Small intestine
It is approximately 1.5 cm in diameter in newborns after 35 weeks of gestational age, and 2.5–3 cm (1 inch) in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm. On CT scans, a diameter of over 2.5 cm is considered abnormally dilated. The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square meters.
Gallstone ileus
First described by Thomas Bartholin in 1654, gallstone ileus is a rare form of small bowel obstruction caused by an impaction of a gallstone within the lumen (small opening) of the small intestine. Such a gallstone enters the bowel via a cholecysto-enteric fistula. The presence of large stones, >2.5 cm in diameter, within the gallbladder are thought to predispose to fistula formation by gradual erosion through the gallbladder fundus. Once a fistula has formed, a stone may travel from the gallbladder into the bowel and become lodged almost anywhere along the GI tract. Obstruction occurs most commonly at the near the distal ileum, within 60 cm proximally to the ileocecal valve. Rarely, gallstone ileus may recur if the underlying fistula is not treated.