Peritoneum

In higher vertebrates, the peritoneum is the serous membrane that forms the lining of the abdominal cavity - it covers most of the intra-abdominal organs. (The corresponding serous membranes in the pleural and pericardial cavities of the thorax are called the pleura and the pericardium respectively.) The peritoneum both supports the abdominal organs and serves as a conduit for their blood and lymph vessels and nerves.

Layers

The peritoneum consists of two layers and a potential space between them:
* The outer layer, called the parietal peritoneum, is attached to the abdominal wall.
* The inner layer, the visceral peritoneum, is wrapped around the internal organs that are located inside the abdominal cavity.
* The potential space between these two layers is the peritoneal cavity; it is filled with a small amount (about 50 ml) of slippery serous fluid that allows the two layers to slide freely over each other.

Structures

There are two main regions of the peritoneum, connected by the epiploic foramen:
* the greater sac (or general cavity of the abdomen), represented in red in the diagrams above.
* the lesser sac (or omental bursa), represented in blue. The lesser sac is divided into two "omenta":
** The lesser omentum (or gastrohepatic) is attached to the lesser curvature of the stomach and the liver.
** The greater omentum (or gastrocolic) 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 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.

Development

The peritoneum develops ultimately from the mesoderm of the trilaminar embryo. As the mesoderm differentiates, one region known as the lateral plate mesoderm splits to form two layers separated by an intraembryonic coelom. These two layers develop later into the visceral and parietal layers found in all serous cavities, including the peritoneum.

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.

Disease states

* Pneumoperitoneum is the presence of gas within the peritoneal cavity, as may occur when a perforation forms in the stomach or intestines, 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.
* Ascites is an accumulation of excess fluid within the peritoneal cavity.

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. The reason for this effect is the high number of arteries and veins in the peritoneal cavity. Through the mechanism of diffusion, waste products are removed from the blood.

Classification of abdominal structures

The structures in the abdomen are classified as intraperitoneal, retroperitoneal or infraperitoneal depending on whether they are covered with visceral peritoneum and have a mesentery or not. Some structures, such as the kidneys, are "secondarily retroperitoneal", meaning that structure developed intraperitoneally but lost its mesentery and thus became retroperitoneal.

Intraperitoneal

*Stomach
* First part of the duodenum [5 cms].
*Pancreas(head and body)
*Jejunum
*Ileum
*Appendix
*Cecum
*Transverse colon
*Sigmoid colon
*Rectum, upper 1/3
*Liver
*Spleen
*In women:
**Uterus
**Fallopian tubes
**Ovaries

Retroperitoneal

*The rest of the duodenum
*Rectum, middle 1/3
*Kidneys
*Pancreas(tail)
*Suprarenal glands (adrenal glands)
*Ureters, renal vessels and gonadal blood vessels
*Ascending colon
*Descending colon

Infraperitoneal

*Rectum, lower 1/3
*Urinary bladder

References

*Tortora, Gerard J., Anagnostakos, Nicholas P. (1984) Principles of Anatomy and Physiology, Harper & Row Publishers, New York ISBN 0-06-046656-1

External links

*
* Overview and diagrams at colostate.edu