---
title: "Fossa Ovalis | Anatomy and Physiology I"
description: "Fossa ovalis is the shallow depression in the interatrial septum where the fetal foramen ovale closed after birth in Anatomy and Physiology I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/fossa-ovalis"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 19"
---

# Fossa Ovalis | Anatomy and Physiology I

## Definition

The fossa ovalis is a shallow depression in the interatrial septum of the heart. It marks the closed opening of the fetal foramen ovale after birth.

## What It Is

The fossa ovalis is the small depression on the interatrial septum, the wall that separates the right and left atria in the heart. In Anatomy and Physiology I, you usually see it as a postnatal landmark that marks where the fetal foramen ovale used to be open.

Before birth, the foramen ovale let blood move from the right atrium to the left atrium. That shortcut made sense in the fetus because the lungs were not yet doing gas exchange. Blood was being routed around the pulmonary circuit, since oxygen came from the placenta instead of the lungs.

After birth, the first breaths change pressure in the heart. Pressure in the left atrium rises as blood returns from the lungs, and the flap that covered the foramen ovale gets pressed shut against the septum. Over time, that closure leaves a thin remnant and a shallow pit, which is the fossa ovalis.

So the fossa ovalis is not a hole. It is the visible trace of a fetal opening that is supposed to seal. That distinction matters because the name can make it sound like blood still passes through there, when in a healthy adult it does not.

When you look at a heart model or diagram, the fossa ovalis is one of the cues that you are looking at the right atrium side of the interatrial septum. It sits as a smooth oval depression and is often surrounded by a raised rim, which is the remnant of the old fetal flap.

A useful way to think about it is as a scar from normal development. The heart changes from a fetal circulation built for the placenta into an adult circulation built for the lungs, and the fossa ovalis is one of the easiest anatomical signs of that transition.

## Why It Matters

The fossa ovalis matters because it connects anatomy to fetal circulation and postnatal circulation in one small structure. If you can identify it, you are not just memorizing a label, you are recognizing evidence of how the heart changes at birth.

It also helps you keep the atria straight on diagrams. A&P questions often ask you to find the interatrial septum, explain how fetal blood bypassed the lungs, or identify the remnant of a closed fetal shunt. The fossa ovalis gives you a landmark for all of that.

This term also supports pathology thinking. If the fetal opening does not close completely, a person may have an atrial septal defect or a patent foramen ovale. Those conditions are not the same as the normal fossa ovalis, but they make the normal structure much easier to remember because they show what happens when closure does not finish correctly.

In lab, the term shows up when you are tracing blood flow through the chambers of the heart, comparing fetal and adult circulation, or labeling a heart model. It is a small structure, but it sits right at the intersection of development, anatomy, and cardiovascular physiology.

## Connections

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

The foramen ovale is the fetal opening that the fossa ovalis marks after closure. In the fetus, it allowed blood to move from the right atrium to the left atrium and bypass the lungs. After birth, that opening should seal, leaving the fossa ovalis as the visible remnant.

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

The fossa ovalis sits on the interatrial septum, the wall between the atria. If you know where the septum is, you can place the fossa ovalis correctly on a heart diagram. That makes it easier to distinguish the right atrial wall from nearby structures.

### Atrial Septal Defect

An atrial septal defect is an abnormal opening in the septum between the atria. The fossa ovalis is normal, but it is closely related because incomplete closure in that area can lead to a defect. This comparison helps you separate a healthy post-birth remnant from a true hole.

### [Atrioventricular Septum](/anatomy-physiology/key-terms/atrioventricular-septum)

The atrioventricular septum is another septal structure you may see while studying heart anatomy, but it is not the same as the interatrial septum. Comparing it with the fossa ovalis helps you keep track of which part of the heart wall you are identifying and what chambers it separates.

## On the AP Exam

A quiz item may show a heart diagram and ask you to label the shallow depression in the interatrial septum, or to explain what fetal structure it marks. You might also get a short question about why the opening is present before birth but not functioning after birth. On lab practicals, this usually appears as a visual ID on a heart specimen or model. In written questions, the move is to connect the fossa ovalis to fetal circulation, then explain that it is the remnant of a closed foramen ovale rather than an active passageway in the adult heart.

## fossa ovalis vs Foramen Ovale

These are closely related, but they are not the same thing. The foramen ovale is the fetal opening between the atria, while the fossa ovalis is the adult depression left behind after that opening closes. If a question asks about blood flow in the fetus, think foramen ovale. If it asks about the adult heart landmark, think fossa ovalis.

## Key Takeaways

- The fossa ovalis is a shallow depression in the interatrial septum of the adult heart.
- It marks the site where the fetal foramen ovale used to be open before birth.
- In a healthy adult, the fossa ovalis is a closed remnant, not an opening for blood flow.
- It is a useful landmark when you are labeling heart anatomy or comparing fetal and adult circulation.
- If closure is incomplete, the area can be associated with an atrial septal defect or other septal opening.

## FAQs

### What is fossa ovalis in Anatomy and Physiology I?

The fossa ovalis is a shallow depression in the wall between the atria of the heart. It is the leftover mark of the fetal foramen ovale after that opening closes after birth. In adult anatomy, it is a landmark, not a passage for blood.

### Is the fossa ovalis the same as the foramen ovale?

No. The foramen ovale is the fetal opening that lets blood bypass the lungs before birth. The fossa ovalis is the remnant left behind after that opening closes. They are connected, but one is the opening and the other is the closed mark.

### Where is the fossa ovalis located?

It is located on the interatrial septum, the wall separating the right and left atria. On a heart model or diagram, you look on the inside of the right atrium side of that septum. It usually appears as a smooth, oval depression with a slight rim.

### Why does the fossa ovalis matter in fetal circulation?

It matters because it shows where the fetal foramen ovale used to allow blood to bypass the lungs. That bypass was normal before birth since the fetus gets oxygen from the placenta, not from breathing air. After birth, the lungs take over and the opening closes, leaving the fossa ovalis.

## Related Study Guides

- [19.1 Heart Anatomy ](/anatomy-physiology/unit-19/1-heart-anatomy/study-guide/rkGmPKJ0SjTIT3Kf)

## About This Document

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