Flatulence is defined in the medical literature as “flatus expelled through the anus” or the “quality or state of being flatulent”, which is defined in turn as “marked by or affected with gases generated in the intestine or stomach
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消化系統(消化道)氣體或胃腸氣、胃腸氣脹,俗稱屁(flatulence),指動物消化道肛門排出的氣體。有研究指出,牛羊等反芻動物放屁所產生的甲烷可能導致全球暖化.  當食物蛋白質含量較高(如肉類),屁中硫化氫、吲哚和糞臭素的含量升高,此時屁有明顯的臭味;當食物澱粉含量較高(如薯類),屁中二氧化碳含量升高,此時屁量明顯增多。因為二氧化碳的增加量明顯大於硫化氫、吲哚和糞臭素,而且氣壓增大時,放屁的氣流速度也增大,放屁的聲音也就更響,但是二氧化碳沒有氣味,所以有「響屁不臭,臭屁不響」和「響屁不臭悶屁臭」的說法。



A considerable amount of gas is present in the gastrointestinal contents of all animals, and much of this is eliminated through the anus as flatus.

Five gases constitute greater than 99% of the gases passed as flatus: CO2, H2, N2, O2 and methane. None of these gases has an odor, and the characteristic odor of feces is due to very small quantities of a few other gases, including hydrogen sulfide, indole 吲哚 (a crystalline organic compound with an unpleasant odor, present in coal tar and in feces) and skatole. There is considerable individual variation in the contribution of each of these gases to total gas, but nitrogen typically predominates. Volume of gas elaborated also varies widely. In normal adult humans, the rate of excretion of gas per rectum ranges between 200 and 2000 ml per day. Ingestion of certain foods, beans being the classical example, is widely recognized to increase the rate of gas production.

Indole is an aromatic heterocyclic organic compound with formula C8H7N. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. Indole is widely distributed in the natural environment and can be produced by a variety of bacteria.
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Skatole or 3-methylindole is a mildly toxic white crystalline organic compound belonging to the indole family. It occurs naturally in feces (it is produced from tryptophan in the mammalian digestive tract) and coal tar and has a strong fecal odor.
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