Physiology Gas is present in the gut as a result of swallowed air, production in the lumen, or diffusion from the blood into the lumen. In an upright person, air rises above the liquid contents of the stomach, comes in contact with the gastroesophageal junction, and is readily belched. In a supine person, air trapped below the stomach fluid tends to be propelled into the duodenum. Belching may be associated with use of antacids. Attributing the relief of ulcer symptoms to belching rather than to antacids, the person may continue to belch to relieve distress.
Gas is produced in the lumen by several mechanisms.
Normal persons incompletely absorb carbohydrates in certain common foods. Celiac disease, tropical sprue, pancreatic insufficiency, and other causes of carbohydrate malabsorption should also be considered in cases of excess colonic gas. The tendency to produce large quantities is familial, appearing during infancy and persisting for life. CO 2 may also be produced by bacterial metabolism, but a more important source is the reaction of HCO 3 and H 2 ions, in which 22.4 mL of CO 2 is released for each mEq of HCO 3 . H 2 ions may be derived endogenously from gastric hydrochloric acid or exogenously from the fatty acids released during digestion of fats, the latter sometimes representing several hundred milliequivalents of H 2 ion.