An “ostomy” is an opening created surgically to allow the bowel to empty through the abdominal wall. Connection of the end of the small bowel and the large bowel in this manner are known, respectively, as ileostomy and colostomy.

The continent, or moderate, ileostomy is a loop of small bowel fashioned under the skin with a nipple valve to prevent spillage. Ileal effluent collects in this reservoir, which must be emptied with a soft rubber catheter. The second type opens directly onto the abdomen, requiring an external ileostomy or colostomy appliance.

The direct type of ileostomy and colostomy poses no danger to the diver. Ensuring a secure fit of the external bag can prevent spillage of fecal material. A continent ileostomy poses theoretical risk if gas swallowed during the dive cannot escape through the ileostomy site.