Peritoneum
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In higher vertebrates, the peritoneum is the membrane that forms the lining of the abdominal cavity - it covers most of the intra-abdominal organs. The corresponding tissues in the pleural and pericardial cavities of the thorax are called the pleura and the pericardium respectively.
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Structure and development
The peritoneum consists of two layers: the outer layer, called the parietal peritoneum, is attached to the wall of the abdominal cavity and the inner layer, the visceral peritoneum, is wrapped around the organs that are located inside the cavity. The potential space between these two layers is the peritoneal cavity; it is filled with a small amount (about 50 ml) of slippery fluid that allows the two layers to slide freely over each other.
Most abdominal organs are attached to the abdominal wall through a mesentery, a part of the peritoneum through which the organ is supplied with blood and lymph vessels and nerves.
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.
Function
The peritoneum both supports the abdominal organs and serves as a conduit for their blood and lymph vessels and nerves.
The peritoneum in human anatomy
Two notable sections of the peritoneum in humans are the omenta, the greater (gastrocolic) omentum and the lesser (gastrohepatic) omentum. They each comprise a double fold of the peritoneum and contain a cavity (omental bursa or lesser peritoneal cavity). The lesser omentum is attached to the lesser curvature of the stomach and the liver. The greater omentum hangs from the greater curve 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 structures in the abdomen are classified as intraperitoneal, intra-retroperitoneal, retroperitoneal or infraperitoneal depending on whether they are covered with visceral peritoneum and have a mesentery or not.
Intraperitoneal structures
- stomach
- jejunum
- ileum
- superior horizontal part of duodenum
- appendix
- spleen
- transverse colon
- sigmoid colon
- rectum, upper 1/3
- liver
- in women:
Intra-retroperitoneal
Retroperitoneal
- the rest of the duodenum
- rectum, middle 1/3
- kidneys
- pancreas
- suprarenal glands
- Ureters, renal vessels and gonadal blood vessels
Infraperitoneal
- rectum, lower 1/3
- urinary bladder
Disease states
- The presence of gas within the peritoneal cavity, as may occur when a perforation forms in the stomach or intestines, is called pneumoperitoneum and heralds a perilous situation.
- Peritonitis refers to inflammation of the peritoneal lining or cavity, as may occur with either a perforation or by spread of infection through the wall of one of the abdominal organs. This too is a serious condition, and often requires emergency surgery.
- An accumulation of excess fluid within the peritoneal cavity is called ascites.
Peritoneal dialysis
In one form of dialysis, the peritoneal dialysis, a special solution is run through a tube into the peritoneal cavity. The fluid is left there for a while to absorb waste products, and then removed through the tube.