The esophagus has natural barriers to acid exposure, one of which is the lower esophageal sphincter (LES). The LES is a muscle at the junction of the esophagus and the stomach, which functions to prevent acid from refluxing back into the esophagus. The most common cause of GERD is transient relaxation of the LES, which can be caused by a number of dietary and environmental factors. A hiatal hernia, when the stomach slips above the diaphragm, can also weaken the LES. In addition, abnormalities in esophageal motility, salvia production, gastric emptying may cause GERD.