One of the clearest illustrations of this principle is found in the condition known as Barrett’s esophagus. This is most commonly caused by gastroesophageal reflux disease (GERD), also known as reflux or, colloquially, heartburn. Normally, stomach acid stays in the stomach and does not back up into the esophagus. The lining of the stomach is designed to withstand the strong acids produced, but the lining cells of the esophagus are not. When stomach acid refluxes upward, the esophageal lining sustains damage, which causes the pain of heartburn. In response, the cell lining of the esophagus
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