peritoneum
Meaning of peritoneum: A single continuous sheet folded so intricately that it both lines the abdomen and wraps the organs inside it.
Definition of peritoneum
The peritoneum is the thin membrane that lines the inside of the abdominal wall and folds inward to cover most of the organs within. It has two continuous layers: the parietal peritoneum against the wall, and the visceral peritoneum wrapped around the organs. Between them lies a narrow space holding a small volume of lubricating fluid.
What is it for?
Two surfaces of membrane, kept apart by a film of fluid, slide over one another with almost no friction. That is what allows loops of the small intestine to move constantly during abdominal digestion without abrading each other. The peritoneum also does structural work. Double folds of it form the mesentery, which suspends the intestines and carries their blood vessels, nerves and lymphatics, and the omentum, an apron of fatty membrane hanging from the stomach that drapes over the intestines and tends to wall off areas of infection or injury. The membrane is rich in immune cells and absorbs fluid readily, which is what makes peritoneal dialysis possible, using the peritoneum itself as the filter when the kidneys cannot do the job.
Which organs are inside it and which are behind it?
| Position | Organs |
|---|---|
| Wrapped by peritoneum | Stomach, most of the small intestine, the transverse and sigmoid colon, liver, spleen |
| Behind the peritoneum | Kidneys, adrenal glands, pancreas, most of the duodenum, the ascending and descending colon, aorta |
Organs lying behind the membrane are described as retroperitoneal, and the distinction matters because disease in that space behaves differently, often producing back pain rather than the pain of an irritated abdominal lining.
Why does the site of pain differ?
The two layers report to different nerve systems. The parietal layer shares the nerve supply of the abdominal wall, so irritation there is sharp and can be pointed to precisely, and it worsens with movement or coughing. The visceral layer has only autonomic nerves that respond to stretch rather than to touch or cutting, so pain from it is dull, poorly located and often felt in the midline regardless of which organ is involved. Appendicitis shows the shift plainly: vague central pain first, then sharp pain in the lower right once the overlying parietal layer becomes involved. Inflammation of the membrane is called peritonitis, and it most often follows infection or leakage from a hollow organ.
Used in a sentence
Imaging was obtained to evaluate for free fluid within the peritoneum.
Good to know
The parietal peritoneum lining the abdominal wall carries nerves that localize pain precisely, while the visceral layer covering the organs does not, which is why abdominal pain often begins vague and central before becoming sharp and localized.
Word origin
Where “peritoneum” comes from
Formed from peri-, from Greek peri, meaning around, and teinein, meaning to stretch, describing a membrane stretched around the abdominal organs.