Each atrium is roughly cube-shaped except for an ear-shaped projection called an atrial appendage, previously known as an auricle.

The right atrium receives and holds deoxygenated blood from the superior vena cava, inferior vena cava, anterior cardiac veins, smallest cardiac veins and the coronary sinus, which it then sends down to the right ventricle through the tricuspid valve, which in turn sends it to the pulmonary artery for pulmonary circulation.

The right atrial appendage (lat: auricula atrii dextra) is located at the front upper surface of the right atrium.

[6] The right atrial appendage is a pouch-like extension of the right atrium and is covered by a trabecula network of pectinate muscles.

[9] LAA anatomy as seen in a CT scan is characterized as being in one of four groups: chicken wing (48%), cactus (30%), windsock (19%), and cauliflower(3%).

The left pulmonary artery passes posterosuperiorly and is separated from the atrial appendage by the transverse sinus.

[9] Because of consequent stroke risk, surgeons may choose to close it during open-heart surgery, using a left atrial appendage occlusion procedure.

The cardiac action potential then spreads across both atria causing them to contract, forcing the blood they hold into their corresponding ventricles.

The foramen ovale is no longer needed and it closes to leave a depression (the fossa ovalis) in the atrial wall.

[21] In human physiology, the atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart during ventricular systole.

But without functioning atria, venous flow becomes pulsatile, and the overall circulation rate decreases significantly.

(4) The "let go" of the atria must be timed so that they relax before the start of ventricular contraction, to be able to accept venous flow without interruption.

The fact that atrial contraction is 15% of the amount of the succeeding ventricular ejection has led to a misplaced emphasis on their role in pumping up the ventricles (the so-called "atrial kick"), whereas the key benefit of atria is in preventing circulatory inertia and allowing uninterrupted venous flow to the heart.

[12] The clots may dislodge (forming emboli), which may lead to ischemic damage to the brain, kidneys, or other organs supplied by the systemic circulation.

Some animals (amphibians and reptiles) have a three-chambered heart, in which the blood from each atrium is mixed in the single ventricle before being pumped to the aorta.

This is considered a primitive arrangement, and many vertebrates have condensed the atrium with the sinus venosus and the ventricle with the conus anteriosus.