These vessels uniting successively, form a single trunk for each lobe, three for the right, and two for the left lung.
The vein from the middle lobe of the right lung generally unites with that from the upper lobe, so that ultimately two trunks from each lung are formed; they perforate the fibrous layer of the pericardium and open separately into the upper and back part of the left atrium.
Occasionally the three veins on the right side remain separate, and not infrequently the two left pulmonary veins end by a common opening into the left atrium. Following from this, it's true that the number of pulmonary veins opening into the left atrium can vary between three and five in the healthy population.
At the root of the lung, the superior pulmonary vein lies in front of and a little below the pulmonary artery; the inferior is situated at the lowest part of the hilus of the lung and on a plane posterior to the upper vein. Behind the pulmonary artery is the bronchus.
Within the pericardium, their anterior surfaces are invested by the serous layer of this membrane.