Despite its name, heartburn has nothing to do with the heart. It’s the burning sensation that a person experiences in the upper abdomen or below the breast bone that is caused by the esophagus becoming irritated from stomach acid flowing backward, or acid reflux. Acid reflux happens when the muscle valve at the end of the esophagus, called the lower esophageal sphincter (LES) fails to close properly, allowing stomach acid to escape into the esophagus. When the acid irritates the esophagus, heartburn occurs.
Most people experience heartburn at some point, and the irritation can last for several hours. But chronic heartburn may be a symptom of GERD (gastroesophageal reflux disease), which can lead to serious problems if left untreated.