The abdomen and chest are separated by a sheetlike muscle called the diaphragm. The esophagus passes through an opening (the hiatus) in the diaphragm to connect to the stomach. A hiatal hernia occurs when part of the stomach bulges out of the abdomen, through the hiatus, and into the chest. When this happens, the lower esophageal sphincter (LES) may move above the diaphragm.
Normally, pressure from the diaphragm muscle helps keep the LES valve closed. When a hiatal hernia occurs, the valve is pushed above the diaphragm so the diaphragm muscle can no longer help keep the valve closed. If the valve cannot prevent stomach acid and juices from backing up into the esophagus, symptoms of heartburn may occur.