Stomach. Computer artwork of the human stomach. The stomach is a hollow bag-like organ of the digestive system which is connected to the oesophagus (at top) and the duodenum, the first part of the of the small intestine (at bottom). The stomach's capacity is about 1.5 litres. The walls are made of layers of longitudinal and circular muscle lined by special glandular cells that secrete gastric juice. A strong muscle at the lower end of the stomach forms a ring called the pyloric sphincter that can close the outlet leading to the duodenum. The stomach continues the digestion of food started in the mouth and also acts as a storage organ.