Most of the abdominal contents lie within the peritoneum, a sac made up of a sheet of dense connective tissue. Some structures lie behind the peritoneum (retroperitoneal). Others go in and out of it, although edges are sealed and there is little or no direct connection between the intra- and retroperitoneal regions.
The liver has a “bare area” at its top where it lies directly against the lower surface of the diaphragm, but the rest of it is intraperitoneal. The ascending and descending colons are both retroperitoneal, while the transverse colon and part of the sigmoid are intraperitoneal; the duodenum, or first portion of the small intestine, is retroperitoneal.
True retroperitoneal structures include the pancreas, the kidneys, ureters and adrenals, the great vessels and the pelvic structures.