Peritoneum

 ( Abdomen ) 

The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in and some invertebrates, such as . It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesothelium supported by a thin layer of connective tissue. This peritoneal lining of the cavity supports many of the abdominal organs and serves as a conduit for their , , and .

The abdominal cavity (the space bounded by the vertebrae, abdominal muscles, diaphragm, and pelvic floor) is different from the intraperitoneal space (located within the abdominal cavity but wrapped in peritoneum). The structures within the intraperitoneal space are called "intraperitoneal" (e.g., the stomach and ), the structures in the abdominal cavity that are located behind the intraperitoneal space are called "retroperitoneal" (e.g., the ), and those structures below the intraperitoneal space are called "subperitoneal" or "infraperitoneal" (e.g., the bladder).

The outer layer, the parietal peritoneum, is attached to the abdominal wall and the pelvic walls.

The inner layer, the visceral peritoneum, is wrapped around the visceral organs, located inside the intraperitoneal space for protection. It is thinner than the parietal peritoneum. The mesentery is a double layer of visceral peritoneum that attaches to the gastrointestinal tract. There are often blood vessels, nerves, and other structures between these layers. The space between these two layers is technically outside of the peritoneal sac, and thus not in the peritoneal cavity.

The potential space between these two layers is the peritoneal cavity, filled with a small amount (about 50 mL) of slippery serous fluid that allows the two layers to slide freely over each other.

The right paracolic gutter is continuous with the right and left subhepatic spaces. The epiploic foramen allows communication between the greater sac and the lesser sac. The peritoneal space in males is closed, while the peritoneal space in females is continuous with the extraperitoneal pelvis through openings of the fallopian tubes, the uterus, and the vagina.

• The greater sac, represented in red in the diagrams above.
• The lesser sac, represented in blue. The lesser sac is divided into two "omenta":
• The lesser omentum (or hepatogastric) is attached to the lesser curvature of the stomach and the liver.
  (1984). 9780060466565, Harper & Row. ISBN 9780060466565
• The greater omentum (or gastrocolic) hangs from the greater curvature of the stomach and loops down in front of the intestines before curving back upwards to attach to the transverse colon. In effect it is draped in front of the intestines like an apron and may serve as an insulating or protective layer.

The mesentery is the part of the peritoneum through which most abdominal organs are attached to the abdominal wall and supplied with blood and lymph vessels and nerves.

In addition, in the pelvic cavity there are several structures that are usually named not for the peritoneum, but for the areas defined by the peritoneal folds:

Structures that are intraperitoneal are generally mobile, while those that are retroperitoneal are relatively fixed in their location.

Some structures, such as the kidneys, are "primarily retroperitoneal", while others such as the majority of the duodenum, are "secondarily retroperitoneal", meaning that structure developed intraperitoneally but lost its mesentery and thus became retroperitoneal.

As an embryo develops, the various abdominal organs grow into the abdominal cavity from structures in the abdominal wall. In this process they become enveloped in a layer of peritoneum. The growing organs "take their blood vessels with them" from the abdominal wall, and these blood vessels become covered by peritoneum, forming a mesentery.

Peritoneal folds develop from the ventral and dorsal mesentery of the embryo.

Image:Gray403.png|Median sagittal section of pelvis, showing the arrangement of fasciæ
Image:Gray1038.png|Horizontal disposition of the peritoneum in the lower part of the abdomen
Image:Gray1125.png|Sagittal section through posterior abdominal wall, showing the relations of the capsule of the kidney
Image:Gray1224.png|Topography of thoracic and abdominal viscera
Image:Gray1039.png|Horizontal disposition of the peritoneum in the upper part of the [[abdomen]]
Image:Cytology of normal mesothelium.jpg|[[Cytology]] of the normal [[mesothelial cell]]s that line the peritoneum, with typical features.Image by Mikael Häggström, MD. Sources for mentioned features:
- 
-  Wright's stain