Middle lobar vein of right lung

The pulmonary vein of the middle lobe (Middle lobe vein; right middle pulmonary vein) is the vein that drains oxygenated blood from the middle lobe of the lung. This vein consists of the junction of medial and lateral branches that collect blood from the corresponding segments of the middle lobe.

Drainage patterns show significant anatomical variation. In the most common pattern (approximately 53-84% of cases), the middle lobe vein drains into the right superior pulmonary vein. However, important variants include direct drainage into the left atrium as a separate ostium (11-17% of cases), drainage into the right inferior pulmonary vein (3-6% of cases), or split drainage where medial and lateral branches drain separately.

When the middle lobe vein drains independently into the left atrium, it has a mean ostial diameter of approximately 8.2-8.6 mm, making it the smallest pulmonary vein ostium in the heart. 

The middle right pulmonary vein as a separate vessel is present in approximately 19-20% of patients.