Problems and diseases affecting the long, complex system by which food is broken into substances to be used in the body and waste products to be excreted. In the digestive system, food enters the mouth, where is it chewed and softened with saliva and then swallowed in the throat and the long tube called the esophagus, where muscle contractions (peristalsis) propel the food into the stomach. The stomach produces hydrochloric acid and various digestive enzymes, mixing these with the food by muscle action for two to four hours.