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 is a sphincter between the esophagus and the stomach that normally stays closed unless you are swallowing something that needs to be passed into the stomach. This is called the gastroesophageal sphincter.
In a normal healthy human, this sphincter prevents things going from the stomach to the esophagus. Even when you are “hanging upside down”.
(If you don’t know what a sphincter is an example: your anus is a sphincter)
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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