What causes heartburn?

When you eat, food passes from your mouth down a tube (about 10 inches long in most people) called the esophagus. To enter the stomach, the food must pass through an opening between the esophagus and stomach. This opening acts like a gate to allow food to pass into the stomach.

Usually, this opening closes as soon as food passes through. But if it doesn't close, acid from your stomach can get through the opening and into your esophagus. This is called reflux. Stomach acid can irritate the esophagus and cause heartburn.