An atrium is one of the two upper chambers of the heart. You have a right atrium and a left atrium, separated by a thin wall called the atrial septum. Their job is to receive blood returning to the heart and pass it down into the larger, more muscular lower chambers (the ventricles), which then pump it out to the body or lungs. Together, the two atria act as the heart’s receiving stations, keeping blood moving in the right direction at the right time.
Right Atrium vs. Left Atrium
The right and left atria sit side by side at the top of the heart, but they handle different blood supplies and differ in structure. The right atrium collects oxygen-poor blood returning from the body through two large veins: the superior vena cava (carrying blood from the head and arms) and the inferior vena cava (carrying blood from the torso and legs). It then sends that blood through the tricuspid valve into the right ventricle, which pumps it to the lungs to pick up fresh oxygen.
The left atrium receives oxygen-rich blood coming back from the lungs through four pulmonary veins. It pushes that blood through the mitral valve into the left ventricle, the heart’s strongest chamber, which sends it out to the rest of the body. Because of the heart’s angled position in the chest, the left atrium sits further back and slightly higher than the right. It also has a noticeably smaller ear-shaped pouch (called the appendage) compared to the right atrium’s larger one, and it lacks a prominent internal ridge that the right atrium has.
How the Atria Fit Into Each Heartbeat
Every heartbeat follows a precise sequence. A cluster of specialized cells called the sinoatrial (SA) node, located in the upper wall of the right atrium, fires an electrical signal that starts the whole process. These cells are unique because they generate impulses on their own, without needing a signal from the brain. This property, called automaticity, is what keeps your heart beating steadily whether you’re awake or asleep. The SA node was first identified over a century ago at the junction of the superior vena cava and the right atrium.
Once the SA node fires, the electrical signal spreads rapidly across both atria, causing them to contract together. The signal then pauses briefly at the atrioventricular (AV) node, a gateway between the upper and lower chambers. That short delay is important: it gives the atria time to finish squeezing blood into the ventricles before the ventricles contract and pump blood out. This final squeeze from the atria, sometimes called the “atrial kick,” tops off the ventricles with extra blood and makes each heartbeat more efficient.
Pressure and Blood Flow
The atria are low-pressure chambers compared to the ventricles. Normal pressure in the right atrium averages about 3 mmHg, with a range of 0 to 8 mmHg. The left atrium runs slightly higher, typically between 2 and 12 mmHg. These low pressures make sense because the atria don’t need to generate much force. They’re simply funneling blood downward through open valves into the ventricles during the relaxation phase of the heartbeat (diastole). Once pressure in the atria exceeds that of the ventricles, the valves open and blood flows through passively for most of the filling period. The atrial contraction at the end adds the final portion.
Two valves guard the exits from the atria. The tricuspid valve (three flaps) sits between the right atrium and right ventricle, while the mitral valve (two flaps) sits between the left atrium and left ventricle. Both valves open to let blood through, then snap shut to prevent it from leaking backward when the ventricles contract.
Atrial Fibrillation
The most common condition affecting the atria is atrial fibrillation, often called AFib. In a normal heart, the SA node sends organized electrical signals that make the atria contract in a smooth, coordinated way. In AFib, the signals become chaotic. Instead of contracting, the atria quiver or tremble, and many of those erratic signals leak through the AV node to the ventricles, producing a fast, irregular heartbeat.
AFib matters for two reasons. First, the irregular rhythm can cause fatigue, shortness of breath, and dizziness. Second, when the atria don’t contract properly, blood can pool and form clots, especially in the left atrial appendage. Over 90% of blood clots in people with AFib originate in this small pouch. If a clot breaks loose, it can travel to the brain and cause a stroke. That connection between AFib and stroke risk is why doctors focus heavily on assessing the mechanical function of the left atrial appendage in people with this condition.
Atrial Septal Defects
An atrial septal defect (ASD) is a hole in the wall between the two atria. It’s a congenital heart defect, meaning it’s present from birth. The most common type, called a secundum defect, occurs in the middle of the atrial septum. Less common types affect the lower part of the wall, the upper part near the veins, or the area around a vein that drains into the right atrium.
Small ASDs often cause no symptoms at all and may be found by chance on an imaging test. Some close on their own during infancy or early childhood. Larger defects allow too much blood to flow from the left atrium into the right atrium, overloading the right side of the heart and the lungs over time. Symptoms, which sometimes don’t appear until adulthood, can include shortness of breath during exercise and unusual tiredness. A large, long-standing ASD can eventually damage both the heart and lungs, and surgery or a catheter-based procedure is typically needed to close it.
How Atrial Size Is Measured
Doctors sometimes measure the size of the left atrium to assess heart health, since an enlarged left atrium can signal problems like high blood pressure, valve disease, or heart failure. The standard measurement is called the left atrial volume index (LAVI), which adjusts for body size. A normal LAVI is 28 mL/m² or less. Values of 29 to 33 are considered mildly enlarged, 34 to 39 moderately enlarged, and 40 or above severely enlarged. An increasing LAVI over time is a warning sign that the heart is under growing strain.