Sagittal section of the abdomen. Intraperitoneal organs (organa intraperitonealia) are completely enveloped by peritoneum viscerale. These organs are the liver (hepar), spleen (splen), stomach (gaster), pars superior duodeni, jejunum, ileum, colon transversum, colon sigmoideum and proximal third of the rectum. Retroperitoneal organs (organa retroperitonalia) are found posterior to the peritoneum in the spatium retroperitoneale with only their anterior walls covered by peritoneum parietale. If they develop and remain outside the peritoneum, they are primary retroperitoneal organs: kidney (ren), glandulae suprarenales and ureter. Other retroperitoneal organs develop inside the peritoneum, but then move posterior to it and fuse with the abdominal wall: pancreas, pars distalis duodeni, colon ascendens/descendens. The great blood vessels (i.e. aorta abdominalis, v. cava inferior) are also retroperitoneal.
