Volatile material (mostly swallowed air, partly digestive by-products) in the digestive tract, which normally contains 150–500 cc of gas.
Air in the stomach is either belched out or passed to the intestines. Some of its oxygen is absorbed into the blood along the way. Carbon dioxide produced by digestion is added. Nitrogen, the major component, is inert and usually passed on. Obstructions in the small intestine can trap gas in distended pockets, causing severe pain. In the large intestine, bacterial fermentation products are added
mostly hydrogen but also methane, hydrogen sulfide, ammonia, and sulfur-containing mercaptans. Excess gas in the colon is eventually expelled from the body.