The stomach

The stomach is a hollow, muscular, sac-like, pear-shaped organ. It is located between the esophagus and the small intestine, lying crosswise in the abdominal cavity, beneath the diaphragm. An adult's stomach is approximately 30.5 cm long and 15.2 cm wide. It can change size and shape according to the amount of food inside. Its capacity is about 0.94 liter.
The stomach stores, dissolves, and partially digests the contents of a meal, then delivers this partially digested food to the small intestine. Simple sugars and alcohol are absorbed directly through the stomach wall.
Food enters the stomach from the esophagus via the cardiac sphincter which prevents food from passing back to the esophagus.
The stomach consists of layers of muscles and nerves. These strong muscles contract and move and mix the contents of the stomach, mashing the food into a wet pulp.
Gastric glands on the epithelium of the stomach lumen secrete gastric acid (also called hydrochloric acid), which contains acids, mucus and enzymes. The gastric acid causes the stomach contents to have a pH of around 1 to 3. During a meal, the rate of hydrochloric acid production increases markedly. Seeing, smelling, tasting, and chewing food sends information through the vagus nerve to the gastric glands, causing them to increase acid production. On average, the stomach produces 2 liters of gastric acid daily. The gastric acid kills most of the bacteria in the food and stimulates hunger. The highly acidic environment in the stomach causes food proteins (large molecules, one of the three types of nutrients used as energy sources by the body) to lose their characteristic folded structure and break down to their components so they can be easily absorbed later in the intestines. The main enzyme in the stomach that functions to break proteins into smaller pieces is called pepsin. So that pepsin doesn't digest the cell that makes it, it is synthesized and secreted in an inactive form called pepsinogen.
Other glands on the epithelium of the stomach secrete mucus, a viscous, slippery fluid, that lubricates and protects the stomach from self-digestion.
The muscular action of the stomach and the digestive action of the gastric acid convert food in the stomach into a semiliquid mixture called chyme, which passes from the stomach into the small intestine.