Stomach. Computer artwork of the stomach and lower oesophagus. The stomach is the first major organ of the digestive system. Food is passed from the mouth to the stomach down the oesophagus (top centre). It is then exposed to the various digestive juices, including hydrochloric acid, pepsin and intrinsic factor. The acid kills bacteria on the food, and also provides an ideal environment for pepsin (an enzyme which breaks down proteins) to work in. Intrinsic factor aids the absorption of vitamin B12. Muscles (white) in the stomach lining grind the food to a creamy liquid called chyme. This is released into the duodenum via the pyloric sphincter (upper left).