What Are the Flaps on the Front of the Atria Called?

The small, ear-shaped flaps projecting from the front of each atrium are called atrial appendages, historically known as auricles. Every heart has two: a left atrial appendage (LAA) and a right atrial appendage (RAA). The name “auricle” comes from the Latin word for “little ear,” and if you look at the heart from the front, the resemblance is easy to see. Although they look like minor anatomical footnotes, these pouches turn out to be surprisingly important, playing roles in blood pressure regulation, hormone release, and, in the case of the left one, stroke risk.

What Atrial Appendages Look Like

Both atrial appendages are muscular pouches that hang off the main atrial chambers, but they are not identical twins. The left atrial appendage is typically a long, narrow, finger-like pouch with a relatively small opening into the left atrium. A large autopsy study of 500 hearts found that about half of all left atrial appendages have two lobes, though the number can range from one to four, and those lobes often sit in different planes of the heart rather than neatly side by side.¹ The interior walls are lined with ridges of muscle called pectinate muscles, and most of these ridges are at least a millimeter wide.

The right atrial appendage is broader and more triangular. It tends to be physically larger than the left, with more pectinate muscles on average, though the muscle density per unit area is actually lower.² Inside the right appendage, two prominent muscle bundles stand out: the crista terminalis (a thick ridge that runs along the junction between the smooth and rough-walled parts of the right atrium) and the sagittal bundle, which connects the crista terminalis to the tip of the appendage.³ These bundles are not just structural scaffolding; they create preferential pathways for electrical signals, helping conduct the heartbeat’s initial impulse from the sinus node forward through the right atrium.

Where They Come From

The atrial appendages are actually the oldest part of the atrial chambers, embryologically speaking. The left atrial appendage develops from the left wall of the primary atrium, which forms during roughly the fourth week of embryonic development.⁴ The smooth-walled portions of the atria that make up most of the chamber volume come later, derived from the pulmonary veins (on the left) and the sinus venosus (on the right). So the wrinkly, muscular appendages are the remnants of the original embryonic atria, while the larger, smooth-walled chambers are evolutionary add-ons.

Leonardo da Vinci was among the first to appreciate that these structures were more than passive conduits. Through careful dissection, he recognized that the upper chambers he called “auricles” were made of tissue that could both expand passively under stress and contract actively, making them functional muscular chambers rather than simple draining tubes.⁵ That insight held up remarkably well over the centuries.

What the Appendages Actually Do

For a long time, surgeons treated the atrial appendages as expendable tissue, something you could clip off during heart surgery without a second thought. Research over the past few decades has complicated that view. The appendages serve at least two distinct functions: they act as pressure-relief valves for the atria, and they produce hormones that help regulate blood volume.

The pressure-relief role is the more intuitive one. The appendages are significantly more stretchy (compliant) than the main atrial chamber walls. In isolated canine left atria, removing the appendage more than doubled the stiffness of the remaining chamber.⁶ Think of the appendage as a small expansion tank: when filling pressures rise, it stretches to accommodate the extra volume, buffering the rest of the heart from sudden pressure spikes. Studies during cardiac surgery have confirmed that clamping the left atrial appendage reduces the atrium’s overall ability to act as a blood reservoir, especially when pressures are elevated.⁷

Animal experiments reinforce this point from a different angle. After surgical removal of the left atrial appendage in dogs, the remaining atrial chamber became measurably stiffer, with the pressure-volume relationship shifting in a way that reduced the atrium’s reservoir capacity at any given filling pressure.⁸

The hormonal role involves natriuretic peptides, particularly atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). These hormones are released when atrial tissue is stretched, and they signal the kidneys to excrete more sodium and water, lowering blood volume and pressure. The atrial appendages are a major source of these peptides.⁹ Eliminating the appendage may therefore impair the body’s ability to respond to fluid overload. Animal data suggest that removing it could reduce cardiac output, blunt thirst responses during dehydration, and potentially promote heart failure under certain conditions.¹⁰ Whether these effects are clinically meaningful in humans is still being studied, but they are a real consideration for doctors weighing the pros and cons of appendage removal.

The Left Atrial Appendage and Stroke

Most of the clinical attention given to atrial appendages focuses on the left one, for a sobering reason: it is the primary site where blood clots form in people with atrial fibrillation (AF). When the atria quiver chaotically instead of contracting in an organized rhythm, blood flow inside the left atrial appendage slows to a near standstill. The appendage’s narrow, irregular shape and muscular ridges create nooks where stagnant blood can pool and clot.¹¹ If a clot breaks free, it enters the left-sided circulation and can travel directly to the brain, causing a stroke.

The pathophysiology lines up neatly with Virchow’s triad, a framework from the 1800s that identifies three factors driving clot formation: abnormal blood flow (stasis), damage to the vessel lining (endocardial dysfunction), and changes in the blood’s clotting tendency (altered hemostasis). All three come together in the left atrial appendage during atrial fibrillation.¹² The remodeling process is not instant; it involves structural changes to the appendage itself, inflammation, and activation of endothelial cells lining the interior.¹³

Four Shapes and What They Mean for Risk

Not all left atrial appendages look alike, and the differences matter. Researchers have classified the most common shapes into four categories, each named for what it resembles on imaging:

Chicken wing: a single dominant lobe that bends sharply, like a folded wing. This is the most common shape, found in roughly 40% of people.
Cactus: a central lobe with secondary lobes branching off in multiple directions.
Windsock: one dominant lobe with secondary lobes trailing off it, somewhat elongated.
Cauliflower: a short, bulbous shape with complex internal ridges and limited overall length.

A multicenter study found that the prevalence of prior stroke or transient ischemic attack differed significantly across these shapes. Among patients with atrial fibrillation, those with the chicken wing shape had just a 4% rate of prior stroke, compared with 12% for cactus, 10% for windsock, and 18% for cauliflower. After adjusting for other known stroke risk factors, the chicken wing shape was associated with roughly 79% lower odds of having had a stroke.¹⁴

A later meta-analysis confirmed the pattern: the chicken wing morphology was associated with about a 41% lower relative risk of stroke compared with the non-chicken-wing shapes combined. The cauliflower shape carried the highest relative risk.¹⁵ The likely explanation is mechanical: the chicken wing’s sharp bend may promote better washout of blood during each heartbeat, while the cauliflower’s complex, stubby interior creates more stagnant pockets.

The Left Atrial Appendage as an Electrical Trigger

Beyond clot formation, the left atrial appendage has earned attention in electrophysiology because it can be a source of the abnormal electrical signals that start or sustain atrial fibrillation. In patients who have already undergone one ablation procedure for AF and come back because the arrhythmia returned, the left atrial appendage appears to be the culprit trigger site in roughly 27% of cases.¹⁶ Electrically isolating the appendage during a repeat procedure can eliminate the arrhythmia in those patients. This is a relatively newer area of study, and the decision to isolate the appendage electrically has to be weighed against the fact that doing so can reduce its contractile function and potentially increase the risk of clot formation within it.¹⁷

How Doctors Look Inside the Appendage

When a doctor suspects a blood clot might be sitting inside the left atrial appendage, the traditional gold-standard test is transesophageal echocardiography (TEE), an ultrasound probe passed down the esophagus to get a close-up view of the left atrium from behind the heart. It is accurate but invasive and uncomfortable.

Cardiac CT has emerged as a strong alternative. A meta-analysis found that CT scanning detects left atrial appendage clots with about 96% sensitivity and 92% specificity overall. When a delayed imaging technique is added, where a second scan is taken a few minutes after the contrast dye injection to distinguish slow-flowing blood from actual clot, the accuracy jumps to about 99%.¹⁸ A separate study in stroke patients confirmed high agreement between two-phase CT angiography and TEE, with an overall concordance above 0.95.¹⁹ For patients who cannot tolerate the swallowed ultrasound probe, CT with delayed imaging is increasingly viewed as a reliable substitute.

Closing Off the Appendage to Prevent Stroke

Because the left atrial appendage is so strongly linked to stroke in atrial fibrillation, a whole category of treatments has developed around sealing it shut. The logic is straightforward: if blood cannot enter the appendage, it cannot pool and clot there. The two main approaches are percutaneous (catheter-based) closure and surgical closure.

On the catheter side, the Watchman device is the most widely studied option. It is a small, parachute-shaped plug delivered through a catheter inserted via the groin. Medium-term follow-up data suggest it may be a viable alternative to long-term blood thinners for patients with nonvalvular atrial fibrillation who cannot tolerate anticoagulant medication.²⁰ A newer competitor, the Amulet device, showed a lower rate of leak around the device at 45 days compared with the Watchman in a randomized trial, though both had low rates of serious complications.²¹

On the surgical side, the appendage can be removed (excised) or sealed off (excluded) during open-heart surgery. Not all methods work equally well. Internal stitches and non-cutting staplers have a track record of incomplete closure, with the appendage sometimes reopening over time. External clip devices like the AtriClip have fared better at achieving durable, complete closure.²² Still, there is no strong evidence yet to support routinely clipping the appendage for every patient undergoing cardiac surgery; the benefit appears most clear for patients already at high stroke risk.

Complications from percutaneous closure, while uncommon, do occur. A review of over 500 procedures identified 11 significant complications requiring intervention, including pericardial effusion (fluid around the heart), clot forming on the device itself, and one case of systemic embolism.²³ These are manageable events in most cases, but they underscore that appendage closure is not a trivial procedure.

How the Left and Right Appendages Compare

The right atrial appendage gets far less clinical attention than the left, mostly because clots forming there do not travel to the brain. The right side of the heart pumps blood to the lungs, so a clot dislodging from the right appendage would head to the pulmonary vasculature rather than the cerebral arteries. That does not make the right appendage unimportant, but it means the consequences of clot formation there are different and generally less devastating.

Structurally, the two appendages have distinct personalities. A comparative anatomical study found that right atrial appendages have more pectinate muscles on average (about 17 versus 11 for the left) and the muscles tend to be longer, but the overall muscle density is lower in the right appendage.²⁴ The wall thickness of the right appendage is not uniform; it is thickest near the crista terminalis and thinnest around the aortic mound, a detail that matters to electrophysiologists who need to place catheters safely without perforating the wall.³

The pectinate muscles inside the right appendage also have a non-uniform trabecular pattern in about 80% of hearts, with crossing and irregular fiber architecture at the junctions between the muscles and the crista terminalis.² This irregular fiber arrangement can create pockets where electrical signals slow down or fragment, which is relevant to certain types of abnormal heart rhythms that originate in or around the right atrium.

Why “Auricle” Still Floats Around in Textbooks

You may encounter both “auricle” and “atrial appendage” in medical literature, and the terminology can be confusing. Historically, “auricle” was used to refer to the entire atrium, not just the appendage. Over time, anatomists narrowed the term to mean specifically the ear-shaped pouch on the front of each atrium, while “atrium” came to designate the whole chamber. Some older textbooks and even some current clinical contexts still use “auricle” and “atrial appendage” interchangeably. In modern anatomical nomenclature, “atrial appendage” (or “auricular appendage”) is the more precise term for the pouch itself, while “auricle” is sometimes reserved for the external, visible portion.

The abbreviations LAA and RAA (left atrial appendage and right atrial appendage) are now standard in cardiology journals, imaging reports, and surgical notes. If you see “LAA closure” or “LAA occlusion” on a medical record, it refers to sealing off the left atrial appendage. The three-dimensional shape, internal muscular architecture, and relationship to surrounding structures like the pulmonary veins, the mitral valve, and the circumflex artery all need to be understood in detail by anyone performing procedures on or near these structures.²⁵ What looks like a simple flap from the outside turns out to be one of the heart’s more complex and consequential neighborhoods.