This is a young patient who has had diarrhea for several years and had right lower quadrant pain associated with it. He presented with a 3 day history of no bowel movement, increased pain and belly distension. You examine the patient and get a flat film. In this patient, there is too much gas. You look at the distal bowel and don't see any gas, so you think mechanical obstruction.
Small bowel is fixed centrally making it easy to differentiate from large bowel.
When the small bowel dilates greater than 3 cm it is abnormal. This is somebody with stenosing Crohn's disease. The inflammation has obstructed the small bowel and the patient needs surgical intervention. When you stand such a patient up in an upright view, they have what is called stair step or step latter appearance from air-fluid levels. Small bowel is a fluid absorptive organ and when it is not functioning well, it does not absorb fluids.