Learning Objective
Understand the structure and layers of the peritoneum, its relationship to abdominal organs, intraperitoneal vs retroperitoneal organs, and its clinical relevance.
The Peritoneum
The peritoneum is a continuous membrane lining the abdominal cavity and covering the abdominal viscera. It supports the organs and provides pathways for blood vessels and lymphatics.
Structure of the Peritoneum
The peritoneum consists of two continuous layers: the parietal peritoneum and the visceral peritoneum, both made of simple squamous epithelial cells called mesothelium.
Parietal Peritoneum
Lines the internal surface of the abdominopelvic wall and is derived from somatic mesoderm. Pain from the parietal peritoneum is well localized and sensitive to pressure, pain, laceration, and temperature.
Visceral Peritoneum
Invaginates to cover most abdominal viscera, derived from splanchnic mesoderm. Pain is poorly localized, sensitive to stretch and chemical irritation, and referred to dermatomes corresponding to the organ’s sensory ganglia.
Activity
Peritoneal Cavity
The peritoneal cavity is a potential space between the parietal and visceral peritoneum containing a thin film of lubricating fluid.
Peritoneal Adhesions
Damage from infection, surgery, or injury can cause fibrous adhesions between organs or peritoneal layers, potentially resulting in volvulus, obstruction, or pain.
Intraperitoneal and Retroperitoneal Organs
Organs are classified by peritoneal relationship:
- Intraperitoneal: Fully covered by visceral peritoneum (stomach, liver, spleen).
- Retroperitoneal: Covered anteriorly only; subdivided into:
- Primarily retroperitoneal: esophagus, rectum, kidneys
- Secondarily retroperitoneal: ascending/descending colon, pancreas (except tail)
Mnemonic: SAD PUCKER – Suprarenal glands, Aorta/IVC, Duodenum, Pancreas, Ureters, Colon, Kidneys, Esophagus, Rectum.
Peritoneal Reflections
The peritoneum forms folds and spaces connecting organs and conveying vessels and nerves. Important structures:
- Mesentery: A double layer connecting organs to the posterior abdominal wall, which provides a pathway for vessels and nerves.
- Omenta: Sheets connecting the stomach/duodenum to organs. The greater omentum has four layers, descends from the stomach/duodenum, and protects organs (“abdominal policeman”). The lesser omentum connects the stomach/duodenum to the liver and contains the hepatogastric and hepatoduodenal ligaments.
- Peritoneal ligaments: Connect the viscera to the abdominal wall (e.g., hepatogastric ligament).
Clinical relevance: Referred Pain
Pain from viscera is poorly localized and referred according to embryologic origin: foregut → epigastrium, midgut → umbilical region, hindgut → pubic region. Retroperitoneal organs (kidney, pancreas) may cause back pain. Diaphragm irritation may result in shoulder-tip pain. Appendicitis pain initially refers to the umbilical region and later localizes to the right lower quadrant as the parietal peritoneum is involved.
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