Anatomy and physiology of the upper gastrointestinal tract, which consists of the esophagus, stomach, and small intestine. Various disorders of the upper GI tract, including esophagitis, gastroesophageal reflux disease, esophageal varices, esophageal cancer, hiatal hernia, pyloric stenosis, gastritis, peptic ulcer disease, Zollinger-Ellison syndrome, small bowel obstruction, and peritonitis.