How Come?: Stomach's acids

How can the stomach secrete acids strong enough to break down metals without injuring itself? asks a reader.

It's a mystery: Some human beings feast on other animals' organs -- liver, kidneys, heart, lungs -- and the stomach obliges by digesting the whole lot. But somehow, our own hardy organ avoids disappearing in the onrush of caustic acids.

From the time we take the first bite of breakfast to our last evening snack, the stomach makes about six cups of digestive "juices." And the key ingredient of this corrosive brew? Metal-dissolving hydrochloric acid.

Only half a percent of the digestive juices are hydrochloric acid, but that's more than enough to digest last night's barbecue. The rest of the digestive liquid is a mix of water, sodium chloride (salt), potassium chloride and some calcium. The acidic mix activates stomach enzymes called pepsins, which break down proteins. Like acid, pepsins are also threats to living cells.

Chew and swallow a bite of food, and a chemical chain reaction begins immediately. The stomach releases gastrin, a hormone that signals cells in the stomach's lining (the parietals) to forge hydrochloric acid from hydrogen and chloride atoms. Food in the stomach -- say, bits of that cheeseburger you just ate -- begin to break down in the acidic juice. Proteins, which are made of linked-up amino-acids, begin to unravel.

Meanwhile, other stomach cells begin churning out a substance called pepsinogen. With the help of hydrochloric acid, water enters pepsinogen, transforming it into protein-chomping pepsin. It's pepsin that severs the bonds between the amino acids, digesting the proteins.

The stomach's environment is so caustic that it kills or inhibits many microorganisms that might harm us. So why isn't the stomach injured by its own chemicals? Luckily, the stomach has an arsenal of built-in defenses.

One way the stomach protects itself: Its lining of epithelial cells is continuously regenerating. While the acid bath is raging, about half a million epithelial cells are shed and replaced -- every minute. Like a snake shedding its old skin, the stomach keeps cells underneath safe from the harsh environment in its churning sac.

Next, there's that all-purpose protective slime the body loves to churn out: mucus. The jack-of-all-trades stomach lining also manufactures its own antacid -- bicarbonate. Finally, the stomach seems to be able to switch its environment from acid to alkaline if need be, using special pH sensor molecules.

But stomach defenses can fail, as when bacteria damage its mucus-y barrier. Then acids can eat through the stomach wall, leaving painful sores known as ulcers.