peritoneal cavity

The peritoneum is a serous membrane consisting of a single layer of mesothelium overlying connective tissue. The parietal peritoneum lines the abdominal and pelvic walls, while the visceral peritoneum covers the abdominal organs. The peritoneal cavity is the potential space between these two layers, normally containing <50 mL of serous fluid.

Organs are classified based on their relationship to the peritoneum: intraperitoneal organs (e.g., stomach, spleen, liver, transverse colon, sigmoid colon) are almost completely covered by visceral peritoneum and suspended by mesentery, while retroperitoneal organs (e.g., kidneys, pancreas, duodenum [except first part], ascending/descending colon, aorta, IVC) lie posterior to the peritoneum. Mnemonic: SAD PUCKER for retroperitoneal structures.

The peritoneal cavity is divided into the greater sac (most of the cavity) and lesser sac (omental bursa), located posterior to the stomach and lesser omentum. These communicate through the epiploic foramen (of Winslow), bounded by the hepatoduodenal ligament, IVC, and caudate lobe of liver.

Clinical significance: - Ascites: pathologic accumulation of fluid in the peritoneal cavity, seen in cirrhosis (portal hypertension), heart failure, nephrotic syndrome, malignancy. - Peritonitis: inflammation of the peritoneum, can be primary (spontaneous bacterial peritonitis, often in cirrhotic ascites, typically E. coli or Klebsiella) or secondary (due to perforation, e.g., appendicitis, diverticulitis). Presents with rebound tenderness, guarding, rigidity. - Peritoneal dialysis utilizes this cavity as a semi-permeable membrane for solute exchange. - The mesenteries and ligaments (e.g., falciform ligament, gastrohepatic ligament) act as conduits for neurovascular structures and can be sites of hernias or metastatic spread (e.g., Krukenberg tumors, Sister Mary Joseph nodule at umbilicus from GI malignancy). - Paracentesis is used to sample peritoneal fluid for diagnostic (SAAG calculation) or therapeutic purposes.