title: Pulmonary vein
text: # Pulmonary vein
  
  The pulmonary veins are the veins that transfer oxygenated blood from the lungs to the heart. The largest pulmonary veins are the four main pulmonary veins, two from each lung that drain into the left atrium of the heart. The pulmonary veins are part of the pulmonary circulation.
  
  ## Structure
  
  There are four main pulmonary veins, two from each lung – an inferior and a superior main vein, emerging from each hilum. The main pulmonary veins receive blood from three or four feeding veins in each lung, and drain into the left atrium. The peripheral feeding veins do not follow the bronchial tree. They run between the pulmonary segments from which they drain the blood.
  
  At the root of the lung, the right superior pulmonary vein lies in front of and a little below the pulmonary artery; the inferior is situated at the lowest part of the lung hilum. Behind the pulmonary artery is the bronchus. The right main pulmonary veins (contains oxygenated blood) pass behind the right atrium and superior vena cava; the left in front of the descending thoracic aorta.
  
  ### Variation
  
  Occasionally the three lobar veins on the right side remain separate, and not infrequently the two left lobar 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 two left lobar veins may be united as a single pulmonary vein in about 25% of people; the two right veins may be united in about 3%.
  
  ## Function
  
  The pulmonary veins play an essential role in respiration, by receiving blood that has been oxygenated in the alveoli and returning it to the left atrium.
  
  ## Clinical significance
  
  As part of the pulmonary circulation they carry oxygenated blood back to the heart, as opposed to the veins of the systemic circulation which carry deoxygenated blood.
  
  On chest X-ray, the diameters of pulmonary veins increases from upper to lower lobes, from 3 mm at the first intercoastal space, to 6 mm just above the diaphragm.
  
  A rare genetic defect of the pulmonary veins can cause them to drain into the pulmonary circulation in whole or in part, this is known as a total anomalous pulmonary venous connection (or drainage), or partial anomalous pulmonary connection, respectively.
  
  ## Additional images
  
  * Computed tomography of a normal lung, with different levels of pulmonary veins.
  * Bronchial anatomy
  * Transverse section of thorax, showing relations of pulmonary artery.
  * Pulmonary vessels, seen in a dorsal view of the heart and lungs.
  