Figure 1.

Embryologic development of the pulmonary veins. (A) Illustration shows that in early gestation, the veins draining the primitive lung buds do not connect to the primitive left atrium, but rather connect to various systemic venous systems (e.g., cardinal venous system, umbilical venous system) through the pulmonary venous plexus; the primitive pulmonary vein appears as an outpouching of the left atrium; (B) illustration shows that after 30 days of gestation the primitive pulmonary vein enlarges and connects to the pulmonary venous plexus, establishing for the first time venous flow from the lung buds to the left atrium; the connections of the pulmonary plexus with the systemic veins atrophy.