Peritoneum and Peritoneal Spaces

Lines the internal surface of the abdominal and pelvic walls (anterior, posterior, lateral, and the undersurface of the diaphragm).

• Somatically innervated by the corresponding segmental nerves of the overlying body wall (intercostal, subcostal, lumbar nerves), pain-sensitive, well-localised, and sensitive to stretch, pressure, cutting, and temperature, similar to parietal pleura.

• Closely invests the abdominal organs (to varying degrees: see intraperitoneal vs retroperitoneal below).
• Autonomically innervated, via the same afferent pathways that accompany the sympathetic supply to the organ it covers, insensitive to touch/cutting, but sensitive to stretching, distension, and ischaemia; pain is typically poorly localised and referred according to the organ's embryological origin (foregut, midgut, hindgut; see Overview section).

• The potential space between parietal and visceral peritoneum, containing a thin film of serous fluid that permits frictionless visceral movement (particularly important for peristalsis).
• In males, a closed cavity; in females, technically continuous with the exterior via the uterine tubes, uterus, and vagina, a pathway of potential clinical significance (e.g., ascending pelvic infection, or transperitoneal spread via the fallopian tubes).

• Organs almost completely covered by visceral peritoneum, typically suspended by a mesentery, allowing relative mobility.
• Includes: stomach, first part of the duodenum (partially), jejunum, ileum, caecum (variable), appendix, transverse colon, sigmoid colon, liver (mostly), spleen, gallbladder (mostly), and (in females) the ovaries and uterine tubes.

• Organs lying posterior to the parietal peritoneum, only partially covered (typically only on their anterior surface); see Overview section for the primary/secondary distinction.
• Includes: kidneys, ureters, suprarenal glands, abdominal aorta, IVC (primary); duodenum (2nd-4th parts), pancreas (head/neck/body), ascending colon, descending colon (secondary).

Mesentery:

• A double layer of peritoneum connecting an intraperitoneal organ to the posterior abdominal wall, carrying the organ's neurovascular supply, lymphatics, and (variably) fat between its two layers.
• The term "mesentery" alone (or "mesentery proper") typically refers specifically to the mesentery of the small intestine (jejunum/ileum).

Greater omentum:

• A large, apron-like fold of peritoneum hanging from the greater curvature of the stomach (and the proximal duodenum), draping down over the transverse colon and small intestine before folding back up to attach to the transverse colon/mesocolon.
• Contains the gastro-omental (gastroepiploic) vessels along its upper border, and a variable amount of fat.
• Functions as the "abdominal policeman": its mobility and tendency to migrate toward and wrap around sites of intra-abdominal inflammation (e.g., appendicitis, perforated viscus) helps to localise infection and limit the spread of peritonitis.

Lesser omentum:

• A smaller peritoneal fold connecting the lesser curvature of the stomach and the first part of the duodenum to the liver (specifically the porta hepatis).
• Two named parts: hepatogastric ligament (thinner, membranous part, between liver and stomach) and hepatoduodenal ligament (thicker, free right border, between liver and duodenum), the hepatoduodenal ligament contains the portal triad (portal vein, hepatic artery proper, common bile duct) within its free edge, and forms the anterior boundary of the epiploic foramen.

Mesocolon:

• The mesentery specifically connecting parts of the colon to the posterior abdominal wall, transverse mesocolon (suspending the transverse colon, containing the middle colic vessels) and sigmoid mesocolon (suspending the sigmoid colon, containing the sigmoid vessels); the ascending and descending colon typically lack a mesocolon in adults, being secondarily retroperitoneal (fused to the posterior wall).

Peritoneal ligaments:

• Numerous named double-layered peritoneal folds connecting organs to each other or to the abdominal wall, generally carrying blood vessels or representing the remnants of embryological structures, e.g., falciform ligament (liver to anterior abdominal wall, containing the ligamentum teres/obliterated umbilical vein), gastrosplenic ligament (stomach to spleen, containing short gastric and left gastroepiploic vessels), splenorenal (lienorenal) ligament (spleen to posterior abdominal wall/left kidney, containing splenic vessels and the tail of pancreas), hepatorenal ligament, and others covered in their respective organ sections.

Greater sac:

• The main, larger part of the peritoneal cavity, extending from the diaphragm to the pelvis, comprising most of the peritoneal cavity outside the lesser sac.

Lesser sac (omental bursa):

• A smaller peritoneal space lying posterior to the stomach and lesser omentum, anterior to the pancreas and posterior abdominal wall structures.
• Communicates with the greater sac via the epiploic foramen (of Winslow): the only connection between the two sacs.
• Bounded: anteriorly by the stomach/lesser omentum/posterior layer of greater omentum; posteriorly by the peritoneum overlying the pancreas, splenic vessels, and left kidney/suprarenal gland; superiorly by the caudate lobe of the liver and diaphragm; inferiorly by the fusion of the greater omentum's layers; laterally by the spleen and gastrosplenic/splenorenal ligaments.

Epiploic foramen (of Winslow):

• The sole opening connecting the greater and lesser sacs.
• Boundaries:
• Anterior: free edge of the lesser omentum (hepatoduodenal ligament), containing the portal triad (portal vein posteriorly, hepatic artery proper anteriorly/to the left, common bile duct anteriorly/to the right).
• Posterior: IVC (retroperitoneal, covered by peritoneum).
• Superior: caudate lobe of the liver.
• Inferior: first part of the duodenum.
• Clinically important as a landmark for controlling haemorrhage from the portal triad (Pringle manoeuvre, compressing the hepatoduodenal ligament through this foramen) and as the route for accessing the lesser sac surgically.

Subphrenic spaces:

• Potential peritoneal spaces between the diaphragm and the liver (right and left subphrenic spaces), separated by the falciform ligament.
• Common sites for postoperative or infective abscess collection, given their dependent position relative to the upper abdomen in certain postures, and their relative isolation once loculated.

Subhepatic space:

• Potential space below the liver; the hepatorenal recess (pouch of Morison), between the liver and right kidney, is the specific right-sided subhepatic recess of greatest clinical importance.

Hepatorenal recess (Morison's pouch):

• The most dependent part of the peritoneal cavity in a supine patient, located on the right side between the liver and the right kidney.
• The classic site where free intraperitoneal fluid (blood, ascites, pus) first collects and pools in a supine patient, making it the key site assessed on the FAST (Focused Assessment with Sonography for Trauma) ultrasound scan for detecting intraperitoneal haemorrhage.

Paracolic gutters:

• Peritoneal grooves lateral (and medial) to the ascending and descending colon, providing pathways for fluid to flow between the upper and lower abdominal peritoneal cavity.
• The right paracolic gutter is generally deeper/wider than the left (which is partially obstructed by the phrenicocolic ligament near the splenic flexure), making it a more efficient conduit for fluid spread, clinically relevant to the pattern of spread of appendiceal or other right-sided intraperitoneal infection/fluid toward the right subphrenic space, and conversely to the more limited spread from left-sided/pelvic sources upward via the left gutter.

Pelvic recesses:

• Rectovesical pouch (males), between the rectum and bladder, the most dependent point of the peritoneal cavity in a male in the upright/standing position.
• Rectouterine pouch (pouch of Douglas) (females), between the rectum and the posterior uterus/vagina, the most dependent point of the peritoneal cavity in a female in the upright position, the classic site for fluid collection (blood, pus, ascites) assessed by posterior vaginal fornix examination or culdocentesis, and a common site for endometriotic deposits and for the initial spread of ovarian malignancy.
• Vesicouterine pouch (females), smaller, between the bladder and anterior uterus, of lesser clinical significance than the rectouterine pouch.

• Parietal peritoneum: somatic, segmental, well-localised pain; visceral peritoneum: autonomic, poorly localised, referred pain according to embryological origin.
• The transition from vague, midline visceral pain to sharp, localised parietal pain (classically illustrated by appendicitis, periumbilical pain progressing to localised right iliac fossa pain as the inflamed appendix irritates the overlying parietal peritoneum) is a key clinical-anatomical correlation running throughout abdominal pathology.

• Peritoneal fluid (and pathological collections: blood, pus, ascites, bile, gastric/bowel contents from perforation) can move relatively freely within the peritoneal cavity, following gravity-dependent pathways influenced by patient position, the peritoneal folds/mesenteries acting as partial barriers or channels, and the paracolic gutters as major conduits between the upper and lower abdomen.
• Normal peritoneal fluid circulation also assists in lymphatic drainage of the peritoneal cavity, predominantly via diaphragmatic lymphatic stomata (small openings in the diaphragmatic peritoneum allowing direct absorption into the diaphragmatic/mediastinal lymphatics), relevant to the rapid, extensive spread of intraperitoneal malignancy (e.g., ovarian cancer) to distant sites via this route.

• Follows the same anatomical pathways as fluid movement generally: paracolic gutters (particularly the right, being deeper), subphrenic and subhepatic spaces (common abscess sites), and the pelvic recesses (dependent pooling in the upright position).
• The greater omentum's mobility helps localise infection by physically migrating to and wrapping around inflamed viscera, though this protective mechanism can be overwhelmed in severe or rapidly progressive peritonitis, or may be less effective/present in young children (relatively underdeveloped omentum).

Ascites:

• Pathological accumulation of fluid within the peritoneal cavity, from causes including portal hypertension (most common, via increased hydrostatic pressure and reduced oncotic pressure), malignancy (peritoneal carcinomatosis, obstructing lymphatic drainage/increasing capillary permeability), infection, and hypoalbuminaemic states.
• Detected clinically by shifting dullness and fluid thrill (large volumes), following the same gravity-dependent distribution pathways.

Peritonitis:

• Inflammation of the peritoneum, from infection (perforated viscus, appendicitis) or chemical irritation (bile, gastric acid, blood).
• Localised peritonitis: parietal peritoneal irritation confined to one region, producing well-localised pain and guarding (e.g., early appendicitis).
• Generalised peritonitis: widespread parietal peritoneal irritation, producing diffuse abdominal pain, rigidity, and a surgical emergency; spreads via the pathways (paracolic gutters, subphrenic spaces).

Intraperitoneal bleeding:

• Follows the same gravity-dependent pooling pattern: Morison's pouch in the supine position (the key FAST scan target), pelvic recesses (Douglas's pouch/rectovesical pouch) in the upright/seated position.
• Free blood is highly irritant to the peritoneum, producing diffuse or localised peritonitic signs depending on the volume and rate of accumulation.

Lesser sac access:

• Surgical access to the lesser sac (e.g., for pancreatic necrosectomy, drainage of a lesser sac collection/pseudocyst) is typically achieved through the gastrocolic ligament (part of the greater omentum, between the stomach and transverse colon) rather than the epiploic foramen (too narrow and anatomically constrained for most surgical purposes), given this provides a much wider working window into the lesser sac.