Peritoneum and lesser sac

Basic arrangement

• The peritoneum is a single serous membrane. Parietal peritoneum lines the wall; visceral peritoneum covers organs; the potential space between is the peritoneal cavity (normally 50–100 mL of fluid).
• Deep to the abdominal muscles, a surgeon opening the abdomen meets, in order: transversus abdominis, transversalis fascia, extraperitoneal (preperitoneal) fat, then parietal peritoneum.
• Intraperitoneal organs hang on a mesentery: stomach, first 2 cm of duodenum, jejunum, ileum, caecum (usually), appendix, transverse and sigmoid colon, liver (except the bare area), spleen, tail of pancreas.
• Retroperitoneal organs: duodenum 2nd–4th parts, head, neck and body of pancreas, ascending and descending colon, upper rectum (front and sides only), kidneys, ureters, adrenals, aorta, IVC.
• Secondarily retroperitoneal organs (duodenum, pancreas, ascending and descending colon) once had a mesentery that fused to the posterior wall; that is why a surgeon can mobilise them along a bloodless plane.

Pain from the peritoneum

• Appendicitis shows both: early periumbilical visceral pain, then right iliac fossa pain once the parietal peritoneum is involved.

Development of the folds

• The ventral mesentery (in the septum transversum region) gives the falciform ligament, lesser omentum and coronary and triangular ligaments of the liver; the liver grows into it.
• The dorsal mesentery gives the greater omentum, gastrosplenic and splenorenal ligaments, mesentery of the small intestine, transverse and sigmoid mesocolons.
• Rotation of the stomach swings the dorsal mesogastrium to the left, creating the lesser sac behind the stomach.

Omenta and ligaments

• The root of the small bowel mesentery crosses, from above down: third part of the duodenum, aorta, IVC, right ureter, right psoas. A bullet across this line damages the superior mesenteric vessels.
• Free edge of the lesser omentum (hepatoduodenal ligament): common bile duct anterior and to the right, hepatic artery proper anterior and to the left, portal vein posterior to both. A cut in the right anterior part leaks bile; a portal vein injury bleeds.
• Bleeding into the substance of the lesser omentum usually comes from the left gastric artery (or right gastric) running between its layers.
• Appendices epiploicae are small fat-filled peritoneal tags on the colon (not on small bowel or rectum).

Lesser sac (omental bursa)

A closed recess behind the stomach, communicating with the greater sac only through the epiploic foramen.

• Recesses: superior (behind the caudate lobe), inferior (between layers of greater omentum, usually obliterated in adults), splenic.
• Posterior gastric ulcers perforate into the lesser sac (pain may go to the back); pancreatic pseudocysts collect here.
• Surgical access: to see the body of the pancreas widely, divide the gastrocolic ligament (greater omentum below the gastroepiploic arcade). Other routes are through the lesser omentum or transverse mesocolon. The epiploic foramen admits only one or two fingers.

Epiploic foramen (of Winslow)

The only natural opening between greater and lesser sacs, at about T12.

• Pringle manoeuvre: a finger in the foramen and thumb in front compress the hepatoduodenal ligament, stopping inflow from the hepatic artery and portal vein. Bleeding that continues comes from vessels the clamp does not control: the hepatic veins or retrohepatic IVC.

Spaces, gutters and spread of fluid

• Hepatorenal pouch (Morison's): between right lobe of liver and right kidney; the most dependent space when supine; first window on a FAST scan; site of subhepatic abscess.
• Subphrenic spaces (right and left): pus after perforation or surgery; swinging fever and shoulder-tip pain.
• Right paracolic gutter connects the hepatorenal pouch with the pelvis. Fluid from a perforated anterior duodenal ulcer can run down it to the right iliac fossa and mimic appendicitis; pelvic or appendicular pus can track up to the right subphrenic space.
• Left paracolic gutter is closed above by the phrenicocolic ligament, so pus tends not to pass to the left subphrenic space.
• Rectovesical pouch (male) and rectouterine pouch of Douglas (female): lowest part when erect; blood from a ruptured ectopic pregnancy or endometriotic cyst collects here; pelvic abscess drains through rectum or posterior fornix.

Clinical patterns

• Pseudomyxoma peritonei: jelly-like mucin filling the cavity, an omental cake and scalloping of the liver surface, with relative sparing of the mobile small bowel. The usual primary is an appendiceal mucinous neoplasm; ovarian masses are usually secondary. Mucin gathers where fluid is absorbed (greater omentum, under the right diaphragm) and in dependent sites.
• Sigmoid volvulus: a long sigmoid mesocolon with a narrow base twists; elderly, constipated or bedridden patients; X-ray shows a dilated "coffee bean" loop rising out of the pelvis.
• Ascites: high serum-ascites albumin gradient (≥ 11 g/L, 1.1 g/dL) means portal hypertension; low gradient suggests malignancy, TB or pancreatitis. Spontaneous bacterial peritonitis: ascitic neutrophils > 250 per mm³.

Remember The epiploic foramen is a finger-hole: portal triad in front, IVC behind, caudate above, duodenum below.

Exam trap The portal vein lies in the anterior wall of the foramen (behind the duct and artery), not behind the foramen; the IVC is the posterior wall.

Exam trap Continued bleeding despite a Pringle manoeuvre points to the hepatic veins or IVC, not the hepatic artery or portal vein.

How FCPS asks about this

• Posterior boundary of the epiploic foramen: inferior vena cava.
• Structure in the right free margin of the lesser omentum: common bile duct (artery to its left, portal vein behind).
• Lowest part of the peritoneal cavity when supine: hepatorenal pouch of Morison; when erect, pouch of Douglas or rectovesical pouch.
• Surgical route to the lesser sac and pancreas: divide the gastrocolic ligament.
• Short gastric versus splenic vessels: gastrosplenic ligament versus splenorenal ligament (with tail of pancreas).
• Mucin filling the peritoneal cavity: pseudomyxoma peritonei from an appendiceal mucinous neoplasm.
• Layer between transversus abdominis and peritoneum: transversalis fascia, then extraperitoneal fat.