Ready to talk about your esophagus? Good! Because one cause of acid reflux involves a part of your esophagus called the lower esophageal sphincter. It’s the valve located between the esophagus and the stomach.) It’s supposed to remain closed to keep food from going back up. But when you have acid reflux, that’s not the case and the acid produced by your stomach can travel up into your esophagus, resulting in acid reflux symptoms like heartburn.