Opening of left superior pulmonary vein

The opening of the left superior pulmonary vein refers to the ostium (or orifice) where the left superior pulmonary vein enters the left atrium of the heart.

In the typical cardiac anatomy (present in approximately 71% of individuals), there are four separate pulmonary vein ostia entering the left atrium: left superior, left inferior, right superior, and right inferior pulmonary veins. The left superior pulmonary vein ostium has a mean diameter of approximately 13.8 ± 2.9 mm in adults. The ostium is located on the posterosuperior wall of the left atrium and is separated from the left atrial appendage by an anatomical landmark called the left lateral ridge, which corresponds to the ligament of Marshall.

However, anatomical variations are common. In approximately 4-5% of cases, the left superior and left inferior pulmonary veins share a common ostium rather than entering separately. 

The pulmonary veins are surrounded by myocardial sleeves that extend from the left atrium onto the adventitial surface of the vein, which are clinically significant as they can serve as triggers for atrial fibrillation.