The digestive system is that it works even when a person is upside down, the small intestine is about 22 feet long, and some animals don't have stomachs. The platypus's food, for example goes straight to its intestine without passing through a stomach.
A person doesn't need gravity to digest his food. Contractions called peristalsis move the food along the digestive tract. This is why astronauts can eat in space.
The growls a stomach makes are called borborygmi. They're the result of the action of peristalsis on the contents of the stomach and the intestines. They're louder when the stomach is empty because there's nothing in it to dampen the sound.
The small intestine is not only very long, but if it is stretched and smoothed out it would cover a tennis court. It is full of folds and villi, which are projections that absorb nutrients. Moreover, the villi are covered with microvilli. The large intestine, which is mainly a holding and drying vat, is about 5 feet long in the average human.
The stomach makes about half a gallon of hydrochloric acid every day. The stomach protects itself by producing a coat of mucus, which is refreshed every two weeks or so.