THE PERITONEUM AND ITS FUNCTIONS
The peritoneum is the thin serous membrane that lines the peritoneal cavity. It is the largest serous surface layer in the human body and its surface area is similar 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 less than 1.016, protein concentration less than 3g/dL, pH between 7.5 and 8, and a white blood cell count less than 3000/μL. The peritoneum is divided anatomically into parietal and visceral components. The parietal peritoneum underlies the anterior, later, and posterior abdominal walls as well as the undersurface of the diaphragm and pelvic basin. The visceral peritoneum is reflected over the viscera within the abdominal cavity.
Once thought to be a passive barrier, the peritoneum is now understood to have numerous functions. The mesothelial cells secrete phosphatidylcholine, which provides a near frictionless environment within the peritoneum and allows intraperitoneal organs to glide over one another during peristalsis and movement. With its large surface area and semi permeable nature, it participates in fluid exchange with the extracellular fluid space at rates of over 500 mL/h. The circulation of peritoneal fluid is directed toward lymphatics on the undersurface of the diaphragm where 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 has a vigorous response to injury and inflammation. Normally sterile, the peritoneum participates in recognizing and eliminating bacteria. Mesothelial cells secrete opsonins that promote bacterial destruction, express CD40 and are aid in antigen presentation, and express intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) which aid in attachment and activation of lymphocytes, granulocytes, and monocytes in response to infectious pathogens. The mesothelial cells secrete tPA under normal conditions which participates in intraperitoneal adhesiolysis. The peritoneum has significant wound healing functions as well, secreting multiple inflammatory mediators including vascular endothelial growth factor (VEGF), PAI, and nitrogen monoxide, TGF beta, and TNF alpha in response to trauma. In response to injury, the peritoneum produces a large proinflammatory response with fibrin deposition and activation of coagulation pathways as well. Imbalance between fibrin deposition and fibrinolysis following peritoneal traumatization can lead to the organization of fibrin deposits between adjacent structures and development of intraperitoneal adhesions which will be discussed further in later sections. Unlike with cutaneous wound healing, following injury to the mesothelium there is uniform recreation of the mesothelial monolayer within 5-10 days.
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