THE PERITONEUM AND ITS FUNCTIONS
The peritoneum is a thin serous membrane that lines the peritoneal cavity. It is the largest serous surface layer in the human body, and its surface area is comparable to the skin. The structure is made up of a single flat layer of mesothelial cells, rich in microvilli. Beneath the mesothelium are a basement membrane and a loose collagen network containing vascularized connective tissue with scattered fibroblasts and macrophages. Normally there is between 5 and 20 mL of free peritoneal fluid. This can vary in women, peaking after ovulation. Normal peritoneal fluid has a specific gravity of less than 1.016, protein concentration of less than 3 g/dL, pH between 7.5 and 8, and white blood cell count (WBC) of less than 3000/µL.
Once thought to be a passive barrier, the peritoneum is now understood to have numerous functions.
The mesothelial cells provide a near-frictionless environment within the peritoneum and allow intraperitoneal organs to glide over one another during peristalsis and movement.
The peritoneum participates in transport of fluid and cells across serosal cavities, which is facilitated by its large surface area and semipermeable nature. Fluid exchange with the extracellular fluid space occurs at rates of over 500 mL/h. The circulation of peritoneal fluid is directed toward lymphatics on the undersurface of the diaphragm. Particulate matter up to 20 µm in size is cleared via stomas in the diaphragmatic mesothelium and emptied into the right thoracic duct.
The peritoneum, usually sterile, participates in recognizing and eliminating bacteria. Mesothelial cells secrete opsonins that promote bacterial destruction, aid in antigen presentation, and express intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1). These are involved in attachment and activation of lymphocytes, granulocytes, and monocytes in response to infectious pathogens.
The mesothelial cells participate in intraperitoneal adhesiolysis by secreting tissue plasminogen activator under normal conditions. Impaired healing and cell transformation lead to formation of serosal adhesions.
The peritoneum has significant wound healing functions and secretes inflammatory mediators in response to trauma and injury, including vascular endothelial growth factor, plasminogen activator inhibitor, nitrogen monoxide, transforming growth factor-β, and tumor necrosis factor-α. The proinflammatory response causes fibrin deposition and activation of coagulation pathways. There is uniform re-creation of the mesothelial monolayer within 5-10 days after injury.
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ANATOMY OF THE PERITONEUM
BOUNDARIES OF THE PERITONEAL CAVITY
The peritoneum is divided anatomically into parietal and visceral components. The parietal peritoneum underlies the anterior, lateral, and posterior abdominal walls as well ...