Editing Flatulence (section)

==Mechanism==

===Production, composition, and odor===
Flatus (intestinal gas) is mostly produced as a byproduct of bacterial fermentation in the gastrointestinal (GI) tract, especially the [[Colon (anatomy)|colon]].<ref name="Tomlin 1991" /> There are reports of [[aerophagia]] (excessive air swallowing) causing excessive intestinal gas, but this is considered rare.<ref name="Hemmink 2009">{{cite journal |last=Hemmink |first=GJ |author2=Weusten, BL |author3=Bredenoord, AJ |author4=Timmer, R |author5= Smout, AJ |title=Aerophagia: excessive air swallowing demonstrated by esophageal impedance monitoring |journal=[[Clinical Gastroenterology and Hepatology]] |date=October 2009 |volume=7 |issue=10 |pages=1127–29 |pmid=19602452 |doi=10.1016/j.cgh.2009.06.029|doi-access=free }}</ref>

Over 99% of the volume of flatus is composed of odorless gases.<ref name="Tangerman 2009" /> These include [[oxygen]], [[nitrogen]], [[carbon dioxide]], [[hydrogen]] and [[methane]]. Nitrogen is not produced in the gut, but a component of environmental air. Patients who have excessive intestinal gas that is mostly composed of nitrogen have aerophagia.<ref name="Levitt 1998" /> Hydrogen, carbon dioxide and methane are all produced in the gut and contribute 74% of the volume of flatus in normal subjects.<ref name="Suarez 1997" /> Methane and hydrogen are [[flammable]], and so [[Fart lighting|flatus can be ignited]] if it contains adequate amounts of these components.<ref name="Mercer2009">{{cite book |last=Mercer |first=Bobby |title=How Do You Light a Fart?: And 150 Other Essential Things Every Guy Should Know about Science |url=https://books.google.com/books?id=gV01_pPEO_AC&pg=PT88 |date=2009-04-18 |publisher=[[Adams Media]] |isbn=9781440519871 |page=71 |access-date=October 2, 2014 }}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Not all humans produce flatus that contains methane. For example, in one study of the [[feces|faeces]] of nine adults, only five of the samples contained [[archaea]] capable of producing methane.<ref>{{cite journal| author=Miller TL|author2=Wolin MJ |author3=de Macario EC |author4=Macario AJ | title=Isolation of ''Methanobrevibacter smithii'' from human faeces| journal=[[Applied and Environmental Microbiology]] | year=1982| volume=43| pages=227–32 |pmid=6798932 |pmc=241804 |doi= 10.1128/AEM.43.1.227-232.1982| issue=1|bibcode=1982ApEnM..43..227M }}</ref> The prevalence of methane over hydrogen in human flatus may correlate with obesity, constipation and irritable bowel syndrome, as archaea that oxidise hydrogen into methane promote the metabolism's ability to absorb fatty acids from food.<ref name="Pimentel2012">{{cite journal |last=Pimentel |first=Mark |author2=Robert P Gunsalus |author3=Satish SC Rao |author4=Husen Zhang |year=2012|title=Methanogens in Human Health and Disease |journal=[[The American Journal of Gastroenterology|The American Journal of Gastroenterology Supplements]] |volume=1 |issue=1 |pages=28–33|doi=10.1038/ajgsup.2012.6 |doi-access=free }}</ref>

The remaining trace (<1% volume) compounds contribute to the odor of flatus. Historically, compounds such as [[indole]], [[skatole]], [[ammonia]] and [[short chain fatty acids]] were thought to cause the odor of flatus. More recent evidence proves that the major contribution to the odor of flatus comes from a combination of volatile [[sulfur]] compounds.<ref name="Tangerman 2009" /><ref name="Suarez 1998">{{cite journal |last=Suarez |first=FL |author2=Springfield, J |author3=Levitt, MD |title=Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour |journal=[[Gut (journal)|Gut]] |date=July 1998 |volume=43 |issue=1 |pages=100–04 |pmid=9771412 |pmc=1727181 |doi=10.1136/gut.43.1.100}}</ref> [[Hydrogen sulfide]], [[methyl mercaptan]] (also known as [[methanethiol]]), [[dimethyl sulfide]], [[dimethyl disulfide]] and [[dimethyl trisulfide]] are present in flatus. The benzopyrrole volatiles [[indole]] and [[skatole]] have an odor of mothballs, <!-- <ref name="Moore 1987" /> ref name not defined--> and therefore probably do not contribute greatly to the characteristic odor of flatus.

In one study, hydrogen sulfide concentration was shown to correlate convincingly with perceived bad odor of flatus, followed by methyl mercaptan and dimethyl sulfide.<ref name="Levitt 1998" /> This is supported by the fact that hydrogen sulfide may be the most abundant volatile sulfur compound present. These results were generated from subjects who were eating a diet high in [[pinto beans]] to stimulate flatus production.

Others report that methyl mercaptan was the greatest contributor to the odor of flatus in patients not under any specific dietary alterations.<ref name="Tangerman 2009" /><!-- <ref name="Moore 1987" /> no ref name defined --> It has now been demonstrated that methyl mercaptan, dimethyl sulfide, and hydrogen sulfide (described as decomposing vegetables, unpleasantly sweet/wild radish and rotten eggs respectively) are all present in human flatus in concentrations above their smell perception thresholds.<ref name="Tangerman 2009" />

It is recognized that increased dietary sulfur-containing amino acids significantly increases the odor of flatus. It is therefore likely that the odor of flatus is created by a combination of volatile sulfur compounds, with minimal contribution from non-sulfur volatiles.<ref name="Levitt 1998" /> This odor can also be caused by the presence of large numbers of microflora [[bacteria]] or the presence of faeces in the rectum. Diets high in protein, especially sulfur-containing amino acids, have been demonstrated to significantly increase the odor of flatus.

===Volume and intestinal gas dynamics===
Normal flatus volume is 476 to 1491 mL per 24 hours.<ref name="Tangerman 2009">{{cite journal |last=Tangerman |first=Albert |title=Measurement and biological significance of the volatile sulfur compounds hydrogen sulfide, methanethiol and dimethyl sulfide in various biological matrices |journal=[[Journal of Chromatography B]] |date=October 1, 2009 |volume=877 |issue=28 |pages=3366–77|doi=10.1016/j.jchromb.2009.05.026|pmid=19505855}}</ref><ref name="Tomlin 1991">{{cite journal |last=Tomlin |first=J |author2=Lowis, C |author3=Read, NW |title=Investigation of normal flatus production in healthy volunteers |journal=[[Gut (journal)|Gut]] |date=June 1991 |volume=32 |issue=6 |pages=665–669 |pmid=1648028 |pmc=1378885 |doi=10.1136/gut.32.6.665}}</ref> This variability between individuals is greatly dependent upon diet. Similarly, the number of flatus episodes per day is variable; the normal range is given as 8–20 per day.<ref name="Levitt 1998">{{cite journal |last=Levitt |first=MD |author2=Furne, J |author3=Aeolus, MR |author4= Suarez, FL |title=Evaluation of an extremely flatulent patient: case report and proposed diagnostic and therapeutic approach |journal=[[The American Journal of Gastroenterology]] |date=November 1998 |volume=93 |issue=11 |pages=2276–81 |doi=10.1111/j.1572-0241.1998.00635.x |pmid=9820415 |s2cid=20991299 }}</ref><!-- <ref name="Levitt 1979" /> ref name not defined--> The volume of flatus associated with each flatulence event again varies (5–375&nbsp;mL).<ref name="Tangerman 2009" /><ref name="Tomlin 1991" /><ref name="Suarez 1997">{{cite journal |last=Suarez |first=F |author2=Furne, J |author3=Springfield, J |author4= Levitt, M |title=Insights into human colonic physiology obtained from the study of flatus composition |journal=[[American Journal of Physiology]] |date=May 1997 |volume=272 |issue=5 Pt 1 |pages=G1028–33 |pmid=9176210|doi=10.1152/ajpgi.1997.272.5.G1028 }}</ref> The volume of the first flatulence upon waking in the morning is significantly larger than those during the day.<ref name="Tangerman 2009" /> This may be due to buildup of intestinal gas in the colon during sleep, the peak in peristaltic activity in the first few hours after waking or the strong prokinetic effect of rectal distension on the rate of transit of intestinal gas.<ref name="Azpiroz 2005">{{cite journal |last=Azpiroz |first=F |title=Intestinal gas dynamics: mechanisms and clinical relevance |journal=[[Gut (journal)|Gut]] |date=July 1, 2005 |volume=54 |issue=7 |pages=893–95 |doi=10.1136/gut.2004.048868|pmid=15951528 |pmc=1774596 }}</ref> It is now known that gas is moved along the gut independently of solids and liquids, and this transit is more efficient in the erect position compared to when supine.<ref name="Azpiroz 2005" /> It is thought that large volumes of intestinal gas present low resistance, and can be propelled by subtle changes in gut tone, capacitance and proximal contraction and distal relaxation. This process is thought not to affect solid and liquid intra-lumenal contents.<ref name="Azpiroz 2005" />

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 faeces, thereby helping detect a need to defecate or to signal the end of defecation.<ref>{{cite journal |author=Read, M. G. |author2=Read, N. W. |title=Role of anorectal sensation in preserving continence |journal=[[Gut (journal)|Gut]] |year=1982 |volume=23 |pages=345–47 |doi=10.1136/gut.23.4.345 |pmid=7076012 |pmc=1419736|issue=4}}</ref>

The sound varies depending on the volume of gas, the size of the opening that the air is being pushed through, which is affected by the state of tension in the [[sphincter muscle]], and the [[force]] or [[velocity]] of the [[gas]] being propelled, as well as other factors, such as whether the gas was caused by swallowed air.<ref name="Totten 2017 j939">{{cite web | last=Totten | first=Sanden | title=There is physics in every fart | website=WHYY | date=February 28, 2017 | url=https://whyy.org/segments/there-is-physics-in-every-fart/ | access-date=February 22, 2024}}</ref><ref name="Shiffer 2018 u561">{{cite web | last=Shiffer | first=Emily J. | title=Fart Noises: Why Some Farts Are Silent and Others Are Loud | website=Men's Health | date=January 2, 2018 | url=https://www.menshealth.com/health/a19545944/fart-noises/ | access-date=February 22, 2024}}</ref> Among humans, flatulence occasionally happens accidentally, such as incidentally to [[cough]]ing<ref name="Cavallari_2017">{{cite journal |last1=Cavallari |first1=Paolo |last2=Bolzoni |first2=Francesco |last3=Esposti |first3=Roberto  |last4=Bruttini |first4=Carlo  |date=2017-09-27 |title=Cough-Anal Reflex May Be the Expression of a Pre-Programmed Postural Action |journal=Frontiers in Human Neuroscience |volume=11 |issue=475 |page=475 |doi=10.3389/fnhum.2017.00475 |doi-access=free |pmid=29021750 |pmc=5624195 }}</ref> 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.{{Citation needed|date=September 2021}}