Editing Flatulence (section)

===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}}