Right atrium

The right atrium is the most anterior and rightward cardiac chamber, serving as the primary receiving chamber for systemic venous blood via the superior and inferior vena cavae and the coronary sinus.

The right atrium  consists of four main components: the principal cavity or venous part (sinus venosus or sinus of venae cavae), the auricula or right atrial appendage, the vestibule, and the interatrial septum, which separates it from the left atrium.

The posterior portion is smooth-walled (sinus venosus), while the anterolateral portion is muscular and trabeculated, forming the right atrial appendage, which is triangular and extends superiorly over the right atrioventricular groove and aortic root.

Key internal structures include the crista terminalis, which demarcates the junction between the smooth and trabeculated regions; the fossa ovalis, a remnant of the fetal foramen ovale; the Eustachian valve at the inferior vena cava orifice; and the coronary sinus ostium.

The right atrium is larger than the left atrium, but its walls are somewhat thinner, measuring about 2 mm.; its cavity is capable of containing about 57 c.c.