The peritoneum is a thin serous membrane that lines the walls of the abdominal and pelvic cavities and clothes the viscera. The peritoneum can be regarded as a balloon against which organs are pressed from outside. The parietal peritoneum lines the walls of the abdominal and pelvic cavities, and the visceral peritoneum covers the organs.
The potential space between the parietal and visceral layers, which is in effect the inside space of the balloon, is called the peritoneal cavity. In males, this is a closed cavity, but in females, there is communication with the exterior through the uterine tubes, the uterus, and the vagina.
Between the parietal peritoneum and the fascial lining of the abdominal and pelvic walls is a layer of connective tissue called the extraperitoneal tissue; in the area of the kidneys, this tissue contains a large amount of fat, which supports the kidneys.