system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the stomach work in space? Do astronauts feel constant need to throw up since the contents are bouncing around inside?
gpt
A stomach is also more a wet bag than a box or bottle. There's no big open space to fill. The contents can slosh around when you fill it up.

You have a sphincter (round muscle like your butthole) that holds things in at each end. There's a lower esophagal sphincter at the top of the stomach that keeps the digesting food down, and a pyloric sphincter at the bottom that lets it into the intestines when it's digested enough to move on.

When that esophagal sphincter gets weak, the result is reflux. It would be very unpleasant to be in space and have that aggravated.