the foramen ovale and preventing mixing after birth in A&P I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/atrial-septum"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 19"
---

# Atrial Septum | Anatomy and Physiology I

## Definition

The atrial septum is the wall that separates the right and left atria of the heart. In Anatomy and Physiology I, you study it as part of fetal heart development and normal chamber separation.

## What It Is

The atrial septum is the tissue wall between the right atrium and left atrium of the heart. In Anatomy and Physiology I, you usually meet it while studying how the fetal heart forms and how blood flow changes after birth.

This septum is not just a simple divider. During development, it forms from two overlapping parts, the septum primum and septum secundum. Those structures grow in a way that leaves a temporary opening called the foramen ovale. That opening matters before birth because fetal blood does not need to go through the lungs yet.

In the fetus, oxygen comes from the placenta, so blood is routed to bypass the lungs, which are not yet doing gas exchange. The foramen ovale lets blood move from the right atrium to the left atrium, so more oxygen-rich blood can be sent to the body. After birth, the first breaths change pressure in the heart, and the flap-like opening closes as the atrial septum becomes a firm divider.

A useful way to think about the atrial septum is as both a structure and a process. Structurally, it is the partition between the atria. Developmentally, it is part of a carefully timed remodeling sequence that helps the heart switch from fetal circulation to postnatal circulation. If that sequence does not finish correctly, the opening can remain partly or fully open.

That is where atrial septal defects come in. A defect in the septum can allow blood to pass between the atria when it should not, which can change pressure patterns and blood flow through the heart. In class diagrams, you may see this as a left-to-right shunt after birth, because left atrial pressure is usually higher than right atrial pressure once the lungs are functioning.

So when you see atrial septum in A&P I, think about chamber separation, fetal bypass of the lungs, and the shift to normal circulation after delivery. It connects anatomy, embryology, and cardiovascular function in one place.

## Why It Matters

The atrial septum matters because it is one of the clearest examples of how heart anatomy and physiology work together. You are not just memorizing a wall in the heart, you are tracing why that wall forms, why it has a temporary opening in the fetus, and what happens when closure goes wrong.

It also gives you a clean way to understand fetal circulation. The lungs are not doing gas exchange before birth, so fetal blood flow is arranged differently from adult blood flow. The atrial septum, through the foramen ovale, helps explain how the fetus routes blood efficiently without using the lungs.

This term also shows up in congenital heart defect discussions. If you can explain the atrial septum, you can make sense of why an atrial septal defect can cause abnormal mixing of blood or extra workload on the heart. That kind of cause-and-effect thinking is a big part of Anatomy and Physiology I, especially when you compare normal structure to a defect.

It is also a helpful visual landmark in heart diagrams and fetal circulation charts. Once you know where the atrial septum sits, it becomes easier to track blood moving through the atria, through the foramen ovale, and eventually into the ventricles and out to the body.

## Connections

### [Foramen Ovale](/anatomy-physiology/key-terms/foramen-ovale)

The foramen ovale is the temporary opening in the atrial septum during fetal life. It lets blood move from the right atrium to the left atrium, bypassing the lungs before birth. After delivery, rising pressure in the left atrium helps close it, and the opening normally seals as circulation changes.

### [Septum Primum](/anatomy-physiology/key-terms/septum-primum)

The septum primum is one of the two embryonic structures that helps form the atrial septum. It grows downward during development and contributes to the flap-like structure associated with the foramen ovale. If you are tracing heart development step by step, this is one of the first pieces to identify.

### [Atrioventricular Cushions](/anatomy-physiology/key-terms/atrioventricular-cushions)

Atrioventricular cushions are another embryonic structure you often see in the same heart-development unit. They help form septal and valve structures in the heart, so they connect to the broader theme of how chambers and openings become separate. They are not the atrial septum itself, but they are part of the same developmental story.

### [Interatrial Septum](/anatomy-physiology/key-terms/interatrial-septum)

Interatrial septum is another name for the wall between the atria. In many class materials, this term is used interchangeably with atrial septum, so it helps to recognize both. If a diagram or lecture note uses the alternate wording, the anatomy being described is the same general partition.

## On the AP Exam

A labeled diagram question may ask you to point to the wall between the atria, identify the foramen ovale, or explain why fetal blood can bypass the lungs. A good answer links structure to function: the atrial septum separates the right and left atria, and the fetal opening in it lets blood move across before birth.

You may also see a short-answer item about congenital heart defects. In that case, you should be ready to explain what happens if the septum does not close properly after birth, especially how abnormal blood flow can occur between atria. When you study heart development, practice naming the septum primum, septum secundum, and foramen ovale in the right order on a diagram.

## Atrial Septum vs Interatrial Septum

These are commonly treated as the same thing in A&P I. Interatrial septum is the more technical name for the wall between the atria, while atrial septum is the shorter, more common term. If a quiz uses one or the other, the structure being referenced is the same partition between the right and left atria.

## Key Takeaways

- The atrial septum is the wall that separates the right atrium from the left atrium.
- During fetal development, it includes a temporary opening called the foramen ovale so blood can bypass the lungs.
- After birth, pressure changes in the heart help the opening close as normal adult circulation begins.
- The septum primum and septum secundum are the embryonic structures involved in forming the atrial septum.
- If the atrial septum does not form or close correctly, blood can move abnormally between the atria.

## FAQs

### What is the atrial septum in Anatomy and Physiology I?

The atrial septum is the wall between the right atrium and left atrium. In A&P I, you usually study it in the context of fetal heart development because it helps explain how blood flow is arranged before and after birth.

### What is the foramen ovale in relation to the atrial septum?

The foramen ovale is a temporary opening in the fetal atrial septum. It allows blood to pass from the right atrium to the left atrium so the fetus can bypass the lungs, which are not yet doing gas exchange.

### How does the atrial septum close after birth?

When a baby takes the first breaths, pressure in the left atrium rises as the lungs fill and begin working. That pressure change helps press the flap closed, and the opening normally seals over time.

### What happens if the atrial septum does not close properly?

If the atrial septum stays open, blood can move between the atria when it should not. That can create abnormal blood flow patterns and may put extra strain on the heart, which is why atrial septal defects matter in heart anatomy.

## Related Study Guides

- [19.5 Development of the Heart ](/anatomy-physiology/unit-19/5-development-heart/study-guide/TbeMMLg8CcS41duk)

## About This Document

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