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 mixture of gases is known as flatus in medical speak, informally as a fart, or simply (in American English) gas, and is 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.
Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide, which is present in higher quantities in those who drink carbonated beverages regularly.
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 odor.
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 as cats. Such 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:
- Nitrogen: 20–90%
- Hydrogen: 0–50%
- Carbon dioxide: 10–30%
- Oxygen: 0–10%
- Methane: 0–10%
As a normal body function, the action of flatulence is an important signal of 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 if an attempt is made to refrain from releasing them.
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 involuntarily 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.
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