Gastroesophageal Reflux Disease (GERD) Heartburn. This medical illustration pictures a body outline with the stomach and esophagus with the mechanism of gastroesophageal reflux. With the stomach contracted, the stomach contents are shown pushing past the weakened lower esophageal sphincter into the esophagus. Labels identify the esophagus, lower esophageal sphincter, acid and stomach contents backing up into esophagus and stomach contracted.