Foreign bodies that fall down the trachea are more likely to enter the right bronchus. Behind the pulmonary artery is the bronchus. It branches off into two bronchial tubes, a left and a right main bronchus. It courses posterior to the superior vena cava and in front of the right bronchus. This is known as a tracheal bronchus, and seen as an anatomical variation. It results from disease of one lung, one major bronchus, or chronic congestive heart failure. In addition to these blood vessels, the aortic arch crosses the left main bronchus. Each bronchopulmonary segment has its own (segmental) bronchus and arterial supply. SCLC may originate from neuroendocrine cells located in the bronchus called Feyrter cells. There is collapse of a main stem bronchus on exhalation. It is possible for something outside of the bronchus to cause the blockage. In situs ambiguus, there is a duplication of either the hyparterial or eparterial bronchus. The defect takes the form of a blind-ended bronchus. This causes the area around the bronchus to appear more prominent on an X-ray.