The Peritoneum – Anatomy, Structure, Functions

The Peritoneum – Anatomy, Structure, Functions

The peritoneum is the serous membrane that lines the abdominal cavity. It is composed of mesothelial cells that are supported by a thin layer of fibrous tissue and is embryologically derived from the mesoderm. The peritoneum serves to support the organs of the abdomen and acts as a conduit for the passage of nerves, blood vessels, and lymphatics. Although the peritoneum is thin, it is made of 2 layers with a potential space between them. The potential space between the 2 layers contains about 50 to 100 ml of serous fluid that prevents friction and allows the layers and organs to glide freely. The outer layer is the parietal peritoneum, which attaches to the abdominal and pelvic walls. The inner visceral layer wraps around the internal organs located inside the intraperitoneal space. The structures bound by the peritoneal cavity may be intraperitoneal or retroperitoneal.

Structure and Function

The boundaries of the peritoneal cavity include:

  • Anterior abdominal muscles
  • Vertebrae
  • Pelvic floor
  • Diaphragm

The peritoneum is comprised of 2 layers: the superficial parietal layer and the deep visceral layer. The peritoneal cavity contains the omentum, ligaments, and mesentery. Intraperitoneal organs include the stomach, spleen, liver, first and fourth parts of the duodenum, jejunum, ileum, transverse, and sigmoid colon. Retroperitoneal organs lie behind the posterior sheath of the peritoneum and include the aorta, esophagus, second and third parts of the duodenum, ascending and descending colon, pancreas, kidneys, ureters, and adrenal glands.

An important space in the peritoneal cavity is the epiploic foramen, also known as the foramen of Winslow. This foramen allows communication between the greater and lesser sacs. It is bordered by the hepatoduodenal ligament anteriorly, the inferior vena cava (IVC) posteriorly, the duodenum inferiorly, and the caudate lobe of the liver superiorly. The foramen provides access to a surgeon, should they need to clamp the hepatoduodenal ligament to stop a hemorrhage or gain anatomical access to the lesser sac. The foramen can also serve as a location for a lesser sac hernia.

The greater omentum loosely hangs from the greater curvature of the stomach and folds over the anterior of the intestine before curving back superiority to attach to the transverse colon. It acts as a protective or insulating layer. The mesentery helps attach the abdominal organs to the abdominal wall and contains many blood vessels, nerves, and lymphatics. Intraperitoneal organs are usually mobile while those in the retroperitoneum are usually fixed to the posterior abdominal wall. The dorsal mesentery also gives off the transverse and sigmoid mesocolons, which are important due to them containing the blood, nerve, and lymphatic supply for related structures.

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The Peritoneum

The peritoneum, the serous membrane that forms the lining of the abdominal cavity, covers most of the intra-abdominal organs.

Key Points

The peritoneum supports the abdominal organs and serves as a conduit for their blood and lymph vessels and nerves.

There are two layers of the peritoneum: 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 intraperitoneal cavity.
The mesentery is the double layer of the visceral peritoneum.

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

The structures in the abdomen are classified as intraperitoneal, retroperitoneal, or intraperitoneal, depending on whether they are covered with visceral peritoneum and are attached by mesenteries.

There are two main regions of the peritoneum connected by the epiploic foramen: the greater sac or general cavity of the abdomen, and the lesser sac or omental bursa.

Intraperitoneal organs and retroperitoneal organs weave in and out of these membranes and serve varying functions. Retroperitoneal structures tend to be more static than intraperitoneal ones.

Key Terms

  • greater omentum: A large fold of visceral peritoneum that hangs down from the stomach.
  • retroperitoneal: Located outside of the peritoneum.
  • serous membrane: A thin membrane that secretes serum that lines an internal body cavity, such as the peritoneum, the pericardium, and the pleura.
  • peritoneum: In mammals, the serous membrane that lines the cavity of the abdomen and is folded over the viscera.
  • intraperitoneal: Within the cavity of the peritoneum.
  • mesentery: The membrane that attaches the intestines to the wall of the abdomen and maintains their position in the abdominal cavity to supply them with blood vessels, nerves, and lymphatics.
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EXAMPLES

In one form of dialysis, called peritoneal dialysis, a glucose solution is sent through a tube into the peritoneal cavity. The fluid is left there for a prescribed amount of time to absorb waste products and then removed through the tube. This form of dialysis is effective because of the high number of arteries and veins in the peritoneal cavity which, through the mechanism of diffusion, remove waste products from the blood.

The peritoneum is the serous membrane that forms the lining of the abdominal cavity of 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.

This is an anatomical drawing, a sagittal section seen through the posterior abdominal wall. It shows how the kidney resides outside the peritoneum. 

The peritoneum and the kidney: A sagittal section through the posterior abdominal wall, showing the kidney residing outside the peritoneum.

Although they ultimately form one continuous sheet, there are two layers of peritoneum and potential space between those layers.

  • 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 intraperitoneal cavity.
  • The potential space between these two layers is the peritoneal cavity. It is filled with a small amount of slippery serous fluid that allows the two layers to slide freely over each other.

The term mesentery is often used to refer to a double layer of the visceral peritoneum. There are generally blood vessels, nerves, and other structures between these layers. The space between the two layers is technically outside of the peritoneal sac, and thus not in the peritoneal cavity.

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The Regions of the Peritoneum

There are two main regions of the peritoneum, connected by the epiploic foramen (also known as the mental foramen or foramen of Winslow). The first is the greater sac or general cavity of the abdomen. The second is the lesser sac or omental bursa.

The lesser sac is divided into two omenta: the gastrohepatic and the gastrocolic. The gastrohepatic omentum is attached to the lesser curvature of the stomach and the liver. The gastrocolic omentum hangs from the greater curve of the stomach and loops down in front of the intestines before curving upwards to attach to the transverse colon. Like a curtain of tissue, it is draped in front of the intestines to insulate and protect them.

This is a midsagittal, cross-section, drawing of the substructures of the peritoneum. In particular, it shows the epiploic foramen, the greater sac or general cavity (red), and the lesser sac or omental bursa (blue). 

Substructures of the peritoneum: This is a midsagittal, cross-section drawing of the epiploic foramen, the greater sac or general cavity (red), and the lesser sac or omental bursa (blue).

Abdomen Structures

The structures in the abdomen are classified as intraperitoneal, retroperitoneal, or intraperitoneal depending on whether they are covered with visceral peritoneum and are attached by mesenteries, such as the mesentery and mesocolon.

Intraperitoneal Structures

Intraperitoneal organs include the stomach, the first five centimeters, and the fourth part of the duodenum, the jejunum, the ileum, the cecum, the appendix, the transverse colon, the sigmoid colon, and the upper third of the rectum.

Other organs located in the intraperitoneal space are the liver, spleen, and tail of the pancreas. In women, the uterus, fallopian tubes, ovaries, and gonadal blood vessels are located in the intraperitoneal.

Retroperitoneal Structures

Retroperitoneal structures include the rest of the duodenum, the ascending colon, the descending colon, the middle third of the rectum, and the remainder of the pancreas. Other organs located in the retroperitoneal space are the kidneys, adrenal glands, proximal ureters, and renal vessels. Organs located below the peritoneum in the subperitoneal space include the lower third of the rectum and the urinary bladder.

Intraperitoneal Structures

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.

This is an anatomical, schematic drawing that shows the peritoneum and the organs and systems surrounded within itself. 

Peritoneum: The peritoneum is illustrated, indicated by blue.

Blood Supply and Lymphatics

The parietal peritoneum receives blood from the abdominal wall vasculature, including the iliac, lumbar, epigastric, and intercostal arteries. The visceral peritoneum receives supply from the superior and inferior mesenteric arteries. The two portions of the peritoneum also differ in their venous drainage: the parietal peritoneum drains into the inferior vena cava while the visceral peritoneum drains into the portal vein.

Nerves

A thorough understanding of the innervation of the peritoneum is important as it has clinical implications. The peritoneum has both somatic and autonomic innervations that help explain why various abdominal pathologies, such as peritonitis or appendicitis present the way they do. The parietal peritoneum receives its innervation from spinal nerves T10 through L1. This innervation is somatic and allows for the sensation of pain and temperature that can be localized. The visceral peritoneum receives autonomic innervation from the Vagus nerve and sympathetic innervation that result in the difficulty to localize abdominal sensations triggered by organ distension.

References

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