Say you drink a glass of water and then hang upside down. How does the stomach keep the water from traveling back up your esophagus?
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There’s a ring of muscle pinching the tube closed just above where it joins the stomach. It’s called a sphincter. It opens to let food/liquid through when you swallow, then closes again and cinches the tube closed.
There’s another one clamping the tube leaving your stomach closed too. Think about it – without that, what’s keeping the stomach contents in the bad and not just falling straight through and out the bottom when you’re upright? If it’s possible to keep that stomach exit sealed against gravity all the time, keeping the entrance sealed on the rare times you’re upside down using the same mechanism (a ring of muscle called a sphincter) is pretty easy!
This kind of thing [benefits a lot from a simple picture](https://medlineplus.gov/ency/images/ency/fullsize/19223.jpg) illustration.
And yes, as Anns alluded, your body uses sphincters to keep poop from falling out of your butt until you want to release it. It’s just a ring of muscle that contracts and keeps the tube sealed. Same thing sealing both the top and bottom tubes attaching to stomach.
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