For people who have GERD, the sphincter between the esophagus and the stomach either opens at the wrong time, or is weak and doesn't completely close off the opening to the stomach. When this happens, stomach juices and food particles can flow back up into the esophagus more frequently than normal.
If stomach juices stay in the esophagus for long periods of time, or if acid reflux happens often, the natural way the esophagus protects itself from stomach juices may simply be overwhelmed. The lining of the esophagus may become irritated, causing heartburn. Over time, GERD can lead to bleeding, the formation of ulcers, or scarring and narrowing of the esophagus.