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Pathophysiology of Intestinal Gas
The pathophys.iology of intestinal gas symptoms is poorly understood; however, minor disruptions of normal physiologic processes of the Gf tract appear to play a role. Each rime food, liquid. and saliva are swallowed, a small amount of air from the atmosphere passes imo the stomach. In the stomach, the swallowed food is mixed with gastric acid, pepsin, and other substances; churned into small fi·agn1ents; and tl1en emptied into the small intestine, where most of the absorption of vitamins, minerals, and digestion products (e.g., food-derived monosaccharides such as glucose) occurs.1" The rare at which the stomach empties va1ies but generally takes about I tO 2 hours. Smooth muscle cona·actions in the small intestine move the l.iquid food fi·agments and air downstream toward the large intestine, where the indigestible liquid waste is mixed with the bacte1 ial flora of the colon. In the colon, most of d1e remaining liquid is absorbed fi·om the mixmre ofliquid waste, bacteria, and intestina.l gas as it is transported toward the rectwll and tempor;uily stored as stool prior to a bowel movement. During a bowel movement, stool is eliminated and intestinal gas is expelled fi·om the rectum as A arus. Gas is produced and removed by various mechanical and biochemical processes during the transport, digestion, and elirnination of food, nutrients, and wast.e fi·om the Gl tract (Figure 15-1). At any given rime, approximately 200 mL of gas resides in the G I tract of most individuals and is excreted at a rate ranging fi·om 500 to 1500 mL/day. The primary gases present in the intestine are nitrogen (N2), hydrogen (H1), methane (CH4), carbon dioxide (C0 2), and o>:ygcn (02), accounting for more than 99% of gas passed through the rectum (Figure 15- '1 ). 11 None of d1ese principal gases causes odor. The remaining 1% of colonic gas consists of volatiJe gases, such as hydrogen sulfide (H2S) and other sulfur-based gases (e.g., methanethjol and dimethyl sulfide), which are produced by bacteria in the colon. These trace gases have an unpleasant smell and are noticeable in quantities as low as a few parts per million. The source and composition of intestinal gas vary throughout the G l tract. Gas in the upper G I rract arises fi:om the swallowing of atmospheric air, which consists primarily of N2 (78% by volume) and 0 2 (20.8% by volume). Most swallowed air in the stomach is subsequently eliminated through belching. In the first portion of the small intestine, C02 is produced when bica rbonate is secreted to neutralize gasttic acid, and fi·om metabolism of dietary substrates (such as f.1t and prorein).11 Most of the C02 that is formed in the upper GI tract beyond the stomach