The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom. It covers most of the intra-abdominal, or coelomic, organs. It is composed of a layer of mesothelial tissue, supported by a thin layer of connective tissue.
The peritoneum provides support and protection for the abdominal organs, and is the main conduit for the associated lymph vessels, nerves, and abdominal arteries and veins.
The abdominal cavity is the open space surrounded by the vertebrae, abdominal muscles, diaphragm, and pelvic floor. Remember not to confuse the abdominal cavity with the intraperitoneal space, which is located within the abdominal cavity and wrapped in peritoneum tissue. For example, a kidney is inside the abdominal cavity, but is retroperitoneal—located outside the peritoneum.