The pulmonary veins are large blood vessels that carry oxygenated blood from the lungs to the left atrium of the heart. In humans there are four pulmonary veins, two from each lung. They carry oxygenated blood, which is unusual since almost all other veins carry deoxygenated blood. The pulmonary veins carry oxygenated blood from the lungs to the left atrium of the heart. In humans there are normally four pulmonary veins, two from each lung. As part of the pulmonary circulation, they carry oxygenated blood back to the heart, as oppose to the veins of the systemic circulation which is carrying deoxygenated blood. 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. Therefore, the number of pulmonary veins opening into the left atrium can vary between three and five in the healthy population. The right pulmonary veins pass behind the right atrium and superior vena cava; the left in front of the descending thoracic aorta. 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.