The peritoneal membrane is a large, thin, serous membrane that lines the abdominal cavity and covers the organs within it. This membrane, composed of a layer of specialized cells called mesothelium supported by connective tissue, creates a smooth, continuous lining that serves to protect and separate the internal structures of the abdomen and pelvis.
Anatomy and Structure of the Peritoneum
The peritoneum consists of two layers: the parietal peritoneum and the visceral peritoneum. The parietal peritoneum lines the abdominopelvic wall. The visceral peritoneum covers most abdominal organs, such as the stomach and intestines.
Between these two layers lies the peritoneal cavity. This cavity contains a small amount of serous fluid, which acts as a lubricant. Folds of the peritoneum also form structures like the omentum and mesentery, which suspend and connect organs while providing pathways for blood vessels, nerves, and lymphatic vessels. Organs fully enclosed by the visceral peritoneum are considered intraperitoneal, including structures like the stomach, spleen, and most of the small intestine. Conversely, organs situated behind the parietal peritoneum, such as the kidneys, pancreas, and parts of the colon, are termed retroperitoneal.
Key Functions of the Peritoneal Membrane
The peritoneal membrane performs several roles within the abdominal cavity. Its smooth, moist surface, lubricated by the serous fluid within the peritoneal cavity, allows organs to glide past one another with minimal friction during movements like digestion and breathing. This lubricating action helps prevent damage to the delicate organ surfaces as they shift. The membrane also supports and suspends the abdominal organs through structures like mesenteries and ligaments, preventing them from collapsing under gravity and maintaining their proper anatomical positions.
Beyond mechanical support, the peritoneum plays a physiological role due to its large, semi-permeable surface area. It facilitates the exchange of fluids, solutes, and waste products between the blood vessels within its layers and the peritoneal cavity. The peritoneum also contributes to the body’s immune defense, acting as a protective barrier against infection. It contains specialized immune cells and lymphatic drainage systems that help to clear foreign particles and fight off potential pathogens that may enter the abdominal cavity.
Role in Medical Procedures
The properties of the peritoneal membrane make it valuable in certain medical treatments, particularly peritoneal dialysis (PD). This procedure uses the peritoneum as a natural filter to remove waste products, excess fluids, and toxins from the blood when kidneys fail. During PD, a sterile dialysis solution, often containing glucose, is introduced into the peritoneal cavity through a surgically implanted catheter.
The dialysis solution remains in the cavity for a “dwell time” (4-6 hours), allowing waste products like urea and creatinine to diffuse from the blood vessels in the peritoneal lining into the solution. Glucose in the dialysate creates osmotic pressure, drawing excess water into the peritoneal cavity. After the dwell time, the fluid, now containing wastes, is drained and discarded, and fresh solution is introduced. This process can be performed manually multiple times a day (continuous ambulatory peritoneal dialysis, CAPD) or automatically by a machine overnight (automated peritoneal dialysis, APD).
Conditions Affecting the Peritoneal Membrane
Despite its protective functions, the peritoneal membrane can be affected by medical conditions. Peritonitis, an inflammation of the peritoneum, often caused by bacterial or fungal infection. This infection can arise from a ruptured appendix, perforated stomach ulcer, or complications from peritoneal dialysis, leading to severe abdominal pain, tenderness, bloating, and fever. Untreated peritonitis can be life-threatening as the infection can spread rapidly throughout the abdomen and into the bloodstream.
Another condition is ascites, which involves the abnormal accumulation of fluid in the peritoneal cavity. This fluid buildup causes abdominal swelling, discomfort, and shortness of breath. Ascites is associated with conditions like severe liver disease, heart failure, or certain cancers, where underlying disease imbalances fluid production and absorption. Peritoneal carcinomatosis refers to the spread of cancer to the peritoneal membrane, where tumors form on its surface. While any cancer can metastasize to the peritoneum, it is most commonly seen with ovarian, colorectal, stomach, or pancreatic cancers.