The stomach can be likened to a storage and processing facility, where the food is prepared for digestion.
This food warehouse can accommodate anything from a light afternoon snack to a five-course meal. Without this large storage capacity, people would have to eat small, frequent meals, and they'd be unable to drink large quantities of liquids at any given time.
But the stomach doesn't just hold food: muscles in the lower stomach also mix that food into a soft mush. This process is aided by the liquids we drink and by saliva, hydrochloric acid, and the enzyme pepsin. Hydrochloric acid and pepsin, produced by the glands that line the stomach, help break down proteins into their constituent amino acids. The stomach mucosa has a defense system, including an overlying layer of mucus and bicarbonate, to protect itself. After mixing, a once-palatable meal is reduced to a thick liquid called chyme.
The other important function of the stomach is delivering the resulting chyme to the small intestine in amounts it can handle. The involuntary contractions that push stomach contents along are governed by nerves in the stomach wall, which transmit electrical impulses to the brain. The nerves that carry impulses from the gastrointestinal (GI) tract, called visceral nerves, recognize stretching, pulling, or expansion (distension) of the muscles in the walls of the digestive tract. Pain can result when these sensations are excessive.