The right atrium is one of the four chambers of the heart, serving as the initial receiving station for deoxygenated blood returning from the entire body. Its purpose is to collect this blood and direct it toward the lungs for reoxygenation, maintaining the flow of the circulatory system. Understanding this upper chamber’s structure and function is key to understanding the heart’s overall pumping mechanism.
Where the Right Atrium Sits and What It Looks Like
The right atrium is situated in the heart’s upper right section, positioned superiorly and slightly anteriorly to the right ventricle. This relatively thin-walled chamber forms the right border of the heart. Its structure is roughly cuboid, featuring an ear-shaped muscular pouch known as the right atrial appendage or auricle, which increases the chamber’s volume.
Three large vessels empty directly into the right atrium. The Superior Vena Cava (SVC) returns deoxygenated blood from the upper body, and the Inferior Vena Cava (IVC) delivers blood from the lower body. The coronary sinus, a third opening, returns venous blood that has supplied the heart muscle itself.
The interior is divided by the crista terminalis, a muscular ridge separating the smooth-walled posterior section (sinus venarum) from the rougher anterior section containing pectinate muscles. A depression on the medial wall, the fossa ovalis, marks the remnant of a fetal hole that allowed blood to bypass the lungs. The opening leading out of the right atrium is the right atrioventricular orifice, guarded by the tricuspid valve, which controls the flow into the right ventricle.
How the Right Atrium Moves Blood
The primary function of the right atrium is to receive deoxygenated blood and pass it to the right ventricle. As the heart relaxes, the atrium passively fills with blood returning from the vena cavae, with the tricuspid valve typically closed initially. As the atrium fills and pressure increases, the tricuspid valve opens, allowing blood to flow into the right ventricle largely due to gravity and pressure changes.
The final approximately 20% of ventricular filling is accomplished by the right atrium’s contraction, often termed the “atrial kick.” This coordinated squeeze is triggered by an electrical impulse originating in the Sinoatrial (SA) Node, located near the junction of the SVC and the right atrium. The impulse spreads across the atrial walls, prompting the muscle to contract and forcing the remaining blood through the tricuspid valve. Once the right ventricle is full, the tricuspid valve snaps shut, preventing backflow when the ventricle pumps blood toward the lungs.
Medical Issues Related to the Right Atrium
Because the right atrium houses the SA Node, it is a common site for electrical rhythm disorders, the most frequent being Atrial Fibrillation (Afib). Afib involves rapid, disorganized electrical signals that cause the atrial muscle to quiver instead of contracting effectively, severely reducing the atrial kick. This chaotic motion can lead to blood pooling and increase the risk of clot formation within the chamber, which can lead to a stroke if carried to the brain.
Structural issues also relate to the right atrium, particularly the Patent Foramen Ovale (PFO), which is a persistence of the fetal opening (foramen ovale). While the fossa ovalis normally seals after birth, a PFO remains open in about 25% of the population, creating a connection between the right and left atria. Although often asymptomatic, a PFO can become medically relevant if blood clots pass through this opening directly into the arterial circulation, bypassing the lungs and causing a stroke. Furthermore, an enlarged right atrium or elevated pressure can indicate underlying problems, such as chronic lung disease, pulmonary hypertension, or heart failure.