When you eat, food travels from your mouth to your stomach through a tube called the esophagus. At the lower end of the esophagus is a small ring of muscle called the lower esophageal sphincter (LES). The LES acts like a one-way valve, allowing food to pass through to the stomach. Normally, the LES closes immediately after swallowing to prevent back-up of stomach juices which have a high acid content. Heartburn (GERD) occurs when the LES does not function properly, allowing acid to flow back and burn the lower esophagus. This irritates and inflames the esophagus, causing heartburn and eventually may damage the esophagus.