Abdominal Peritoneum

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Abdominal Peritoneum

Definition and Structure

The peritoneum is the largest and most complex serous membrane in the human body. Its surface area (~2 m²) nearly equals that of the skin. It consists of a single layer of flattened polyhedral cells called mesothelium, resting on a thin layer of fibroelastic tissue. Beneath the mesothelium lies a rich network of lymphatic vessels and capillary blood vessels through which all absorption and exudation occur.

The abdominal wall layers deep to the muscles are, from outside in:

Transversalis fascia
Extraperitoneal (preperitoneal) fascia/fat
Parietal peritoneum (innermost lining)

Transverse section of the abdominal wall showing layers from skin to visceral peritoneum

Fig: Transverse section showing abdominal wall layers - note the parietal peritoneum as the innermost layer, and visceral peritoneum wrapping the gut tube. (Gray's Anatomy for Students)

Two Layers of Peritoneum

Layer	Location	Nerve Supply	Pain Character
Parietal peritoneum	Lines the abdominal and pelvic walls	Somatic afferents (T7-L1 spinal nerves)	Severe, well-localized pain
Visceral peritoneum	Covers the abdominal viscera	Visceral afferents (autonomic)	Poorly localized, referred to midline

The Peritoneal Cavity

The parietal peritoneum forms a continuous sac. This sac is:

Closed in men
Has two openings in women - where the uterine tubes open, providing a pathway to the exterior (a common route for pelvic infection to ascend into the peritoneal cavity)

In health, only a few millilitres of pale yellow, slightly viscid peritoneal fluid are present. This fluid contains lymphocytes and other leukocytes and serves to lubricate the viscera, facilitating smooth movement and peristalsis.

Greater Sac vs. Lesser Sac (Omental Bursa)

The greater and lesser omentum divide the peritoneal cavity into two compartments:

Greater sac: The main peritoneal space
Lesser sac (omental bursa): Located behind the lesser and greater omentum, stomach, and transverse colon. Communicates with the greater sac through the omental (epiploic) foramen of Winslow

CT peritoneogram - sagittal section showing the lesser omentum, lesser sac, transverse mesocolon, greater omentum, small bowel mesentery, sigmoid mesentery, greater sac, and rectouterine pouch

Fig: CT peritoneogram (sagittal) illustrating the mesenteries, greater and lesser sacs, and omenta. L = Liver, S = Stomach, C = Colon. (Sabiston Textbook of Surgery)

Intraperitoneal vs. Retroperitoneal Organs

Abdominal viscera are classified based on their relationship to the peritoneum:

Intraperitoneal - suspended in the cavity by mesenteries (e.g., stomach, jejunum, ileum, transverse colon, sigmoid colon, spleen, liver)
Retroperitoneal - outside the peritoneal cavity, covered by peritoneum on only one surface (e.g., kidneys, ureters, aorta, ascending and descending colon, duodenum 2nd-4th parts, pancreas, rectum)

A further subdivision:

Preperitoneal: Between the anterior abdominal wall muscles and the peritoneum (e.g., preperitoneal fat in the inguinal region)
Retroperitoneal: Between the peritoneum and the posterior abdominal wall

Peritoneal Folds

Throughout the peritoneal cavity, folds of peritoneum connect organs to each other or to the abdominal wall. They develop from the embryonic dorsal and ventral mesenteries. There are three types:

1. Omenta

Two-layered peritoneal folds passing from the stomach/duodenum to other viscera:

Greater Omentum (from dorsal mesentery):

A large, apron-like fold attached to the greater curvature of the stomach and first part of the duodenum
Drapes inferiorly over the transverse colon and loops of small bowel
Turns posteriorly, ascending to fuse with the superior surface of the transverse colon and anterior layer of the transverse mesocolon
Always contains fat (which can become substantial); carries the right and left gastro-omental vessels
Clinically known as the "policeman of the abdomen" - it migrates to inflamed areas, walling off infection/inflammation
A major site for transcoelomic metastatic spread (especially ovarian carcinoma → "omental cake")

Lesser Omentum (from ventral mesentery):

Extends from the lesser curvature of the stomach and first part of the duodenum to the inferior surface of the liver
Divided into:
- Hepatogastric ligament (medial part - between stomach and liver)
- Hepatoduodenal ligament (lateral part - between duodenum and liver; free right border forms the anterior boundary of the omental foramen)
The hepatoduodenal ligament encloses three vital structures: portal vein, hepatic artery proper, and bile duct (Pringle's manoeuvre compresses this)

2. Mesenteries

Double-layered peritoneal folds attaching viscera to the posterior abdominal wall; carry vessels, nerves, and lymphatics:

Mesentery	Associated Organ	Attachment
Mesentery (proper)	Jejunum and ileum	Duodenojejunal flexure → ileocaecal junction (~15 cm root, oblique)
Transverse mesocolon	Transverse colon	Anterior surface of pancreas/posterior abdominal wall
Sigmoid mesocolon	Sigmoid colon	Posterior abdominal/pelvic wall

All three are derivatives of the dorsal mesentery.

3. Peritoneal Ligaments

Folds connecting organs to the abdominal wall or to each other (e.g., falciform ligament connecting liver to anterior wall, coronary ligament, splenorenal ligament, gastrosplenic ligament).

Peritoneal Fluid Circulation

Peritoneal fluid circulates in a clockwise orientation. During expiration, intra-abdominal pressure falls and fluid travels upward toward the diaphragm. Particulate matter and bacteria are absorbed within minutes into the lymphatic network via "pores" in the diaphragmatic peritoneum. This circulation pattern explains why abscesses can develop at sites remote from the primary disease. The most common collection sites are:

Pelvis (rectovesical/recto-uterine pouch - pouch of Douglas) - gravity when upright
Right subphrenic/subhepatic space (pouch of Morrison/hepatorenal fossa) - gravity when supine
Right paracolic gutter

Innervation of the Peritoneum

Parietal peritoneum of the abdominal wall: T7-T12 and L1 spinal nerves (somatic) - pain is well-localised to the affected quadrant; stimulation causes protective guarding and rigidity
Visceral peritoneum: Visceral afferents via autonomic nerves - pain is poorly localised, typically referred to the midline, described as dull/crampy
The pelvic parietal peritoneum is innervated by obturator nerve branches and sacral nerves

Functions of the Peritoneum

In health:

Visceral lubrication (frictionless movement)
Fluid and particulate absorption
Immunological surveillance

In disease:

Pain perception (primarily parietal component)
Inflammatory and immune responses
Fibrinolytic activity
Can produce large volumes of fluid (ascites) or inflammatory exudate (peritonitis)

Clinical Relevance

Peritonitis - inflammation of the peritoneum; classified as localised or diffuse. Causes include:

Gastrointestinal perforation (peptic ulcer, appendix, diverticulum)
Transmural translocation (pancreatitis, ischaemic bowel, primary bacterial peritonitis)
Exogenous contamination (trauma, drains, peritoneal dialysis)
Female genital tract infection (pelvic inflammatory disease)
Haematogenous spread (rare)

Peritoneal dialysis: The large vascular surface area (~2 m²) makes the peritoneum an effective dialysis membrane.

Peritoneal metastases: Direct transcoelomic spread is common from GI and gynaecological cancers; fluid circulation determines implant distribution (pelvis, right paracolic gutter, hepatorenal fossa most affected).

Sources: Gray's Anatomy for Students (9780323934237); Bailey and Love's Short Practice of Surgery, 28th ed. (9780367548117); Sabiston Textbook of Surgery, 21st ed. (9780443124341); Schwartz's Principles of Surgery, 11th ed. (9781259835346)

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