Chapter 3 The Digestive System 111 Hiatal Hernia A hiatal hernia occurs when a portion of the stomach protrudes (bulges) through the diaphragm (Figure 3.4). The diaphragm, the major muscle involved in the breathing process, separates the thoracic (chest) cavity from the abdominal cavity. The esophagus normally enters the abdominal cavity through an opening in the diaphragm. If the opening is weakened or enlarged, the stomach may herniate (bulge) upward through the diaphragm into the thoracic cavity. A large hiatal hernia causes heartburn, chest pain, belching, and nausea. Figure 3.3 Gastroesophageal reflux disease (GERD) is characterized by backflow of stomach acids into the esophagus. Esophagus Lower esophageal sphincter closed Stomach Duodenum Liquid Lower esophageal sphincter open, allowing reflux Diaphragm Figure 3.4 A—Normal muscle of the abdominal wall. B—In a hiatal hernia, a portion of the stomach bulges through the diaphragm. Flap valve Duodenum Stomach Hiatal hernia Esophagus Diaphragm Lower esophageal sphincter Duodenum Stomach Esophagus Diaphragm Lower esophageal sphincter A B