When we think about barotrauma, we usually consider damage to the lungs and upper air passages - the ears and sinuses - and forget that it can affect that other gas-containing organ in the body, the gut.
Fortunately this happens only rarely, for two reasons. First, the total amount of gas in the gut does not usually change much during a dive. Second, the gas is in a very flexible container. As pressure increases during descent, the gas is compressed and the abdominal wall pushed inwards. On ascent, as pressure is reduced, the gas re-expands to fill its original volume.
Gas is produced slowly in the gut, so barotrauma is likely to occur only if more gas has entered the gut during the dive, and cannot be vented on ascent.
This can happen when we swallow air. Air-swallowing is common enough in everyday life, but if one does it at depth, there is potential for considerable volume increase on ascent.
If the diver is unable to get rid of this gas by belching, the stomach can become painfully distended. The stomach wall may be traumatised, and I recently heard of a case in which a small tear occurred in the stomach, allowing the gas to pass out into the abdominal cavity. The diver required an operation.
Failure to belch to relieve the increased volume can occur when the muscular valve separating the bottom of the gullet from the top of the stomach will not relax. In practice divers tend to experience the opposite problem, when they have a valve that is too lax.
Hydrostatic pressure on the abdomen increases during immersion, pushing the acidic contents of the stomach into the gullet and causing heartburn.
This produces a burning pain in the chest and sometimes an unpleasant taste in the mouth. The condition may be helped by taking antacids before the dive.