The pulmonary vein carries blood from lungs to heart while the pulmonary Artery carries blood from heart to lung thus this makes a cycle for transfer of blood from lungs to heart.
The pulmonary artery carries CO2 rich blood to the lungs and the pulmonary vein carries O2 rich blood to the heart. The pulmonary artery is the only artery in the entire body to carry CO2 rich blood and the pulmonary vein is the only vein in the entire body to carry O2 rich blood.
Pulmonary vein carries oxygenated blood from lungs to heart while pulmonary artery carries deoxygenated blood from heart to lungs.