The abdomen contains a large membrane called the peritoneum, which lines the abdominal wall and covers most abdominal organs. Composed of peritoneal cells, its primary roles are to support and protect the organs. It provides insulation, lubrication, and structural support while also housing blood vessels, lymphatic pathways, and nerves.
The Peritoneal Environment
The peritoneum consists of two layers. The outer layer, the parietal peritoneum, adheres to the abdominal and pelvic walls. The inner layer, the visceral peritoneum, folds over and encases organs like the liver, stomach, and intestines. The potential space between these layers is the peritoneal cavity, which contains a small amount of serous fluid.
This peritoneal fluid, normally totaling between 50 and 100 mL, allows the abdominal organs to move smoothly against one another. The fluid is composed of water, electrolytes, and various leukocytes, or white blood cells. Secreted by the cells lining the peritoneum, it acts as a lubricant and contains antibodies that help defend against infection. The structure of the peritoneal ligaments and mesenteries divides the cavity into several compartments, which influences fluid circulation.
Primary Cell Types and Normal Functions
The peritoneal environment is maintained by several types of cells. The most prominent are mesothelial cells, which form a single, flat layer that constitutes the surface of the peritoneum. These cells produce the lubricating fluid that reduces friction between organs by secreting substances like phosphatidylcholine and hyaluronic acid. Mesothelial cells also regulate the transport of fluids and particles across the peritoneal membrane and are involved in tissue repair.
Beyond the mesothelial lining, the peritoneal cavity hosts a population of immune cells for its defense system. Peritoneal macrophages act as a “clean-up crew” by engulfing cellular debris, foreign particles, and invading pathogens. Lymphocytes, including B-cells and T-cells, reside in the cavity and mount targeted immune responses. Mast cells also participate in inflammatory and allergic reactions by releasing signaling molecules.
Involvement in Disease Processes
The functions of peritoneal cells can be disrupted by disease. A common condition is peritonitis, the inflammation of the peritoneum, frequently caused by a bacterial or fungal infection. An infection can occur following the rupture of an organ, such as the appendix. In response, peritoneal cells orchestrate an immune reaction, recruiting large numbers of leukocytes to fight the pathogens.
Cancer also involves the peritoneal environment. Some cancers, like malignant mesothelioma, are primary tumors that arise directly from the mesothelial cells. More often, the peritoneum becomes the site of secondary cancers through peritoneal carcinomatosis. This occurs when cancer cells from other organs, such as the colon, ovaries, or stomach, spread into the peritoneal cavity and grow into new tumors.
A related complication is ascites, the abnormal accumulation of fluid within the peritoneal cavity. This condition develops when the fluid transport function of the peritoneal cells is disrupted, such as in cases of liver cirrhosis, heart failure, or cancer. The presence of ascites can be a sign of advanced disease and requires medical intervention.
Clinical Significance in Diagnosis and Treatment
The peritoneum’s properties are utilized for both diagnosis and treatment. In treating kidney failure, the peritoneum can be used as a natural filter in a procedure called peritoneal dialysis. This treatment involves introducing a dialysis solution into the peritoneal cavity through a catheter. Waste products from the blood diffuse across the peritoneal membrane into this solution, which is then drained from the body.
The fluid within the peritoneal cavity is also a diagnostic tool. A procedure known as paracentesis involves using a needle to draw a sample of peritoneal fluid from the abdomen for laboratory analysis. This fluid can be analyzed to detect signs of disease. For instance, cytology can identify cancer cells to diagnose peritoneal carcinomatosis, while culturing the fluid can check for bacteria or fungi to confirm peritonitis.