INTESTINAL GAS

volatile material contained within the digestive tract. Intestinal gas consists principally of swallowed air and partly of by-products of digestion. In humans the digestive tract contains normally between 150 and 500 cubic centimetres of gas. During eating, air is swallowed into the stomach; this is either eructated or passed on to the intestines. Gas in the stomach contains approximately 15 to 16 percent oxygen and 5 to 9 percent carbon dioxide; the rest is nitrogen. The air that is breathed contains about 21 percent oxygen; thus, some of the swallowed oxygen is absorbed by the blood capillaries in the stomach. Carbon dioxide is formed by reduction of the food by the stomach's gastric juices. Nitrogen is an inert element that is neither used nor absorbed as a gas, and it is usually passed on. The small intestine absorbs some of the carbon dioxide and oxygen and rapidly passes the remaining gas to the large intestine. If obstructions occur in the small intestine, gas pockets can accumulate containing as much as 3,500 cubic centimetres of gas. These pockets distend the small intestine, causing severe pain. Normally, gas passes through the small intestine with the regular intestinal movements. In the large intestine, or colon, the gas volume is usually 100 to 200 cubic centimetres. Most of the oxygen has been removed, and the amount of carbon dioxide has increased. New gases formed from bacterial fermentation are added in the colon. Of the new gases produced, hydrogen is the major component. Some of this is absorbed by the blood and released through the lungs during breathing. Other gas products formed are methane, hydrogen sulfide, ammonia, and various sulfur-containing mercaptans. Excess gas in the colon is eventually passed from the body by a process known as flatulence. Certain foods, such as beans, pork, onions, cabbage, and cauliflower, because of their high sulfur content, are known to increase gas production. Cows and other animals that eat mainly plant life have a larger volume of intestinal gas than do human beings and may produce several litres of gas daily.