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Greater_and_lesser_sac_1035gif
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The Epiploic Foramen , Greater Sac or general cavity (red) and Lesser Sac , or omental bursa (blue)
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Horizontal disposition of the Peritoneum in the upper part of the Abdomen (Areas outlined in blue and red correspond to above diagram)
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Peritoneum
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In higher
Vertebrate s, the 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 .
The peritoneum consists of two layers and a potential space between them:
- The outer layer, called the , is attached to the Abdominal Wall .
- The inner layer, the , is wrapped around the Internal Organ s that are located inside the abdominal cavity.
- The potential space between these two layers is the ; 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.
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.
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.
- 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.
In one form of
Dialysis , the , 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.
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.
- Tortora, Gerard J., Anagnostakos, Nicholas P. (1984) ''Principles of Anatomy and Physiology'', Harper & Row Publishers, New York ISBN 0-06-046656-1
