Peritoneum, Omenta & Peritoneal Spaces

The peritoneum is the largest serous membrane of the body, and its complex folds, spaces, and recesses dictate how infection, fluid, and tumour spread within the abdomen. For NEET PG, this topic sits at the crossroads of gross anatomy and surgery — boundaries of the omental bursa, the policeman of the abdomen, and where pus collects after a perforated appendix are perennial favourites.

Basic structure & classification

The peritoneum is a thin, glistening serous membrane lined by a single layer of mesothelium resting on loose connective tissue. It has two continuous layers:

• Parietal peritoneum — lines the abdominal and pelvic walls. Derived from somatopleuric mesoderm; supplied somatically by spinal nerves (phrenic, lower intercostals, subcostal, iliohypogastric). It is therefore sensitive to pain, touch, temperature, and pressure, and pain is well localised (e.g., shifting of pain to McBurney's point in appendicitis once parietal peritoneum is involved).
• Visceral peritoneum — covers the organs. Derived from splanchnopleuric mesoderm; supplied autonomically by the same nerves as the underlying viscus. It is insensitive to touch/cutting but sensitive to stretch and chemical irritation, with poorly localised (referred, midline) pain.

The potential space between the two layers is the peritoneal cavity, normally containing only a thin film (~50 mL) of serous lubricating fluid.

High-yield: Parietal peritoneum = somatic innervation = sharp, localised pain. Visceral peritoneum = autonomic = dull, diffuse, referred pain. This explains the classic initial periumbilical pain → later right iliac fossa pain sequence of acute appendicitis.

Sex difference

The peritoneal cavity is a closed sac in males but open in females — the abdominal ostia of the uterine tubes communicate with the cavity. This is why infection (e.g., gonococcal/chlamydial PID) can ascend to cause perihepatitis (Fitz-Hugh-Curtis syndrome) and why air can be insufflated transvaginally.

Intraperitoneal vs retroperitoneal organs

This classification is one of the most repeatedly tested points.

Category	Definition	Organs
Intraperitoneal	Almost completely covered by visceral peritoneum, suspended by a mesentery	Stomach, jejunum, ileum, transverse colon, sigmoid colon, caecum (mostly), appendix, spleen, liver, tail of pancreas, transverse mesocolon
Primarily (1°) retroperitoneal	Developed and remained behind the peritoneum	Kidneys, ureters, adrenal glands, aorta, IVC, gonadal vessels
Secondarily (2°) retroperitoneal	Began intraperitoneal then fused to posterior wall (fusion fascia)	Duodenum (2nd–4th parts), ascending colon, descending colon, head/neck/body of pancreas
Infraperitoneal/subperitoneal	Below the peritoneal reflection in pelvis	Lower rectum, urinary bladder, distal ureters

High-yield mnemonic for retroperitoneal organs — "SAD PUCKER": Suprarenal (adrenal), Aorta/IVC, Duodenum (2–4), Pancreas (except tail), Ureters, Colon (ascending & descending), Kidneys, Esophagus (abdominal part), Rectum (lower).

Peritoneal folds: mesenteries, omenta, ligaments

A mesentery is a double-layered fold connecting a viscus to the posterior wall, carrying its vessels, nerves, and lymphatics. An omentum connects the stomach/proximal duodenum to another organ. A peritoneal ligament connects two organs or an organ to the wall.

The mesenteries

• The mesentery (of small intestine) — root ~15 cm, runs obliquely from the duodenojejunal flexure (L2, left of midline) to the ileocaecal junction (right sacroiliac joint), crossing the 3rd part of duodenum, aorta, IVC, right ureter, and right psoas.
• Transverse mesocolon, sigmoid mesocolon, mesoappendix (contains the appendicular artery, a branch of the ileocolic — an end artery, hence appendix is prone to ischaemic gangrene).

The omenta

Feature	Greater omentum	Lesser omentum
Attachment	Greater curvature of stomach → hangs down → folds back to transverse colon	Lesser curvature of stomach & first 2 cm duodenum → porta hepatis
Layers	4 layers of peritoneum	2 layers
Parts	Gastrophrenic, gastrosplenic, gastrocolic ligaments	Hepatogastric + hepatoduodenal ligaments
Key contents	Right & left gastro-omental (gastroepiploic) vessels along its attached border	Free right border = hepatoduodenal ligament containing the portal triad
Function	"Policeman of the abdomen" — walls off infection	Forms anterior boundary of lesser sac

High-yield: The free margin of the lesser omentum (hepatoduodenal ligament) transmits the portal triad → bile duct (anterior right), hepatic artery proper (anterior left), portal vein (posterior). This margin forms the anterior boundary of the epiploic foramen (of Winslow) and is where the Pringle manoeuvre clamps to control hepatic haemorrhage.

Greater omentum — "policeman/gendarme of the abdomen"

The greater omentum is a fatty apron rich in macrophages (milky spots) and a mobile blood supply. It migrates to and adheres around sites of inflammation (e.g., a perforated appendix or inflamed gallbladder), walling off the focus and limiting generalised peritonitis. In children it is short and underdeveloped, so they are more prone to generalised peritonitis after appendicular perforation.

High-yield: A common surgery MCQ — "Why does a child develop generalised peritonitis more readily after a perforated appendix?" → Short, underdeveloped greater omentum that cannot reach and wall off the appendix.

Lesser sac (omental bursa) & the epiploic foramen

The peritoneal cavity is divided into a large greater sac and a smaller diverticulum, the lesser sac (omental bursa), which lies behind the stomach and lesser omentum. The two communicate only through the epiploic foramen (foramen of Winslow / omental foramen).

Boundaries of the epiploic foramen — must memorise

Direction	Boundary
Anterior	Free margin of lesser omentum (hepatoduodenal ligament) with the portal triad
Posterior	Inferior vena cava (covered by parietal peritoneum)
Superior	Caudate lobe of liver
Inferior	First part of duodenum (and horizontal part of hepatic artery)

High-yield: Anterior = portal triad; Posterior = IVC; Superior = caudate lobe; Inferior = 1st part of duodenum. The relationship of the bile duct (anterior) and portal vein (posterior) to the foramen explains why a finger can be passed into the foramen and the hepatoduodenal ligament compressed between finger and thumb in the Pringle manoeuvre.

Boundaries & recesses of the lesser sac

• Anterior wall: lesser omentum, posterior surface of stomach, and the gastrocolic ligament.
• Posterior wall: structures of the posterior abdominal wall covered by peritoneum — pancreas, left suprarenal gland, upper pole of left kidney, transverse mesocolon.
• Superior recess lies behind the liver around the caudate lobe; inferior recess lies between the layers of the greater omentum (largely obliterated in adults).
• Splenic recess extends to the left towards the hilum of the spleen between the gastrosplenic and splenorenal ligaments.

Clinical relevance: Fluid from a posterior gastric ulcer perforation or a pancreatic pseudocyst collects in the lesser sac. A pseudocyst classically forms in the lesser sac behind the stomach following acute pancreatitis.

High-yield: Pancreatic pseudocyst → lesser sac (behind stomach). The posterior wall relation of the pancreas to the lesser sac is why pancreatitis tracks fluid here. Drainage is classically via cystogastrostomy into the posterior stomach wall.

Subdivisions of the greater sac — supracolic & infracolic

The transverse mesocolon divides the greater sac into:

• Supracolic compartment — above the transverse mesocolon (stomach, liver, spleen). Contains the subphrenic and subhepatic spaces.
• Infracolic compartment — below the transverse mesocolon, split by the root of the mesentery into right and left infracolic spaces.

These compartments communicate laterally via the paracolic gutters.

Subphrenic (suprahepatic) and subhepatic spaces

Space	Location	Drains pus from
Right subphrenic	Between diaphragm & right lobe of liver, right of falciform ligament	Perforated duodenal ulcer, appendix, gallbladder
Left subphrenic	Between diaphragm & left lobe/spleen/stomach	Perforated gastric ulcer, splenectomy bed, anastomotic leak
Right subhepatic / hepatorenal pouch (of Morison)	Between liver, right kidney, & hepatic flexure	Most dependent part of peritoneal cavity in supine patient
Left subhepatic	Effectively the lesser sac

High-yield: Hepatorenal pouch of Morison is the lowest (most dependent) part of the peritoneal cavity in the supine position, so it is the commonest site for pus/fluid collection and the first space examined on FAST ultrasound in trauma (free fluid = haemoperitoneum).

High-yield: The right subphrenic space is the commonest site of a subphrenic abscess; fluid from a perforated duodenal ulcer or ruptured appendix tracks up the right paracolic gutter to reach it.

Paracolic gutters & fluid tracking

The paracolic gutters are peritoneal recesses lateral to the ascending and descending colon.

• Right paracolic gutter is deeper and continuous superiorly with the right subhepatic and subphrenic spaces and inferiorly with the pelvis. It is the main channel for fluid spread between the pelvis and the right subphrenic space.
• Left paracolic gutter is shallower and its superior extension is blocked by the phrenicocolic ligament, limiting upward spread of pus to the left subphrenic region.

Fluid-tracking flow (perforated appendix, supine patient): Appendiceal pus → right paracolic gutter → ascends to right subhepatic (Morison) & right subphrenic spaces (upward, supine) → or descends to the pelvis (pouch of Douglas) when erect → right subphrenic / pelvic abscess.

High-yield: Because the phrenicocolic ligament blocks the top of the left gutter, infection from the appendix preferentially reaches the right subphrenic space, not the left. The old surgical aphorism: "pus from a perforated appendix ascends the right paracolic gutter."

Pelvic peritoneal recesses

In the pelvis the peritoneum forms the most dependent pouches in the erect/sitting position:

• Rectovesical pouch in males (between rectum and bladder).
• Rectouterine pouch (of Douglas) in females — the lowest point of the peritoneal cavity in females; pus here is drained per rectum/posterior fornix (culdocentesis).
• Vesicouterine pouch in females, anterior to the uterus.

High-yield: Lowest point of peritoneal cavity — supine = hepatorenal pouch of Morison; erect = rectovesical pouch (male) / pouch of Douglas (female). Pelvic abscess after appendicitis collects in the pouch of Douglas → causes tenesmus, mucous diarrhoea, and a boggy mass on PR.

Peritoneal recesses & internal hernias

Small fossae where bowel can become trapped (internal hernia):

• Duodenal recesses — superior, inferior, paraduodenal (left paraduodenal hernia is commonest internal hernia).
• Ileocaecal (superior & inferior) and retrocaecal recesses — the retrocaecal recess is the commonest site of the appendix (retrocaecal position ~65–75%).
• Intersigmoid recess.

Clinical correlations & investigations

• Peritonitis: Inflammation of peritoneum. Board-stiff rigidity, guarding, rebound tenderness (signs of parietal peritoneal irritation). Erect chest X-ray shows gas under the diaphragm in hollow-viscus perforation.
• Ascites: Pathological fluid in the peritoneal cavity; tested by shifting dullness (>1.5 L) and fluid thrill; paracentesis at the junction of lateral one-third and medial two-thirds of a line from umbilicus to ASIS (to avoid inferior epigastric vessels).
• Investigation of choice for free fluid/abscess: CECT abdomen is the modality of choice for localising intra-abdominal collections; ultrasound (FAST) is the bedside first-line in trauma.
• Peritoneal dialysis uses the large surface area of peritoneum as a semipermeable membrane.
• Carcinomatosis peritonei & Sister Mary Joseph nodule (umbilical metastasis along the obliterated umbilical vein/ligamentum teres) — sign of intra-abdominal malignancy. Krukenberg tumour = transcoelomic spread of gastric signet-ring carcinoma to ovaries.

High-yield: Transcoelomic (peritoneal) spread routes — gastric cancer → ovary (Krukenberg); umbilicus (Sister Mary Joseph nodule); pouch of Douglas (Blumer's shelf, palpable on PR).

Management / drug-of-choice pointers (surgical context)

• Generalised peritonitis: Resuscitation, broad-spectrum IV antibiotics covering gut flora (e.g., a third-generation cephalosporin + metronidazole, or piperacillin-tazobactam), and source control by laparotomy/laparoscopy with peritoneal lavage.
• Subphrenic abscess: Image-guided (USG/CT) percutaneous drainage is first-line; open drainage if inaccessible.
• Hepatic haemorrhage control: Pringle manoeuvre — clamp hepatoduodenal ligament at the epiploic foramen.

Key differentials / distinctions to keep clear

• Subphrenic abscess vs basal pneumonia/pleural effusion — both cause shoulder-tip pain (phrenic, C3–C5 referral); CT distinguishes.
• Lesser sac collection (pancreatic pseudocyst) vs greater sac ascites — location behind stomach on CT.
• Retroperitoneal vs intraperitoneal bleed — retroperitoneal (e.g., AAA, renal) may be missed on FAST.

Recently asked / exam angle

• Boundaries of the epiploic foramen of Winslow — anterior, posterior, superior, inferior (recurrent single-best-answer).
• Contents of the free margin of the lesser omentum / portal triad arrangement — bile duct anterior-right, hepatic artery anterior-left, portal vein posterior.
• Most dependent part of peritoneal cavity in supine patient → Morison's pouch (and erect → pouch of Douglas).
• Why pus from perforated appendix reaches right subphrenic space → right paracolic gutter; left gutter blocked by phrenicocolic ligament.
• Secondarily retroperitoneal organs — ascending/descending colon, duodenum (2–4), pancreas (except tail).
• Pancreatic pseudocyst location → lesser sac.
• Policeman of the abdomen → greater omentum; children get generalised peritonitis due to short omentum.
• Structure forming posterior boundary of epiploic foramen → IVC.
• Image/CT-based: identify the lesser sac, paracolic gutters, or falciform ligament on a cross-section.

Rapid revision

1. Parietal peritoneum = somatic = sharp localised pain; visceral = autonomic = dull referred pain.
2. Peritoneal cavity is a closed sac in males, open in females (via uterine tube ostia).
3. Retroperitoneal organs mnemonic — SAD PUCKER.
4. Greater omentum = 4 layers = "policeman of the abdomen"; lesser omentum = 2 layers.
5. Hepatoduodenal ligament (free margin of lesser omentum) carries the portal triad = anterior boundary of epiploic foramen.
6. Epiploic foramen: Anterior = portal triad, Posterior = IVC, Superior = caudate lobe, Inferior = 1st part duodenum.
7. Pringle manoeuvre clamps the hepatoduodenal ligament to control liver bleeding.
8. Morison's (hepatorenal) pouch = most dependent peritoneal space when supine = first FAST window.
9. Erect lowest point = rectovesical pouch (male) / pouch of Douglas (female).
10. Right paracolic gutter is deep and continuous up to the subphrenic space; left is blocked by the phrenicocolic ligament.
11. Pancreatic pseudocyst collects in the lesser sac behind the stomach.
12. Transcoelomic spread → Krukenberg tumour (ovary), Sister Mary Joseph nodule (umbilicus), Blumer's shelf (pouch of Douglas).