Science Puzzle
Why Doesn’t Your Stomach Digest Itself?
Your stomach is filled with hydrochloric acid, about as acidic as lemon juice or more so, and an enzyme called pepsin that breaks down protein. Together they can start to digest a steak. But the wall of the stomach is made largely of protein too.
Why doesn't your stomach digest itself?
The Answer
The stomach protects itself in three ways: a thick coat of slimy mucus that neutralises the acid before it reaches the wall, an enzyme that's only switched on once it's away from the cells that make it, and a lining that's replaced every few days.
The inside of the stomach is covered in a layer of mucus a fraction of a millimetre thick. The cells beneath it pour bicarbonate, an alkali, into the mucus. Acid trying to get through is neutralised on the way, so although the middle of the stomach is fiercely acidic, the surface of the wall sits in nearly neutral conditions.
Pepsin is made in an inactive form that can't digest anything. It's only switched on when it meets the acid out in the stomach, away from the cells that made it. And the cells of the stomach lining don't last long: they're replaced every few days, so any damage is quickly repaired.
When the defences fail, the result is an ulcer, a sore where the wall has been eaten into. For a long time ulcers were blamed on stress and spicy food. In the 1980s two Australian doctors, Barry Marshall and Robin Warren, showed that most are caused by a bacterium that weakens the mucus layer. Marshall proved it by drinking a culture of the bacteria himself, and the pair won a Nobel Prize in 2005.
The principle: Self-protection in digestion. The stomach shields its wall with mucus and an alkaline layer, makes its enzyme in an inactive form and constantly renews its lining, so it digests food but not itself.