The Esophagus

The esophagus is located at the center of the chest, in front of the spine. It is marked at the beginning by the upper esophageal sphincter/pharyngoesophageal sphincter and marked at the end by the lower esophageal sphincter/cardiac sphincter. Both sphincters usually stays shut to prevent air and stomach acid from entering the esophagus. 

Even if the human is upside down, the esophagus is still able to push the bolus through due to peristalsis - a series of wave like muscle contractions. Peristalsis is apparent throughout the entirety of the esophagus.