Your stomach produces hydrochloric acid strong enough to reach a pH of around 1.5 to 2 — acidic enough to corrode many metals and dissolve bone.
The obvious question is why it does not dissolve the stomach.
The answer is a layer of mucus roughly half a millimetre thick, secreted continuously, loaded with bicarbonate that neutralises acid at the wall while leaving the interior acidic. It is a chemical buffer maintained in real time.
Even that is not enough on its own. The cells lining the stomach are replaced roughly every three to five days. Your stomach does not so much resist the acid as outrun it — rebuilding the wall faster than the acid can take it down.
When that balance fails, you get an ulcer. For most of the twentieth century doctors blamed stress and spicy food. Then in 1984, Barry Marshall drank a culture of Helicobacter pylori, gave himself gastritis, and demonstrated that most ulcers are bacterial. He received a Nobel Prize for it in 2005.
There is something quietly impressive in it: the part of you that handles the most corrosive chemistry in your body survives by refusing to stay the same for even a week.
