Final answer:
Retroperitoneal organs are those in direct contact with the posterior abdominal and pelvic walls, only partially covered by the peritoneum. They include the duodenum, pancreas, ascending and descending colons, and the rectum. The peritoneum, which supports these organs, is divided into the parietal and visceral layers with a lubricated cavity in-between.
Step-by-step explanation:
The organs you are referring to, which are in direct contact with the posterior abdominal and pelvic walls and only covered on their anterolateral surfaces with visceral peritoneum, are known as retroperitoneal organs. During fetal development, certain digestive structures such as the first portion of the small intestine (duodenum), the pancreas, portions of the large intestine (ascending and descending colon), and the rectum are completely or partially located behind the peritoneum, making them retroperitoneal. The peritoneum itself is a serous membrane that comprises a parietal peritoneum, lining the abdominal wall, and a visceral peritoneum, which envelops the abdominal organs, with the peritoneal cavity lying in between. Retroperitoneal organs are therefore situated outside the peritoneal cavity, between the parietal peritoneum and the abdominal wall, and due to their positioning, they are afforded protection by reducing friction that could cause inflammation.