Atria

The atria are the two upper chambers of the heart that receive blood returning to the heart and transfer it to the ventricles. The right atrium collects systemic venous blood, while the left atrium collects oxygenated pulmonary venous blood. The atrial walls are generally thinner than those of the ventricles, and each chamber contains an auricle (atrial appendage) that can be smooth or irregular due to pectinate muscles.

The right atrium receives blood from three major veins: the superior and inferior venae cavae and the coronary sinus. Its interior has a smooth posterior wall and a rough anterior wall formed by pectinate muscles, separated by the crista terminalis. The interatrial septum features a shallow fossa ovalis, a remnant of the fetal foramen ovale. The sinoatrial node, the natural pacemaker, is located in the right atrium near the superior vena cava opening.

The left atrium receives blood from the lungs via the four pulmonary veins. It is largely smooth-walled, with pectinate muscles confined to the left atrial appendage. Like the right atrium, it contributes to the heart’s conduction system and sits adjacent to the mitral valve, which anchors to the atrioventricular annulus.

Atria function by collecting venous return and transferring blood to the ventricles through the atrioventricular valves (tricuspid on the right, mitral on the left). During atrial systole, they actively contribute to ventricular filling, the so-called atrial kick, which helps optimize cardiac output, especially when ventricular diastolic function is impaired. In fetal life, the foramen ovale provides an alternative pathway for blood flow, and its closure forms the fossa ovalis after birth.