The left atrium is thicker than the right. It helps to form the left side and back of the base of the heart. The left auricle is visible anteriorly, upon the left of the pulmonary artery. The left auricle is more nodulated and irregular in outline than the right. It is constricted at its junction with the atrium. Its inner surface, with the exception of the pectinate muscles in its auricle, is smooth. It presents the four openings of the pulmonary veins; some small openings of veins, the foramina venarum minimarum, and the left atrioventricular orifice below. Two of the pulmonary veins open on its left side and two on the right; either or both pairs may unite before their entrance. The left pair unites more frequently than the right. A slight depression exists in the atrial septum opposite the side of the fossa ovalis in the other atrium.

Atrium sinistrum

Atrium cordis sinistrum

Atrium gauche

Linker Atrium

Atrio izquierdo