Peritoneum, Mesentery, and Omentum: Intraperitoneal vs Retroperitoneal Organs Guide

The peritoneum is a single continuous serous membrane lining the abdominal cavity, with the parietal peritoneum attached to the abdominal wall and the visceral peritoneum wrapping the abdominal organs. The potential space between these two layers is the peritoneal cavity, normally containing only a thin film of lubricating fluid (about 50 mL). Organs suspended within this cavity by mesentery are called intraperitoneal; organs that lie behind (posterior to) the parietal peritoneum are called retroperitoneal. Two double-layered peritoneal folds are named independently: the greater omentum, a fat-laden apron hanging from the greater curvature of the stomach over the intestines, and the lesser omentum, connecting the stomach's lesser curvature to the liver. Behind the stomach and lesser omentum is the lesser sac (omental bursa), connecting to the greater sac through the epiploic foramen of Winslow at the free edge of the hepatoduodenal ligament. The rest of the peritoneal cavity is the greater sac. The high-yield mnemonic for retroperitoneal organs is SAD PUCKER: Suprarenal (adrenal) glands, Aorta and IVC, Duodenum (2nd and 3rd parts), Pancreas (except tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus, and Rectum. Everything else in the abdomen is intraperitoneal or partially so. This content is for educational purposes only and does not constitute medical advice.

The parietal peritoneum is innervated by the same somatic nerves that supply the overlying abdominal wall — intercostal nerves T7-T12 and branches of the lumbar plexus. This means parietal peritoneum transmits sharp, well-localized pain. When you press on the abdomen in appendicitis and release (rebound tenderness), the pain at the point of release comes from inflammation of the parietal peritoneum. The patient can point to exactly where it hurts. The visceral peritoneum is innervated by autonomic fibers that travel with the organ's blood supply. Pain from visceral peritoneum is dull, poorly localized, and referred along embryologic segment lines: foregut pain refers to the epigastrium, midgut pain to the periumbilical region, and hindgut pain to the suprapubic region. This is why early appendicitis (inflamed visceral peritoneum of the midgut appendix) causes vague periumbilical pain that later migrates to the right lower quadrant when the parietal peritoneum becomes inflamed. Key clinical application: the 'migration of pain' in appendicitis — from diffuse periumbilical pain (visceral) to sharp RLQ pain (parietal) — is one of the most reliable clinical signs of acute appendicitis. The shift indicates the inflammation has extended from the organ to the overlying parietal peritoneum at McBurney's point. The diaphragmatic peritoneum has additional innervation from the phrenic nerves (C3-C5), which is why irritation of the diaphragmatic peritoneum (from subphrenic abscess or blood) causes referred pain to the shoulder — the 'Kehr sign' of splenic rupture or ruptured ectopic pregnancy.

The greater omentum is a four-layered apron of peritoneum and fat that hangs from the greater curvature of the stomach, drapes over the transverse colon and small bowel, and extends inferiorly to reach near the pelvic brim. It has several important functions: (1) immunologic — contains lymphoid tissue that responds to peritoneal infection, (2) mechanical — can seal off inflamed or perforated areas ('the abdominal policeman'), and (3) thermal insulation and cushioning for abdominal organs. Clinically, the greater omentum often adheres to a diseased structure. A ruptured appendix or perforated duodenal ulcer is frequently wrapped in omentum, which limits peritonitis from spreading. Surgeons can pack a questionable bowel anastomosis with omentum to reinforce healing. The omentum's migratory ability is mediated by peritoneal fluid currents and its vascular supply from the right and left gastroepiploic arteries. The lesser omentum connects the lesser curvature of the stomach and the first part of the duodenum to the liver. It has two anatomically named components: - Hepatogastric ligament: connects the stomach to the liver. Contains the left and right gastric arteries. - Hepatoduodenal ligament: connects the duodenum to the liver. Contains the portal triad — portal vein, hepatic artery proper, and common bile duct. This is the structure you compress during the Pringle maneuver to control hepatic bleeding. The free edge of the hepatoduodenal ligament forms the anterior border of the epiploic foramen of Winslow. Through this foramen you can palpate the portal triad between your thumb (in the lesser sac) and fingers (in the greater sac) — a classical surgical maneuver.

The peritoneal cavity is divided by the lesser omentum, stomach, and gastrocolic ligament into two communicating compartments: the greater sac (most of the peritoneal cavity) and the lesser sac (also called omental bursa), which lies posterior to the stomach and lesser omentum. Borders of the lesser sac: - Anterior: posterior wall of the stomach, lesser omentum, and part of the gastrocolic ligament - Posterior: pancreas, posterior abdominal wall, and aorta - Superior: diaphragm and caudate lobe of liver - Inferior: transverse mesocolon - Left: spleen (the splenorenal and gastrosplenic ligaments bound this side) - Right: epiploic foramen of Winslow The lesser sac is clinically important because fluid collections (abscess, pancreatic pseudocyst, blood) can be sequestered here and require specific drainage approaches. Pancreatitis-related pseudocysts often form in the lesser sac because the pancreas sits just posterior. The epiploic (omental) foramen of Winslow is the sole communication between lesser and greater sacs. Its borders: - Anterior: hepatoduodenal ligament (portal triad inside) - Posterior: inferior vena cava - Superior: caudate lobe of liver - Inferior: first part of the duodenum You can pass a finger through this opening during laparotomy — placing the IVC posterior to your finger and the portal triad anterior. Pringle maneuver compresses the triad by pinching the hepatoduodenal ligament between thumb (through the foramen into the lesser sac) and finger (in the greater sac) — temporarily occluding hepatic inflow to control bleeding from a liver injury.

Intraperitoneal organs are suspended by mesentery and wrapped in visceral peritoneum on all sides. Retroperitoneal organs lie between the parietal peritoneum and the posterior abdominal wall — the peritoneum covers only their anterior surface. Truly intraperitoneal organs: - Stomach - First part of duodenum (duodenal cap) - Jejunum and ileum - Cecum and appendix - Transverse colon - Sigmoid colon - Liver (most of it) - Gallbladder (except its upper surface against the liver) - Spleen - Tail of pancreas (within the splenorenal ligament) Primarily retroperitoneal organs (SAD PUCKER): - S: Suprarenal (adrenal) glands - A: Aorta and inferior vena cava - D: Duodenum (2nd, 3rd, and 4th parts) - P: Pancreas (head, neck, body; tail is intraperitoneal) - U: Ureters - C: Colon (ascending and descending) - K: Kidneys - E: Esophagus (abdominal portion) - R: Rectum (upper third partially; middle and lower are infraperitoneal) Secondarily retroperitoneal: organs that were intraperitoneal in development but migrated posteriorly and 'fused' with the posterior wall, losing their mesentery. Examples include the ascending colon, descending colon, and much of the duodenum and pancreas. The embryologic origin affects blood supply — ascending and descending colon are supplied by the SMA and IMA respectively, carried along their original mesenteric pathways, even though the mesentery is no longer functional. Clinical relevance: retroperitoneal hemorrhage (from pancreatitis, aortic rupture, renal trauma) can be clinically occult because blood collects behind the peritoneum without causing obvious peritoneal signs. Grey Turner sign (flank bruising) and Cullen sign (periumbilical bruising) suggest tracking of retroperitoneal blood. Pancreatic malignancies often present late because the retroperitoneal location allows local invasion before symptoms develop.

Mesenteries are double layers of peritoneum that suspend intraperitoneal organs from the posterior abdominal wall and carry their neurovascular bundles. Major mesenteries: The mesentery (proper) supports the small bowel (jejunum and ileum). Root of the mesentery runs obliquely from the duodenojejunal junction (left side of L2) to the ileocecal junction (right iliac fossa). The root contains the superior mesenteric artery and its branches. The transverse mesocolon suspends the transverse colon from the anterior surface of the pancreas. Contains the middle colic artery (SMA branch). The sigmoid mesocolon suspends the sigmoid colon. Contains the sigmoid arteries (IMA branch) and superior rectal artery. The attachment is inverted V-shape on the posterior pelvic wall. The mesoappendix suspends the appendix from the terminal ileum. Contains the appendicular artery (branch of the ileocolic artery from the SMA). Other important peritoneal ligaments (named folds carrying vessels or organs): Gastrosplenic ligament: stomach to spleen. Contains the short gastric arteries and left gastroepiploic artery. Splenorenal ligament: spleen to left kidney/posterior wall. Contains the splenic artery and vein, and the tail of the pancreas. Gastrocolic ligament: stomach to transverse colon. Contains the right and left gastroepiploic arteries. Part of the greater omentum. Gastrophrenic ligament: fundus of the stomach to the diaphragm. Contains no major vessels. Falciform ligament: from the anterior abdominal wall to the liver, dividing right and left lobes. Contains the ligamentum teres (obliterated umbilical vein), which can recanalize in portal hypertension and drain umbilical collateral flow (causing caput medusae). Coronary and triangular ligaments: attach the liver to the diaphragm. Contain no major vessels but define the bare area of the liver, where the liver lacks peritoneal covering and directly contacts the diaphragm. Ligamentum venosum: remnant of the ductus venosus, running between the caudate lobe and the left lobe of the liver. Median umbilical ligament: remnant of the urachus, running from the bladder to the umbilicus in the midline. Medial umbilical ligaments: remnants of the obliterated umbilical arteries, running on either side of the midline from the bladder to the umbilicus. Lateral umbilical folds: contain the inferior epigastric arteries. Form the lateral borders of the inguinal triangle.

In supine patients, the pelvis (specifically the rectouterine pouch of Douglas in women or rectovesical pouch in men) is the most dependent portion of the peritoneal cavity — fluid gravitates there first. This is why pelvic exam or transvaginal ultrasound detects small amounts of peritoneal fluid (blood from a ruptured ectopic pregnancy, peritoneal fluid from ascites). In upright patients, peritoneal fluid distributes differently and the subphrenic spaces (between liver/spleen and the diaphragm) become important collection sites. Major peritoneal recesses and spaces: Morison pouch (hepatorenal recess): space between the right lobe of the liver and the right kidney. The most dependent part of the upper peritoneal cavity in supine position. Fluid from the pelvis can track up the right paracolic gutter to Morison pouch. FAST exam (Focused Assessment with Sonography for Trauma) checks Morison pouch to detect free fluid in suspected abdominal trauma. Subphrenic spaces: bilateral potential spaces between the diaphragm and the liver (right) or spleen/stomach (left). Subphrenic abscess after abdominal surgery classically presents with referred shoulder pain from phrenic nerve irritation. Paracolic gutters: grooves between the ascending/descending colon and the lateral abdominal wall. The right paracolic gutter provides a pathway for fluid to move from the pelvis up to the liver and subphrenic space. The left is shorter because of the phrenicocolic ligament that attaches the splenic flexure to the diaphragm. Rectouterine pouch of Douglas (women): between the uterus and rectum. Most dependent pelvic recess in women. Accessible via culdocentesis (posterior vaginal fornix puncture) in emergency settings. Rectovesical pouch (men): between the bladder and rectum. Most dependent pelvic recess in men. Vesicouterine pouch (women): between the bladder and uterus. Smaller and less clinically significant than the rectouterine pouch. Lesser sac: as described above, can sequester fluid from pancreatic pathology. Clinical application: in peritoneal dialysis, dialysate is infused into the peritoneal cavity through a catheter tip positioned in the pelvic recesses (usually the rectouterine pouch in women). The large surface area of the peritoneum (about 1-2 m², comparable to body surface area) enables effective solute exchange with systemic circulation.

Peritonitis: inflammation of the peritoneum from infection, chemical irritation (bile, blood, urine, pancreatic juice), or mechanical injury. Classic exam findings: diffuse tenderness, rebound tenderness, involuntary guarding ('board-like abdomen'), absent bowel sounds (reflex ileus), fever, tachycardia, leukocytosis. CT or plain radiograph may show free air (perforation) or free fluid. Management is usually surgical — the source of contamination must be controlled. Spontaneous bacterial peritonitis (SBP): a specific peritonitis occurring in cirrhotic patients with ascites, typically from bacterial translocation across the gut wall. Diagnosis requires paracentesis with ascitic fluid WBC > 250 cells/μL (PMNs). Treatment: third-generation cephalosporin (ceftriaxone). Ascites: accumulation of fluid in the peritoneal cavity. Causes include cirrhosis (most common in US), right heart failure, nephrotic syndrome, peritoneal carcinomatosis, and tuberculosis. Evaluation includes paracentesis with SAAG (serum-ascites albumin gradient): SAAG > 1.1 suggests portal hypertension; SAAG < 1.1 suggests non-portal causes like peritoneal malignancy or infection. Peritoneal carcinomatosis: diffuse seeding of tumor throughout the peritoneal cavity. Common in advanced ovarian, gastric, and colon cancers. The peritoneum's large surface area and mobility of tumor cells within peritoneal fluid make the entire peritoneum a single 'compartment' for metastatic spread. Symptoms: ascites (often hemorrhagic), bowel obstruction, malnutrition. Krukenberg tumor: gastric adenocarcinoma metastasizing to the ovaries via the peritoneal cavity (or possibly lymphatics). Bilateral ovarian masses with characteristic signet-ring cells. High-yield. Sister Mary Joseph nodule: umbilical metastasis from intra-abdominal malignancy, typically gastric, colon, or ovarian. Tumor spreads through the median umbilical ligament and nearby lymphatics. Sign of advanced disease. Cullen sign and Grey Turner sign: periumbilical and flank ecchymoses (bruising) from retroperitoneal hemorrhage, classically associated with severe hemorrhagic pancreatitis but also seen with ruptured ectopic pregnancy, ruptured aortic aneurysm, and other causes of retroperitoneal bleeding. Pringle maneuver: compression of the hepatoduodenal ligament (portal triad) through the epiploic foramen of Winslow to control hepatic hemorrhage during surgery. Up to 60 minutes of warm ischemia is generally tolerated; longer requires cold perfusion techniques. High-yield exam points: pain pattern differences (visceral referred pain vs somatic localized pain), the SAD PUCKER mnemonic for retroperitoneal organs, the borders and contents of the epiploic foramen of Winslow, Morison pouch as the most dependent upper peritoneal space in supine position, and the clinical syndromes (SBP, Krukenberg tumor, Sister Mary Joseph nodule).