GERD is the result of a disordered valve mechanism between the esophagus and the stomach.

When the valve is working correctly, it opens during swallowing to allow food to enter the stomach and then closes to prevent food and stomach secretions from moving backward into the esophagus. When it fails to close correctly, the stomach contents—which are acidic and contain digestive secretions—can flow back into the esophagus.

This reverse flow (reflux) of food, acids, and the digestive enzyme pepsin, can cause damage to the esophageal lining and resulting heartburn.