The stomach is a bag like organ located on the left side of the upper abdomen. This is the largest part of our gastrointestinal tract. It works as a reservoir for the food as food reaches the end of the esophagus. The stomach secretes acid and enzymes that digest food. Ridges of muscle tissue called rugae line the stomach. The stomach muscles contract periodically, churning food to enhance digestion. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.