Your stomach lining replaces itself every few days

Your stomach is, by any reasonable definition, a chemistry hazard you carry around all day. It secretes hydrochloric acid that drives the contents down to a pH of roughly 1.5 to 3.5 — corrosive enough to dissolve metal and certainly enough to eat through living tissue. The obvious question is why the stomach doesn’t simply digest itself.

Part of the answer is a defensive coating. The stomach wall is lined with a thick, bicarbonate-rich mucus that forms a physical barrier and chemically neutralizes acid right at the surface. But mucus alone wouldn’t last. The cells underneath take a relentless beating, so the body’s solution is brute-force replacement.

The surface epithelium of the stomach is completely replaced every 3 to 6 days.

That renewal runs off stem cells tucked in the necks of the gastric glands. They divide constantly, sending fresh cells migrating up toward the surface to shed and down toward the gland base as they mature. It’s an assembly line that never stops: damaged lining cells are sloughed off and swapped out before the acid can do lasting harm.

Different cell types turn over at very different speeds — surface mucous cells last only days, while the deeper acid- and enzyme-producing cells can persist for months. The net effect is a tissue in permanent reconstruction, trading durability for disposability. It’s cheaper, evolution apparently decided, to keep rebuilding the wall than to armor it against the acid you make on purpose.