Intestinal floras differ from person to person. Some bacteria produce hydrogen, while others consume it. In one person out of three, an organism called Methanobrevibacter smithii converts hydrogen to methane. The presence of this organism and the methane-producing trait are a result of early environment. Spouses do not share the trait with one another. Another product of fermentation, carbon dioxide, is also released when hydrochloric acid reacts with bicarbonate in the intestines. However, this gas is quickly absorbed. Hydrogen, carbon dioxide, methane and swallowed nitrogen comprise 99% of colon gas. The remaining 1% consists of trace gases that compensate for their small quantities by their strong odors. Smelly gases include hydrogen sulfide ammonia, skatole, indole and volatile fatty acids. 

Borborygmi is the name given to the noises generated as air and fluid gurgle through the gut. Farting and borborygmi do not account for bloating. 

9.3.1 MECHANISM 

During inspiration, the normally negative intraesophageal pressure draws in ambient air. Forced inspiration against a closed glottis (intentionally closed windpipe) draws in even more air. The air may be forced out again as intra-esophageal pressure increases with expiration. Adolescents love to shock their elders with voluntary belching. As a practical application, those who have lost their larynx because of cancer put this learnable skill to use in generating esophageal speech. More commonly, aerophagia is an unwanted habit in those who repeatedly belch in response to other gut symptoms. 

Some air is ingested with each swallow, perhaps more with food. Nervous patients undergoing abdominal x-rays accumulate more intestinal gas than those who are relaxed. Other mechanisms of aerophagia include thumb sucking, gum chewing, drinking carbonated drinks, rapid eating and wearing poor dentures. Stomach gas has the same composition as the atmosphere. 

In achalasia, where the lower esophageal sphincter cannot relax, the stomach is gasless. In bowel obstruction or a gastrocolic fistula colon gases reach the stomach. Sometimes gastric stasis permits bacteria to grow and produce hydrogen in the stomach. Normally, gastric gas is swallowed air. 

Belching is to bring forth wind noisily from the stomach. The word burp means to "cause to belch," as one would burp a baby, but colloquially, the terms are used interchangeably. A belch after a large meal is a physiologic venting of air from the stomach. A meal stretching the muscle of the stomach, which can stretch to accommodate food, causes distress with little increase in intragastric pressure. A satisfying belch eases the discomfort. Some individuals seem unduly sensitive to intragastric pressure. People with gastroenteritis, heartburn or ulcers swallow more frequently. If release of gas transiently relieves the distended feeling, a cycle of air swallowing and belching may be established. The swallow-belch cycle may continue long after the original discomfort is forgotten. 

Of course, venting gas is important, as those unable to do so will attest. When the lower esophageal sphincter is reinforced by antireflux surgery, belching may be impossible. Bedridden patients such as those recovering from surgery may trap air in the stomach. In the supine position gastric contents seal the gastroesophageal junction so that air cannot escape until the subject assumes the prone position. 

While a patient may insist that his or her stomach is producing prodigious amounts of gas, in reality air is drawn into the esophagus and released. A little may even reach the stomach. Some can belch on command, and the inspiration against a closed glottis is demonstrable. Most sufferers are relieved to have their habit pointed out, but some are incredulous. Quitting the habit is often difficult. Repeated and intractable belching is termed eructio nervosa. 

9.4.1 MECHANISM 

Those complaining of bloating and distention are often convinced that it is due to exess intestinal gas. Although the sensation may induce aerophagia, it seldom results from it. Farting may temporarily relieve bloating, but intestinal gas production does not cause it. Research has demonstrated that gas volume in bloaters is not abnormal. Despite visible distention, x-rays and computerized tomography (CT) show no large collections of intestinal gas. The distention disappears with sleep and general anesthesia. 

Gut hypersensitivity may explain the sensation of abdominal bloating. The hypersensitive gut feels full at lower than normal filling, and abdominal muscles relax to accommodate the perceived distention. The stomach is and feels distended with normal amounts of air. 

Abdominal girth of female irritable bowel syndrome (IBS) patients complaining of distention may increase 3 to 4 cm over an eight-hour day. CT has demonstrated the change in profile despite unchanged gas content or distribution. There were no corresponding changes in control subjects. Lumbar lordosis (arching of the spine) is sometimes increased. When women deliberately protrude their abdomens, the configuration is different from when they are bloated, so a conscious mechanism poorly explains increased abdominal girth. Perhaps abdominal muscles are weakened. The reality of the phenomenon is indisputable; the mechanism remains a mystery. 

Bloating occurs in 30% of adults and is frequent in 10%. Amongst those with the irritable bowel syndrome and dyspepsia the figures are much higher. It is often the most troublesome feature of these conditions. Typically, the abdomen is flat upon awakening, but distends progressively during the day, only for the distention to disappear with sleep. Women complain of the need to let out their clothing and sometimes volunteer "It’s as if I’m six months pregnant." Many report that bloating occurs quickly, in some cases within a minute. It is often aggravated by eating and relieved by lying down. Menstrual periods and stress affect a few cases. Usually, it is most obvious in the lower abdomen, but many report it near the umbilicus or all over the belly. 

Observable bloating has been called hysterical nongaseous bloating, pseudotumor or pseudocyesis (false pregnancy). If distention is present at the time of the examination (more likely late in the day), the phenomenon is likely functional. There is no abdominal tympany to suggest gaseous intestines, and sometimes the distended abdomen can be mistaken for ascites or a tumor.