---
title: "Foramen Ovale | Anatomy and Physiology I"
description: "Foramen ovale is the fetal heart opening that shunts blood from the right atrium to the left atrium, bypassing the lungs before birth in A&P I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/foramen-ovale"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 19"
---

# Foramen Ovale | Anatomy and Physiology I

## Definition

The foramen ovale is a fetal opening between the right and left atria that lets blood bypass the lungs before birth. In Anatomy and Physiology I, it shows how fetal circulation changes after delivery.

## What It Is

In Anatomy and Physiology I, the foramen ovale is an opening in the interatrial septum of the fetal heart that lets blood move from the right atrium into the left atrium. That route bypasses the lungs, which are not yet doing gas exchange before birth.

This shortcut makes sense in fetal circulation. A fetus gets oxygen from the placenta, not from breathing air. Because the lungs are fluid-filled and collapsed, pulmonary blood vessels are tightly constricted, so blood has a hard time moving through them. The foramen ovale gives part of that blood a faster path to the left side of the heart, where it can be pumped out to the body.

The flow pattern matters. Oxygen-rich blood from the placenta travels through the umbilical vein, enters fetal circulation, and is directed toward the heart. Much of it is guided through the foramen ovale so the most oxygenated blood can reach the left atrium, left ventricle, and then the brain and heart. That is one reason fetal circulation is organized differently from postnatal circulation.

The opening does not stay open forever. After birth, the first breaths expand the lungs and lower pulmonary vascular resistance. More blood moves into the lungs, the pressure in the left atrium rises, and the pressure difference pushes the flap of the foramen ovale closed. In many people this closure becomes permanent as the septa fuse.

If the opening does not seal completely, the result is a patent foramen ovale, or PFO. A PFO can stay silent for life, but in some cases it can allow a clot to pass from the right side of the heart to the left side and enter systemic circulation. That is why the foramen ovale shows up not just in fetal development, but also in discussions of congenital heart anatomy and certain adult cardiovascular risks.

## Why It Matters

The foramen ovale is one of the clearest examples of how fetal anatomy matches fetal function. Instead of forcing blood through inactive lungs, it reroutes circulation so the fetus can use the placenta as the gas-exchange organ. That makes it a great checkpoint for understanding why the fetal heart and great vessels are built with temporary shortcuts.

It also connects directly to birth transition. When you study the first breath, falling pulmonary vascular resistance, and rising left atrial pressure, the foramen ovale gives you a concrete structure to attach those changes to. If you can trace why it closes, you can usually explain the bigger switch from fetal to neonatal circulation.

This term also shows up when you compare normal development with congenital heart problems. If the foramen ovale remains open, it is easier to understand how a small anatomical remnant can matter in a clinical setting, especially when pressure changes or a clot are involved.

## Connections

### Fetal Circulation

The foramen ovale is one of the main fetal shunts in fetal circulation. Together with other bypass routes, it helps blood avoid the lungs and liver so oxygen from the placenta reaches the tissues that need it most. If you understand fetal circulation as a whole, the foramen ovale makes more sense as part of a system, not an isolated hole in the heart.

### [Ductus Arteriosus](/anatomy-physiology/key-terms/ductus-arteriosus)

Both the foramen ovale and ductus arteriosus help reroute fetal blood away from the lungs, but they do it in different places. The foramen ovale sends blood from right atrium to left atrium, while the ductus arteriosus connects the pulmonary trunk to the aorta. On diagrams and exam questions, students often need to tell those two shunts apart.

### [Atrial Septum](/anatomy-physiology/key-terms/atrial-septum)

The foramen ovale sits in the atrial septum, the wall that separates the right and left atria. In fetal life, part of that septal region functions as a flap-like opening. After birth, the septum usually seals and leaves the fossa ovalis as a remnant. This connection helps you recognize where the structure is located and what remains afterward.

### Patent Ductus Arteriosus

Patent ductus arteriosus is another failure of a fetal shunt to close after birth, so it is a useful comparison for foramen ovale problems. The two defects are not the same structure, but both involve leftover fetal circulation patterns. Comparing them helps you sort out which vessel or opening is supposed to close and what happens if it does not.

## On the AP Exam

A quiz or lab practical may ask you to identify the foramen ovale on a heart diagram, explain its function in fetal circulation, or predict what changes after birth. You might also see a short case about a newborn and need to connect the first breath to decreased pulmonary vascular resistance and closure of the opening.

In written answers, the move is simple: name the structure, say that it shunts blood from the right atrium to the left atrium before birth, and explain that it closes as lung expansion changes pressure in the heart. If the question mentions a persistent opening, connect it to patent foramen ovale and possible circulation problems.

## foramen ovale vs Ductus Arteriosus

The foramen ovale and ductus arteriosus are both fetal shunts, but they bypass the lungs in different ways. The foramen ovale is an opening between the atria, while the ductus arteriosus is a vessel between the pulmonary trunk and the aorta. If a question asks where blood is moving, that distinction usually gives away the answer.

## Key Takeaways

- The foramen ovale is a fetal opening between the right and left atria that lets blood bypass the lungs before birth.
- It matters because fetal lungs are not yet used for gas exchange, so the placenta does the oxygen work instead.
- After birth, lung expansion lowers pulmonary resistance and the pressure change helps the opening close.
- If the foramen ovale does not fully close, it becomes a patent foramen ovale, which can matter in some cardiovascular cases.
- On diagrams, remember that this structure is in the atrial septum, not the ventricle or the great vessels.

## FAQs

### What is foramen ovale in Anatomy and Physiology I?

The foramen ovale is a fetal opening in the interatrial septum that lets blood move from the right atrium to the left atrium. It helps blood bypass the nonfunctioning fetal lungs before birth. After delivery, it usually closes as breathing begins and pressures in the heart change.

### What is the difference between the foramen ovale and ductus arteriosus?

Both are fetal shortcuts, but they are in different places. The foramen ovale connects the atria, while the ductus arteriosus connects the pulmonary trunk to the aorta. If you are labeling a heart diagram, that location is the fastest way to tell them apart.

### Why does the foramen ovale close after birth?

The first breaths expand the lungs and drop pulmonary vascular resistance, so more blood flows to the lungs. That change raises pressure on the left side of the heart relative to the right side. The pressure shift presses the flap of the foramen ovale shut, and it usually seals over time.

### What happens if the foramen ovale does not close completely?

If it stays open, it is called a patent foramen ovale. Many people never notice it, but in some cases it can let a clot cross from the right side of the heart to the left side and enter systemic circulation. That is why it shows up in some cardiovascular and stroke discussions.

## Related Study Guides

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

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