Final answer:
The gastro-esophageal sphincter prevents food from coming back up the esophagus into the mouth when a person is turned upside down.
Explanation:
When a person is turned upside down, food in the esophagus does not come back out into the mouth because of the action of the gastro-esophageal sphincter. This sphincter, located at the stomach end of the esophagus, opens in response to swallowing and the pressure exerted by the bolus of food, allowing the bolus to enter the stomach. When there is no swallowing action, the sphincter remains shut, preventing the contents of the stomach from traveling back up the esophagus.
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