functional or mechanical blockage of the alimentary canal. Functional blockage occurs when the muscles of the intestinal wall fail to contract normally in the wavelike sequence (peristalsis) that propels the intestinal contents. Mechanical obstructions include a narrowing of the channel (stricture), adhesions, tumours, the presence of a foreign object, pressure from outside, hernia, volvulus, and intussusception. In a hernia a loop of intestine protruding from the abdomen may be compressed at the point where it passes through the abdominal wall. A volvulus is a twisting of the intestine upon itself, and an intussusception is the telescoping of a section of intestine into an adjacent portion.

The loss of fluids and chemicals through vomiting is frequent with obstructions that are located high in the intestinal canal. Obstructions of the lower reaches of the small intestine (ileum) and of the large intestine are inaccessible to evacuation by vomiting. In such cases, the intestine above the obstruction becomes distended by accumulated material and swallowed air; the pressure in the intestinal channel may compromise the blood supply and cause death of tissue in the walls of the intestine, and the walls may become abnormally permeable, allowing noxious agents to escape into the abdominal cavity and the bloodstream. The signs and symptoms of intestinal obstruction, as well as the treatment, depend upon the nature of the obstruction and its location.