Structure
The peritoneum is a single continuous sheet of mesothelium on loose connective tissue, arranged as a parietal layer on the body wall and a visceral layer on the organs.
Between the two layers is the peritoneal cavity, a potential space holding only a small volume of lubricating fluid. In males the cavity is closed. In females it communicates with the exterior through the uterine tubes, uterus and vagina, which is why pelvic infection can reach the peritoneum.
Intraperitoneal and retroperitoneal organs
An intraperitoneal organ is almost entirely covered by visceral peritoneum and usually hangs from a mesentery. A retroperitoneal organ lies behind the parietal peritoneum and is covered on its anterior surface only. Primary retroperitoneal organs developed there; secondary retroperitoneal organs began with a mesentery that later fused to the posterior wall.
| Intraperitoneal | Secondary retroperitoneal | Primary retroperitoneal |
|---|---|---|
| Stomach, first 2 cm of duodenum, jejunum, ileum, transverse colon, sigmoid colon, liver, spleen, tail of pancreas, appendix | Duodenum (parts 2 to 4), head, neck and body of pancreas, ascending and descending colon, upper rectum | Kidneys, adrenal glands, ureters, aorta, inferior vena cava |
Folds of peritoneum
A mesentery is a double layer attaching an organ to the posterior wall and carrying its vessels. An omentum is a double layer running from the stomach to another organ. A peritoneal ligament is a double layer joining two organs, or an organ to the wall. The root of the small bowel mesentery runs obliquely from the duodenojejunal flexure, left of L2, to the right sacroiliac joint, crossing the third part of the duodenum, aorta, inferior vena cava and right ureter.
Greater and lesser sacs
The greater sac is the main peritoneal cavity; the lesser sac (omental bursa) is the recess behind the stomach and lesser omentum, and the two communicate only through the epiploic foramen.
Omenta
The lesser omentum runs from the lesser curvature of the stomach and the first part of the duodenum to the liver. Its hepatogastric part carries the left and right gastric vessels; its free right edge, the hepatoduodenal ligament, carries the portal triad. The greater omentum is a four-layered apron hanging from the greater curvature, folding back up to fuse with the transverse colon. It carries the gastro-omental (gastroepiploic) vessels and tends to wall off inflamed organs.
Epiploic foramen
The epiploic foramen (of Winslow) is the entrance to the lesser sac, admitting one or two fingers.
- Anterior: free edge of the lesser omentum, containing the bile duct (right), hepatic artery proper (left) and portal vein (behind both)
- Posterior: inferior vena cava and right crus of the diaphragm
- Superior: caudate process of the liver
- Inferior: first part of the duodenum
Lesser sac
Behind the lesser sac lie the pancreas, left kidney, left adrenal gland and aorta; in front lie the lesser omentum and posterior stomach. On the left it is closed by the gastrosplenic and splenorenal ligaments. Its superior recess reaches up behind the caudate lobe; its inferior recess between the layers of the greater omentum is usually obliterated in adults.
Compartments and recesses
The transverse mesocolon divides the greater sac into supracolic and infracolic compartments, and fluid moves between them along the paracolic gutters to collect in the dependent recesses.
- Subphrenic spaces: between the diaphragm and the liver, right and left of the falciform ligament.
- Hepatorenal pouch (Morison's pouch): between the right lobe of the liver and the right kidney; the lowest part of the upper abdomen in a supine patient, and the first place free fluid shows on FAST scanning.
- Paracolic gutters: lateral to the ascending and descending colon. The right gutter opens freely into the hepatorenal pouch; the left is partly closed above by the phrenicocolic ligament.
- Pelvic pouches: the rectovesical pouch in males; the vesicouterine and rectouterine (Douglas) pouches in females. The rectovesical or rectouterine pouch is the lowest part of the cavity in an upright patient.
Blood supply and innervation
Parietal peritoneum takes its blood and nerves from the overlying body wall, while visceral peritoneum shares those of the organ it covers.
Parietal peritoneum of the anterior and lateral walls is supplied by the lower intercostal nerves (T7 to T11), the subcostal nerve (T12) and the iliohypogastric and ilioinguinal nerves (L1). It is sensitive to pain, temperature, touch and pressure, and pain arising from it is sharp and well localised. The central diaphragmatic peritoneum is supplied by the phrenic nerves (C3 to C5), so irritation there is felt at the shoulder tip. The peripheral diaphragm is supplied by the lower intercostal nerves. Pelvic parietal peritoneum is supplied mainly by the obturator nerve.
Visceral peritoneum has autonomic afferents that respond to stretch and ischaemia, not to cutting. Its pain is dull, poorly localised and referred to the midline by embryological origin: foregut to the epigastrium, midgut to the umbilicus, hindgut to the suprapubic region.
Clinical relevance
The peritoneum explains the pattern of abdominal pain, the spread of infection and several surgical manoeuvres.
- Appendicitis: vague periumbilical pain from visceral peritoneum shifts to the right iliac fossa once the parietal peritoneum is inflamed.
- Perforated duodenal ulcer: fluid runs down the right paracolic gutter and can mimic appendicitis.
- Pringle manoeuvre: a finger through the epiploic foramen and thumb in front compress the hepatoduodenal ligament to control bleeding from the liver.
- Ascites and paracentesis: a needle lateral to the rectus sheath avoids the inferior epigastric vessels.
- Peritoneal dialysis: uses the large surface area of the membrane for exchange.
- Pancreatic pseudocyst: collects in the lesser sac behind the stomach.