The pulmonary veins return the aerated blood from the lungs to the heart.
They are usually four in number, two right and two left. Occasionally, however, there are three pulmonary veins on the right side, the result of the vein from the middle lobe of the right lung opening separately into the left auricle instead of joining as usual the upper of the two right pulmonary veins. The relations of the pulmonary veins to the pulmonary arteries and bronchi in the lungs are given with the Anatomy of the Lungs. At the root of the lung the pulmonary veins on both sides are arranged as an upper and lower branch, an anterior descending branch of the bronchus passing between them. The upper vein on the right side is larger than the lower, and usually receives the vein from the middle lobe of the right lung. The lower vein on the left side is larger than the upper. Both the upper and lower veins lie in front of the pulmonary artery and on a lower plane, and run almost horizontally inwards and forwards to the left auricle. As they pierce the pericardium they receive a reflexion from the serous layer of that membrane. Their relations within the pericardium are given with the Anatomy of the Heart. At the root of the lung their relations to the surrounding structures are similar to those of the pulmonary artery. A separate description is not required.