system
Given the following post, respond as if you're commenting on Reddit:
human
What is it about the cells in our stomach lining that allows them to avoid being damaged by our stomach acid?

Basically, how does our stomach acid not cause us problems or even eventually seep into our abdominal cavity?
gpt
What is different about the cells in the stomach that stops them being attacked by acid? Nothing, or nothing much.

Your stomach is much more than just a dumb bag filled with digestive juices and acid. Your stomach is an organ filled with sensors and and control mechanisms, and it is connected to a network of "brain cells" AKA neurons called the enteric nervous system, with the sole goal of making sure your digestion works smoothly. There is also a complex collection of hormones and paracrine factors (aka short range hormones) making sure things are functioning properly.

In your stomach lining, there are cells that sense acid, and they communicate to cells that release the hormone gastrin. Gastrin tells a specific class of cells in your stomach to release acid. No gastrin, no acid. If the stomach starts getting to acidic, then gastrin release is turned down, turning down the acid release.

But the contents of the stomach are still very acidic, and they're still full of enzymes whose job it is to chew up proteins. So how does the stomach protect itself from that? With alkaline mucous. There are cells in the stomach that secrete mucous and bicarbonate ions, basically constantly. As acid diffuses into the mucous, it reacts with the biocarbonate ions, and is nutralized, and then that "layer" of mucous is washed away into the contents of the stomach. And this process just runs constantly, like a conveyer belt: mucous being secreted, moving out towards the contents of the stomach, reacting with the acid, and then falling off into the contents of the stomach. If the mucous isn't being secreted fast enough, then the acid will start to difuse all the way to the stomach lining, which is when the cells sense the acid, and send signals that control the rate of acid secretion, mucous secretion and bicarbonate secretion.

These kind of feedback loops are found throughout the body. The gastrointestinal system is full of them.