The peritoneum is a closed serous membrane sac that lines the walls of the abdomen like the pleura lines the thoracic walls. The abdominal organs and vessels develop outside this sac and grow into (invaginate) it becoming surrounded by a layer of visceral peritoneum. This relationship is referred to as intraperitoneal (circumperitoneal). Most of the gastrointestinal system exhibits the intraperitoneal relationship.

Some organs found in the abdomen do not grow into the sac very far and are only covered on one side by peritoneum. This relationship is called retro-peritoneal (behind), and applies to most of the organs of the urinary and reproductive systems.

Like the pleural space in the thoracic cavity, the peritoneal cavity is only a potential space between two peritoneal layers containing a small amount of lubricating serous fluid. The serous fluid prevents friction between abdominal organs that move against each other. The mobility of the abdominal organs depends to a great extent on their peritoneal coverings. Intraperitoneal organs of the G.I. system can move with great freedom while the retroperitoneal urogenital organs are less mobile.

Pathological conditions can cause the peritoneal cavity to become distended with tissue fluid, blood, bile, or fecal material.

Specialized portions of the peritoneum suspend and anchor the gastrointestinal organs. A double-layered mesentery suspends the intestines from the posterior abdominal wall. Vessels and nerves traveling to and from the intestines pass between these two layers.

The quadruple-layered greater omentum appears to be extra peritoneum as it drapes over the abdominal contents on their anterior surface. This odd peritoneal structure usually contains fat and lymph vessels. It has a great deal of mobility and can wrap itself around inflamed areas to wall them off and prevent the spread of infection. For this reason, the greater omentum is sometimes called the policeman of the gut. Lucky for us, it doesn't carry a belly club.