A sphincter muscle is found at each end of the esophagus to ensure one-way movement of food. The sphincter at the upper end of the esophagus is the pharyngoesophageal sphincter. The gastroesophageal sphincter, also called the cardiac sphincter, is at the lower end of the esophagus. It is important because it prevents acids from the stomach from being forced into the esophagus and irritating the lining of the esophagus.