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http://en.wikipedia.org/wiki/Flatulence
Flatulence is the presence of a mixture of gases in the digestive tract of mammals. Such a mixture of gases is known as flatus, and is expelled from the rectum in a process also known as flatulence, or as 'breaking wind' or 'farting'.
Explanation
Flatus is expelled under pressure through the anus, whereby, as a result of the voluntary or involuntary relaxation of the anal sphincter, the rapid evacuation of gases from the lower intestine occurs. Essentially this happens when the flatus pressure inside the rectum exceeds the anal sphincter's ability to restrain it. Depending upon the relative state of the sphincter (relaxed/tense) and the positions of the buttocks, this often results in a crackling or trumpeting sound, but gas can also be passed quietly. The olfactory components of flatulence include skatole, indole, and sulfurous compounds.[1] The non-odorous gases are mainly nitrogen (ingested), carbon dioxide (produced by aerobic microbes or ingested), and hydrogen (produced by some microbes), as well as lesser amounts of oxygen (ingested) and methane (produced by anaerobic microbes).[2]
Composition of flatus gases
Nitrogen is the primary gas released. Carbon dioxide is often present, especially in persons who drink carbonated beverages in quantity. Methane and hydrogen, lesser components, are flammable, and so flatus can be ignited. 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.[3] Similar results are found in samples of gas obtained from within the rectum.
The gas released during a flatus event frequently has a foul odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulfur compounds such as hydrogen sulfide (rotten egg smell) and carbonyl sulfide that are the result of protein breakdown. The incidence of odoriferous compounds in flatus increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats or dogs. Flatulence odor 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 odorless) by percentage are:[4]
* Nitrogen - 20% - 90%
* Hydrogen - 0% - 50%
* Carbon Dioxide - 10% - 30%
* Oxygen - 0% - 10%
* Methane - 0% - 10%
Mechanism of action
The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks. 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; on other occasions, flatulence can be voluntarily elicited by tensing the rectum or "bearing down" and subsequently releasing the anal sphincter, resulting in the expulsion of a flatus.
Flatus is brought to the rectum via peristalsis, which also causes feces to descend from the large intestine, and may cause a similar feeling of urgency and discomfort. Nerve endings in the rectum usually enable individuals to distinguish between flatus and feces, [5] although loose stool can confuse the individual, occasionally resulting in accidental defecation.
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 procaryotes normally or abnormally present in the gastrointestinal tract.
Flatulence-producing foods are typically high in certain polysaccharides (especially oligosaccharides such as inulin) and include beans, lentils, dairy products, onions, garlic, scallions, leeks, radishes, sweet potatoes, cashews, Jerusalem artichokes, oats, wheat, yeast in breads, and other vegetables. Cauliflower, Broccoli, cabbage and other cruciferous vegetables that belong to the Brassica family 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 oligosaccharide (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by microorganisms that inhabit the digestive tract. These oligosaccharides pass through the upper intestine largely unchanged, and when these reach the lower intestine, bacteria feed on them, producing copious amounts of flatus.[6] In the case of those with 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.[6] In the field of mountaineering, High Altitude Flatus Expulsion was first noticed over two hundred years ago.
Health effects
As a normal body function, the action of flatulence is an important signal of normal bowel activity and hence is often documented by nursing staff following surgical or other treatment of patients. However, symptoms of excessive flatulence can indicate the presence of irritable bowel syndrome or some other organic disease. In particular, the sudden occurrence of excessive flatulence together with the onset of new symptoms provide reason for seeking further medical examination.
Flatulence is not poisonous; it is a natural component of various intestinal contents. However, discomfort may develop from the build-up of gas pressure. In theory, pathological distension of the bowel, leading to constipation, could result if a person holds in flatulence.
Not all flatus is released from the body via the anus. When the partial pressure of any gas component of the intestinal lumen is higher than its partial pressure in the blood, that component enters into the bloodstream of the intestinal wall by the process of diffusion. As the blood passes through the lungs this gas can diffuse back out of the blood and be exhaled. If a person holds in flatus during daytime, it will often be released during sleep when the body is relaxed. Some flatus can become trapped within the feces during its compaction and will exit the body, still contained within the fecal matter, during the process of defecation.
