After you swallow food, it gets squeezed through the tube of the esophagus toward the stomach, where it passes through a trap door at the base of the esophagus, called the esophageal sphincter, which opens to allow the food to pass and then closes behind it. The stomach is filled with acids; the lining of the stomach is built to withstand these acids, but the lining of the esophagus isn’t. So if anything goes wrong with this process and the acidic contents of the stomach make their way back into the esophagus, the resulting pain is called heartburn.
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