The fossa ovalis is a thin, oval-shaped depression in the wall between the heart’s two upper chambers (the right and left atria). It marks the spot where the foramen ovale, an opening that allowed blood to bypass the lungs during fetal life, once existed. In most people, this opening seals shut after birth, leaving behind the shallow indent visible on the right atrial side of the septum. The structure is far from a mere anatomical footnote: it plays a starring role in fetal survival, remains unsealed in roughly a quarter of adults, and serves as the preferred entry point for several important cardiac procedures.
What the Fossa Ovalis Looks Like
If you could peer inside the right atrium, you would see a smooth, slightly concave area bordered by a raised muscular ridge. That ridge is called the limbus (sometimes referred to historically as the “annulus of Vieussens,” after the French anatomist Raymond de Vieussens, who first described its prominent margin in the late 1600s).1PubMed Central. Raymond de Vieussens (1641–1715): connoisseur of cardiologic anatomy and pathological forms thereof The depression itself is the fossa ovalis, and its floor is a paper-thin membrane derived from an embryological structure called the septum primum. The muscular rim around it comes from a different structure, the septum secundum.2PubMed. Multimodality imaging anatomy of interatrial septum and mitral annulus
In a study of cadaveric hearts, the fossa was oval-shaped in about 82% of specimens, with an average width of roughly 14.5 mm and height of roughly 12.6 mm. The raised rim was present in 90% of hearts, and about a fifth had a noticeable recess tucked beneath the rim’s edge.3Indian Heart Journal. Morphological study of fossa ovalis and its clinical relevance Measurements obtained during actual cardiac procedures tend to differ slightly depending on the imaging method, with one intracardiac echocardiography study reporting an average vertical diameter closer to 18.5 mm.4PubMed. Shifting of puncture site in the fossa ovalis during radiofrequency catheter ablation: intracardiac echocardiography-guided transseptal left heart catheterization These differences partly reflect whether the heart is relaxed (cadaver) or beating (live patient), and whether the tissue is being gently stretched by a catheter.
Why Fetuses Need a Hole in the Heart
Before birth, a baby’s lungs are filled with fluid and aren’t yet exchanging oxygen. The placenta does that job instead, sending oxygen-rich blood back to the fetus through the umbilical vein. That oxygenated blood needs to reach the left side of the heart so it can be pumped out to the brain and body, but it arrives on the right side. The foramen ovale solves this problem. It is a flap-like opening in the atrial septum that lets the incoming oxygenated blood stream across from the right atrium into the left atrium, bypassing the lungs almost entirely.
The mechanics are elegant. Blood returning from the placenta enters the right atrium and hits a ridge called the crista dividens, which splits the flow. A substantial portion is directed through the foramen ovale into the left atrium.5Seminars in Fetal and Neonatal Medicine. Physiology of the fetal circulation – Section: Foramen ovale This isn’t just a trickle. In near-term fetal sheep, the foramen ovale accounts for the majority of the left ventricle’s output, meaning most of the blood the left side of the heart pumps came through that opening.6PubMed. Foramen ovale blood flow and cardiac function after main pulmonary artery occlusion in fetal sheep Without it, the fetus simply could not deliver enough oxygenated blood to its developing organs.
How the Opening Closes After Birth
The moment a newborn takes its first breaths, the lungs expand and blood begins flooding into the pulmonary circulation. This dramatically drops the resistance in the lung’s blood vessels, which in turn increases blood flow returning to the left atrium. The resulting rise in left atrial pressure, combined with the loss of the directed placental blood flow that had been streaming into the right atrium, pushes the thin flap of septum primum against the muscular rim of the septum secundum, effectively sealing the foramen ovale shut.7Pediatric Research. Transitional circulation and hemodynamic monitoring in newborn infants – Section: Foramen ovale This happens within the first few breaths and heartbeats, making the foramen ovale functionally closed almost immediately.
True anatomical closure, where the tissue actually fuses together permanently, takes longer. Studies in mice have tracked the progressive tissue fusion after birth, showing that the flap eventually adheres to the surrounding septum to ensure the two atrial chambers stay separated so that oxygen-depleted blood on the right side doesn’t mix with oxygenated blood on the left.8PubMed Central. Progressive anatomical closure of foramen ovale in normal neonatal mouse hearts In humans, this fusion process can stretch over months or years. And in a sizable fraction of people, it never completes at all.
When the Opening Never Fully Seals
A patent foramen ovale, or PFO, is the term for a foramen ovale that remains at least partially open in adulthood. “Patent” simply means open or unobstructed. This is remarkably common. In autopsy studies, roughly one in four adults has a PFO. The numbers vary slightly depending on how you look: autopsy series report about 24% prevalence, while imaging studies using different techniques put the figure anywhere from about 15% to 31%.9PubMed Central. Epidemiology of Patent Foramen Ovale in General Population and in Stroke Patients: A Narrative Review The variation partly depends on the sensitivity of the detection method used. Ethnicity may also play a role: a Japanese autopsy study found a PFO prevalence of only about 14%, lower than the roughly 25% typically reported in Western populations.10Circulation Journal. Prevalence of Patent Foramen Ovale in the Japanese Population – Autopsy Study
For most people with a PFO, the flap sits quietly in place and never causes trouble. The pressure in the left atrium is normally a bit higher than in the right, which keeps the flap pushed closed most of the time. But certain situations, like straining, coughing hard, or bearing down, can briefly raise right atrial pressure enough to let a small amount of blood slip through from right to left. In the majority of people, this is completely harmless and goes unnoticed for an entire lifetime. The interesting finding from evolutionary biology is that this incomplete closure may simply be a quirk of how mammals with placentas build their atrial septum, using two overlapping flaps rather than a single solid wall. Researchers have described this two-septum approach as inherently “inefficient” at achieving permanent closure.11Wiley Online Library (Anatomical Record). Evolution and Development of the Atrial Septum
PFO and Stroke Risk
The reason doctors care about PFOs is paradoxical embolism. Normally, a small blood clot forming in a leg vein would travel to the right side of the heart and get caught in the lungs, where it would cause a pulmonary embolism rather than a stroke. But if a PFO is open, that clot can sneak across to the left atrium, get pumped into the arterial circulation, and lodge in a brain artery, causing a stroke. This is especially relevant in so-called cryptogenic strokes, which are strokes with no obvious identifiable cause.12PubMed Central. Patent Foramen Ovale and Cryptogenic Stroke: Integrated Management
For years, PFO-related stroke was considered primarily a young person’s problem. A landmark study published in the New England Journal of Medicine challenged that assumption, finding that the association between PFO and cryptogenic stroke exists in older patients too, not just younger ones.13PubMed. Patent foramen ovale and cryptogenic stroke in older patients Still, it is worth emphasizing that having a PFO does not mean you will have a stroke. The vast majority of people with a PFO never experience any neurological event. The risk becomes clinically significant mainly when a PFO is found in someone who has already had a stroke that can’t be explained by more common causes like atrial fibrillation or blocked arteries.
Closing a PFO to Prevent Recurrent Stroke
For patients who have already had a cryptogenic stroke and are found to have a PFO, the question becomes whether to close it. Randomized controlled trials now provide solid evidence that device-based PFO closure reduces the risk of another stroke compared to medication alone in selected patients.14PubMed. Patent Foramen Ovale Management for Secondary Stroke Prevention: State-of-the-Art Appraisal of Current Evidence A meta-analysis pooling the trial data found that PFO closure cut recurrent stroke risk substantially, though the absolute annual risk of recurrence was low in both groups: about 0.6% per year with closure versus about 1.2% per year with medication.15European Heart Journal. Patent foramen ovale closure vs. medical therapy for cryptogenic stroke: a meta-analysis of randomized controlled trials
The closure procedure itself is minimally invasive. A small umbrella-like device is threaded through a vein, guided into the heart, and deployed across the PFO to plug it. Over time, tissue grows over the device. The procedure does carry its own modest risks, including a small chance of new-onset atrial fibrillation, so the decision to close a PFO is individualized rather than routine.
PFO Versus an Atrial Septal Defect
People sometimes confuse a PFO with an atrial septal defect (ASD), but they are fundamentally different. A PFO is a remnant of normal fetal anatomy; it results from incomplete fusion of two overlapping flaps. A secundum ASD, which is the most common type of ASD, is a true hole in the fossa ovalis region where tissue is actually missing. The distinction matters because their effects on the heart differ dramatically. An ASD typically causes continuous left-to-right blood flow because the left atrium usually operates at higher pressure, which overloads the right side of the heart and can lead to enlarged right chambers and elevated lung pressures. A PFO, by contrast, usually causes only intermittent right-to-left leaking during brief spikes in right atrial pressure, and the right chambers remain normal-sized.16PubMed Central. Should we consider patent foramen ovale and secundum atrial septal defect as different steps of a single anatomo-clinical continuum?
On imaging, the two can also look different. A PFO appears as a channel-like passage between the overlapping flaps, with any contrast dye flowing toward the bottom of the heart. An ASD looks more like a hole punched through a membrane, with contrast flowing straight through perpendicular to the septum.17Journal of Computer Assisted Tomography. Interatrial Shunt Detected in Coronary Computed Tomography Angiography: Differential Features of a Patent Foramen Ovale and an Atrial Septal Defect Accurate distinction is critical, because management paths diverge considerably: ASDs often require closure regardless of symptoms once the shunt is significant, whereas PFOs are typically left alone unless they are linked to a specific clinical problem.
Migraine with Aura and Decompression Sickness
Beyond stroke, a PFO has been linked to two conditions that might seem surprising: migraine with aura and decompression sickness in divers. The migraine connection is strongest when a PFO is paired with an atrial septal aneurysm (ASA), a condition in which the thin floor of the fossa ovalis balloons back and forth with each heartbeat. In a large observational study, the combination of PFO and ASA was found in about 18% of people who experienced migraine with aura, compared to only about 6% of people without migraine. A PFO without an aneurysm, however, was not significantly associated with migraine with aura on its own.18PubMed Central. Patent Foramen Ovale With Atrial Septal Aneurysm Is Strongly Associated With Migraine With Aura: A Large Observational Study This suggests the aneurysm component is doing something important, possibly facilitating larger or more frequent right-to-left passage of microemboli or vasoactive chemicals that trigger aura.
Atrial septal aneurysms themselves are worth noting. A large French multicenter study found that over half of these aneurysms overlap the fossa ovalis region, the vast majority protrude into the right atrium, and about 58% of patients with one had a history of embolic events.19International Journal of Cardiology. Atrial septal aneurysm. Morphological characteristics in a large population Their high mobility appears to be the feature most associated with embolic risk.
For divers, the issue is nitrogen bubbles. During ascent, dissolved nitrogen comes out of solution and forms tiny bubbles in venous blood. Normally these get filtered out by the lungs. If a PFO lets them cross to the arterial side, the bubbles can reach the brain, spinal cord, or inner ear, causing decompression sickness. A meta-analysis found that divers with right-to-left shunts were more than five times as likely to experience decompression sickness compared to those without.20EuroIntervention. European position paper on the management of patients with patent foramen ovale. Part II – Decompression sickness, migraine, arterial deoxygenation syndromes and select high-risk clinical conditions This is why some diving-medicine professionals recommend PFO screening for divers who have had unexplained decompression illness.
The Fossa Ovalis as a Gateway for Cardiac Procedures
Cardiologists and electrophysiologists depend on the fossa ovalis every day. The thin floor of the fossa is the preferred puncture site for transseptal access, a technique in which a catheter is pushed through the atrial septum from the right atrium into the left. This is the standard approach for procedures like catheter ablation of atrial fibrillation, mitral valve interventions, and left atrial appendage closure. The membrane at the fossa ovalis is the only part of the septum thin enough to puncture safely; the surrounding muscular rim and the rest of the septum are too thick.
During the procedure, a specialized needle (the Brockenbrough needle) is advanced against the fossa, creating a visible “tenting” of the membrane on imaging before the puncture is made.21PubMed. Radiofrequency puncture of the fossa ovalis for resistant transseptal access Real-time three-dimensional echocardiography has significantly improved the ability to visualize the septum’s anatomy during these procedures, giving operators an en-face view of the fossa and its borders.22PubMed. Imaging the atrial septum using real-time three-dimensional transesophageal echocardiography: technical tips, normal anatomy, and its role in transseptal puncture Newer techniques have even used the unique electrical characteristics of the fossa ovalis tissue to guide the puncture in three dimensions without needing fluoroscopy or echocardiography at all.23PubMed. 3-Dimensional Transseptal Puncture Based on Electrographic Characteristics of Fossa Ovalis: A Fluoroscopy-Free and Echocardiography-Free Method
Precise puncture location matters. One study found that in 89% of patients the needle drifted toward the upper edge of the fossa during the procedure, a tendency operators need to anticipate to avoid puncturing outside the safe zone.4PubMed. Shifting of puncture site in the fossa ovalis during radiofrequency catheter ablation: intracardiac echocardiography-guided transseptal left heart catheterization The stakes are real. A misplaced puncture can hit the aortic root, the thick muscular wall above the fossa, creating a potentially life-threatening perforation that requires emergency repair.24PubMed Central. Transcatheter Repair of Iatrogenic Aortic Perforation Complicating Transseptal Puncture for a Catheter Ablation of Atrial Arrhythmia
Balloon Septostomy in Newborns
Sometimes, instead of closing a hole in the fossa ovalis, doctors need to create or enlarge one. Babies born with certain congenital heart defects, most classically transposition of the great arteries (where the aorta and pulmonary artery are connected to the wrong ventricles), depend on mixing between the atrial chambers to survive.25PubMed Central. Balloon atrial septostomy through internal jugular vein in a 45-day-old child with transposition of great arteries In this defect, oxygenated and deoxygenated blood circulate in two separate loops that barely communicate, and the baby turns dangerously blue.
A balloon atrial septostomy, first developed by William Rashkind in the 1960s, addresses this by threading a balloon catheter through a vein into the right atrium, across the fossa ovalis, and into the left atrium. The balloon is then inflated and pulled back forcefully, tearing open the thin septum primum floor to create a larger opening. The immediate effect is dramatic: in one study, oxygen saturation jumped by an average of nearly 19 percentage points after the procedure.26Canadian Journal of Cardiology. Balloon Atrial Septostomy: Does the Balloon Size Matter? This buys time until definitive surgical repair can be performed.
Unusual Tissue Inside the Septum
The fossa ovalis region has turned out to contain more than just ordinary heart muscle. In a detailed histological study of 40 human hearts, researchers found clusters of unusual cells in the fossa ovalis, its rims, and the flap valve in 95% of specimens. These cells looked different from regular heart muscle cells: some were tortuous and horseshoe-shaped, others were rounded with pale cytoplasm. Under further analysis, the cells tested positive for markers typically associated with the heart’s electrical conduction system, and electron microscopy confirmed that they had structural features resembling pacemaker-like cells.27PLOS ONE. Evidence of Specialized Tissue in Human Interatrial Septum: Histological, Immunohistochemical and Ultrastructural Findings The functional significance of these cells is still being worked out, but their presence raises the possibility that the fossa ovalis region plays some role in the heart’s electrical activity that hasn’t been fully appreciated.
The Foramen Ovale in Other Species
Humans are not the only mammals with a fossa ovalis, but the structure varies across species in ways that reflect different evolutionary pressures. A comparative anatomy study of cetaceans (whales and dolphins) found that the tissue fold derived from the septum primum at the foramen ovale looked different in toothed whales versus baleen whales. In toothed whales, it formed a tunnel-like structure with thin threads anchoring it to the septum. In baleen whales, it was more sleeve-like, with a network of threads covering the far end. Strikingly, similar structures were found in newborn hippopotamuses, which makes sense given that hippos are among the closest living relatives of whales.28PubMed Central. Comparative anatomy of the foramen ovale in the hearts of cetaceans
The equine heart offers another useful comparison. In horses, the fossa ovalis has been measured at roughly 21 mm by 11 mm, with a prominent limbus visible in all specimens studied.29Scientific Reports. Morphological variations of the interatrial septum and potential implications in equine cardiology These morphological variations across species are more than curiosities: they help veterinary cardiologists understand how shunts and septal defects behave in animals they treat, and they give human cardiologists a broader evolutionary lens for understanding why our own septum is built the way it is, and why it so often fails to close completely.