Peritoneum Structure and Anatomy of the Abdominal Cavity
The peritoneum is a continuous serous membrane that lines the abdominal and pelvic cavities. It serves as a critical protective barrier and structural support system for the organs within the abdomen, facilitating movement and providing a pathway for blood vessels, nerves, and lymphatics.
The Layers of the Peritoneum
The peritoneum consists of a single sheet that folds back on itself to create two distinct layers and a potential space between them known as the peritoneal cavity.
Parietal Peritoneum
The outer layer, the parietal peritoneum, is attached to the abdominal and pelvic walls. A specialized outpouching of this layer, called the vaginal process, gives rise to the tunica vaginalis, which is the serous membrane covering the male testis.
Visceral Peritoneum
The inner layer, the visceral peritoneum, is thinner than the parietal layer and wraps directly around the visceral organs for protection. When this layer forms a double fold that attaches organs to the gastrointestinal tract, it is called the mesentery. These double layers often house essential blood vessels, nerves, and other structures; because they are enclosed, this space is technically outside the peritoneal sac.
The peritoneal cavity itself is a potential space filled with approximately 50 mL of slippery serous fluid. This lubrication allows the visceral and parietal layers to slide freely against one another during organ movement.
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Key Facts
- Two Layers: The peritoneum consists of the parietal (outer) and visceral (inner) layers.
- Lubrication: About 50 mL of serous fluid prevents friction between layers.
- Connectivity: Peritoneal folds (omentums, mesenteries, and ligaments) connect organs to each other or the abdominal wall.
- Gender Differences: The peritoneal space is closed in males but open to the extraperitoneal pelvis in females via the vagina, uterus, and fallopian tubes.
- Mobility: Intraperitoneal organs are generally mobile, whereas retroperitoneal organs are relatively fixed.
Subdivisions and Peritoneal Folds
The peritoneal cavity is divided into two primary regions: the greater sac and the lesser sac. These two areas communicate through a small opening called the omental (or epiploic) foramen.
The Omenta
The lesser sac is further defined by two "omenta":
- Lesser Omentum (Hepatogastric): Connects the liver to the lesser curvature of the stomach.
- Greater Omentum (Gastrocolic): Hangs from the greater curvature of the stomach, draping over the intestines like an apron before attaching to the transverse colon. It provides insulation and protection.
The Mesenteries
The mesentery is the specific portion of the peritoneum that anchors most abdominal organs to the abdominal wall, serving as the conduit for blood, lymph vessels, and nerves.
| Structure | From | To | Key Contents |
|---|---|---|---|
| Greater Omentum | Greater curvature of stomach | Transverse colon | Gastroepiploic vessels, fat |
| Lesser Omentum | Lesser curvature of stomach | Liver | Portal vein, hepatic artery, bile duct |
| Mesentery Proper | Small intestine | Posterior abdominal wall | Superior mesenteric artery, lymph nodes |
| Falciform Ligament | Liver | Diaphragm/Abdominal wall | Round ligament of liver |
| Broad Ligament | Uterus | Pelvic wall | Mesovarium, mesosalpinx, mesometrium |
Classification of Abdominal and Pelvic Structures
Organs are classified based on their relationship with the visceral peritoneum and their attachment via mesenteries:
- Intraperitoneal: Fully covered by visceral peritoneum and generally mobile (e.g., stomach, jejunum, ileum, spleen, liver).
- Mesoperitoneal: Attached by a mesentery (e.g., ascending and descending colon).
- Retroperitoneal (Extraperitoneal): Located behind the peritoneum and relatively fixed. These can be primarily retroperitoneal (e.g., kidneys, adrenal glands) or secondarily retroperitoneal, meaning they were once intraperitoneal but lost their mesentery during development (e.g., most of the duodenum).
- Infraperitoneal (Subperitoneal): Located below the peritoneal cavity (e.g., urinary bladder, distal ureters).
Developmental Origins
The peritoneum originates from the mesoderm of the trilaminar embryo. Specifically, the lateral plate mesoderm splits to form two layers separated by an intraembryonic coelom. As abdominal organs grow into the cavity from the abdominal wall, they are enveloped in peritoneum. The blood vessels accompanying these organs are also covered, creating the mesenteries. Peritoneal folds eventually develop from the embryonic ventral and dorsal mesenteries.
Frequently Asked Questions
What is the difference between the parietal and visceral peritoneum?
The parietal peritoneum is the outer layer that lines the abdominal and pelvic walls, while the visceral peritoneum is the thinner inner layer that wraps directly around the internal organs.
What is the function of the greater omentum?
The greater omentum acts as a protective and insulating layer, draping over the intestines like an apron to provide a barrier and support.
How does the peritoneal cavity differ between males and females?
In males, the peritoneal space is closed. In females, it is continuous with the extraperitoneal pelvis through openings in the vagina, uterus, and fallopian tubes.
What does "secondarily retroperitoneal" mean?
It refers to organs that developed as intraperitoneal structures during embryonic growth but later lost their mesentery, resulting in them becoming fixed against the posterior abdominal wall.
What is the purpose of the serous fluid in the peritoneal cavity?
The approximately 50 mL of serous fluid acts as a lubricant, allowing the visceral and parietal layers to slide smoothly over each other without friction.