---
title: "Septum Primum | Anatomy and Physiology I"
description: "Septum primum is the first atrial septum in fetal heart development, forming the foramen ovale pathway and helping shift circulation at birth."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/septum-primum"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 28"
---

# Septum Primum | Anatomy and Physiology I

## Definition

Septum primum is the first thin membrane that helps divide the fetal atria in Anatomy and Physiology I. It forms part of the foramen ovale opening, which lets blood bypass the lungs before birth.

## What It Is

Septum primum is the first thin sheet of tissue that grows down inside the developing heart to help separate the right and left atria. In Anatomy and Physiology I, you usually meet it while tracing how the fetal heart changes from a single pumping tube into a four-chambered organ.

At first, the atria are not fully separated. The septum primum grows from the roof of the common atrium toward the atrioventricular cushions, leaving an opening below it called the foramen primum. As that opening closes, small spaces appear higher in the septum primum and merge into the foramen ovale. That opening is not a mistake, it is part of normal fetal circulation.

The reason the fetus needs this opening is simple: the lungs are not working yet. Blood coming back to the heart from the placenta needs a way to bypass the pulmonary circuit, since the fetal lungs have high resistance and are filled with fluid. The foramen ovale lets blood move from the right atrium to the left atrium, so oxygenated blood from the placenta can reach the body more efficiently.

After the baby is born, the first breaths change everything. The lungs expand, pulmonary vascular resistance drops, and pressure in the left atrium rises. That pressure pushes the septum primum against the septum secundum, closing the foramen ovale functionally. Later, the two septa may fuse and leave a fossa ovalis as the adult remnant.

A helpful way to think about septum primum is as the first flap in a two-part atrial partition. It does not work alone, and it is not the final adult wall between the atria. It is the early fetal structure that makes the temporary shunt possible and then helps seal it off when breathing starts.

## Why It Matters

Septum primum shows up any time you are connecting fetal anatomy to newborn circulation. If you know what it does, the whole birth transition makes more sense, especially why the heart can work before the lungs are fully online and why that arrangement has to change fast after delivery.

It also gives you a clean way to explain common congenital heart findings. If the septum primum does not form normally, or if the atrial septal structures fail to close the opening properly, blood can continue to pass between the atria when it should not. That can affect oxygen delivery and may lead to a murmur or later cardiac issues.

This term also connects several other heart-development topics. Septum primum does not develop in isolation. It is tied to the atrial septum, the septum secundum, the foramen ovale, and the pressure changes that happen at birth. Once you can place it in that sequence, embryology questions become much easier to track.

In lab images, diagrams, and test questions, septum primum is often the structure you identify when the prompt asks about the first atrial septal partition or the flap that helps form the foramen ovale. If you can trace its position and purpose, you can explain both fetal shunting and postnatal closure without guessing.

## Connections

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

The foramen ovale is the opening associated with the septum primum that lets blood pass from the right atrium to the left atrium before birth. Septum primum forms the flap-like part of this fetal route, so the two terms are usually taught together. If you see a question about blood bypassing the lungs in the fetus, this is the pathway to name.

### [Septum Secundum](/anatomy-physiology/key-terms/septum-secundum)

Septum secundum forms after the septum primum and overlaps it, creating the one-way arrangement that keeps fetal shunting efficient. The septum primum acts more like a valve flap, while the septum secundum adds the partial cover that makes closure possible after birth. Together, they explain why the foramen ovale can function prenatally and close later.

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

The atrial septum is the final wall that separates the right and left atria. Septum primum is the first piece in building that wall, so it is one step in a longer developmental sequence rather than the whole structure. If you are studying heart development, this is the larger anatomical category that septum primum belongs to.

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

Atrioventricular cushions help form the lower part of the atrial septum and the atrioventricular septum. Septum primum grows toward these cushions during development, and that relationship helps close the foramen primum early in fetal life. If the cushions develop abnormally, atrial partitioning can be affected too.

## On the AP Exam

A quiz question may show a fetal heart diagram and ask you to identify the structure that first partitions the atria or forms the flap associated with the foramen ovale. A short-answer item might ask you to trace what happens when the baby takes the first breath, so you would connect septum primum to rising left atrial pressure and closure of the fetal shunt.

In labeling exercises, look for the first thin septal membrane growing toward the atrioventricular cushions. In case-based questions about persistent opening between the atria, you would use septum primum as part of the explanation for an atrial septal defect or incomplete closure of the fetal pathway. The best answers move in order: fetal blood flow, birth transition, then postnatal closure.

## septum primum vs Septum Secundum

These are easy to mix up because both are involved in forming the atrial septum. Septum primum is the first septal flap and helps create the foramen ovale, while septum secundum forms beside it and overlaps it to support closure after birth. If a question asks about the earliest atrial partition, choose septum primum.

## Key Takeaways

- Septum primum is the first structure that helps divide the fetal atria.
- It forms the flap associated with the foramen ovale, which lets blood bypass the lungs before birth.
- After birth, rising left atrial pressure pushes the septum primum against the septum secundum and closes the opening functionally.
- The adult remnant of this fetal opening is the fossa ovalis.
- If atrial septal development goes wrong, blood can keep shunting between the atria and create a congenital heart defect.

## FAQs

### What is septum primum in Anatomy and Physiology I?

Septum primum is the first membrane that develops to separate the atria in the fetal heart. It helps form the foramen ovale, which allows blood to bypass the lungs before birth. After birth, it contributes to closure of that opening.

### How is septum primum different from septum secundum?

Septum primum forms first and acts like a flap over the fetal opening between the atria. Septum secundum forms later and overlaps it, creating the structure that helps the foramen ovale close after birth. If you are asked which comes first, septum primum is the answer.

### What does septum primum do before birth?

Before birth, it helps keep a passage open between the right and left atria so blood can move around the nonfunctioning fetal lungs. That shunt is normal in fetal circulation because the placenta, not the lungs, is doing the oxygen exchange.

### What happens if septum primum does not close properly?

If the atrial opening does not close the way it should, blood can keep moving between the atria after birth. That can be part of an atrial septal defect, which may be found on imaging, during a physical exam, or in a case study about abnormal heart sounds.

## Related Study Guides

- [28.5 Adjustments of the Infant at Birth and Postnatal Stages ](/anatomy-physiology/unit-28/5-adjustments-infant-birth-postnatal-stages/study-guide/Go04YK5yLiBAXVS2)
- [19.5 Development of the Heart ](/anatomy-physiology/unit-19/5-development-heart/study-guide/TbeMMLg8CcS41duk)

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