Both ends of the esophagus has circular muscles called sphincters. The whole esophagus is also just a long tube of muscle. So after you swallow, the top sphincter closes, this prevents food from coming up. Then the tube rhythmically contracts top-to-bottom to push the food down. Once it gets to the bottom, the bottom sphincter opens up to let food go into the stomach and quickly shuts to prevents gastric juice from leaking backwards.
These sphincters are always closed and only open to let food pass. So doesnt matter if you’re upside down or in outer space, nothing really comes back up unless (1) you never swallowed it fully or (2) something’s wrong with the muscles in general or (3) theres is immense pressure pushing it back up like when you vomit.
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