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Flatulence
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Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply (in American English) gas. The gases are expelled from the rectum in a process colloquially referred to as "passing gas", "breaking wind" or "farting". Flatus is brought to the rectum by the same peristaltic process which causes feces to descend from the large intestine. The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks.
Composition
Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide. The lesser component gases methane and hydrogen are flammable, and so flatus containing adequate amounts of these can be ignited. However, not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane. Similar results are found in samples of gas obtained from within the rectum.
The gas released during a flatus event frequently has an unpleasant odour. For many years, this was thought to be due to skatole and indole, which are byproducts of the digestion of meat. However, gas chromatography testing in 1984 revealed that sulfur-containing compounds, such as methanethiol, hydrogen sulfide (rotten egg smell) and dimethyl sulfide, were also responsible for the smell.
The incidence of odoriferous compounds in flatulence emissions increases from herbivores, such as cattle, through omnivores to carnivorous species. Such odour can also be caused by the presence of large numbers of microflora bacteria and/or the presence of feces in the rectum.
The major components of the flatus, which are odourless, by percentage are:
- Nitrogen: 20–90%
- Hydrogen: 0–50%
- Carbon dioxide: 10–30%
- Oxygen: 0–10%
- Methane: 0–10%
Physiology
Flatus is brought to the rectum by the same process which causes feces to descend from the large intestine (peristaltic movement), and may cause a similar feeling of urgency and discomfort. Researchers investigating the role of sensory nerve endings in the anal canal did not find them to be essential for retaining fluids in the anus, and instead speculate that their role may be to distinguish between flatus and feces, thereby helping detect a need to defecate or to signal the end of defecation.
The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled, as well as other factors, such as water and body fat. The auditory pitch (sound) of the flatulence outburst can also be affected by the anal embouchure. Among humans, flatulence occasionally happens accidentally, such as incidentally to coughing or sneezing or during orgasm; on other occasions, flatulence can be voluntarily elicited by tensing the rectum or "bearing down" on stomach or bowel muscles and subsequently relaxing the anal sphincter, resulting in the expulsion of flatus.
Causes
Intestinal gas is composed of varying quantities of exogenous sources (air that is ingested through the nose and mouth) and endogenous sources (gas produced within the digestive tract). The exogenous gases are swallowed (aerophagia) when eating or drinking or increased swallowing during times of excessive salivation (as might occur when nauseated or as the result of gastroesophageal reflux disease). The endogenous gases are produced either as a by-product of digesting certain types of food, or of incomplete digestion. Anything that causes food to be incompletely digested by the stomach and/or small intestine may cause flatulence when the material arrives in the large intestine, due to fermentation by yeast or prokaryotes normally or abnormally present in the gastrointestinal tract.
Flatulence-producing foods are typically high in certain polysaccharides, (especially oligosaccharides such as inulin). Those foods include beans, lentils, dairy products, onions, garlic, scallions, leeks, turnips, rutabagas, radishes, sweet potatoes, potatoes, cashews, Jerusalem artichokes, oats, wheat, and yeast in breads. Cauliflower, broccoli, cabbage, Brussels sprouts and other cruciferous vegetables that belong to the genus Brassica are commonly reputed to not only increase flatulence, but to increase the pungency of the flatus.
In beans, endogenous gases seem to arise from complex oligosaccharides (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by gut flora – microorganisms (methane-producing archaea; Methanobrevibacter smithii) that inhabit the digestive tract. These oligosaccharides pass through the upper intestine largely unchanged, and when they reach the lower intestine, bacteria feed on them, producing copious amounts of flatus.
In the case of people who have lactose intolerance, intestinal bacteria feeding on lactose can give rise to excessive gas production when milk or lactose-containing substances have been consumed.
Interest in the causes of flatulence was spurred by high-altitude flight and the space program; the low atmospheric pressure, confined conditions, and stresses peculiar to those endeavours were cause for concern. In the field of mountaineering, the phenomena of high altitude flatus expulsion was first recorded over two hundred years ago.
Some infections, such as giardiasis, are also associated with flatulence.
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