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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', 'trumping' 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]

COMPISITION 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 or during orgasm; 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 by the same process which 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.

SOCIAL CONTEXT
In many cultures, human flatulence in public is regarded as embarrassing and repulsive, even to the point of being a taboo subject. People will often strain to hold in the passing of gas when in polite company, or position themselves to conceal the noise and smell. In other cultures it may be no more embarrassing than coughing. It is even a sign of happiness in some cultures.

While the act of passing flatus is generally considered to be an unfortunate occurrence in public settings, flatulence may, in casual circumstances, be used as either a humorous supplement to a joke, or as a comic activity in and of itself.

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