Peritoneum and Retroperitoneum

The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). The peritoneum supports and suspends the organs within the abdominal cavity and provides an important conduit for the neurovasculature supplying these organs. There are several peritoneal folds, known as mesenteries, omenta, and ligaments. The greater and lesser omenta divide the peritoneal cavity into greater and lesser sacs, which are important anatomic spaces within the cavity. Organs located behind the posterior parietal peritoneum are known as retroperitoneal, while organs that protrude into the cavity and are fully covered by visceral peritoneum are known as intraperitoneal.

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Overview

Definition

The peritoneum is a serous membrane lining the abdominopelvic cavity and covering its organs. 

  • Peritoneal cavity: the cavity encased by the peritoneal membrane
  • Function: 
    • Supports and suspends the organs in their proper location within the abdominopelvic cavity
    • Allows organs to move freely and smoothly
    • Serves as a protective conduit for the neurovasculature supplying organs suspended within the cavity (e.g., protects arteries running from the aorta to the intestines)

Development

The peritoneum develops from the mesoderm of the trilaminar embryo:

  • The lateral plate mesoderm splits to form 2 layers separated by an intraembryonic coelom:
    • Splanchnic layer (also known as the visceral layer)
      • Adjacent to the endoderm (which gives rise to the primitive gut tube)
      • Gives rise to visceral peritoneum covering the internal organs
      • Gives rise to mesentery, connecting the gut tube to the abdominal walls
    • Somatic layer (also known as the parietal layer) 
      • Adjacent to surface ectoderm
      • Gives rise to parietal peritoneum lining the abdominal wall
  • Mesenteries suspend the primitive gut tube within the developing abdominal cavity:
    • Ventral mesentery: 
      • Anchors gut tube to the anterior abdominal wall
      • Gives rise to lesser omentum
    • Dorsal mesentery: 
      • Anchors gut tube to the posterior abdominal wall
      • Remains as mesentery
      • Also gives rise to the greater omentum
    • Contains the blood vessels that supply the primitive gut tube as it grows away from the abdominal aorta

Peritoneum and the Peritoneal Cavity

Anatomy of the peritoneum

  • Histologically composed of: 
    • Mesothelial cells 
    • Thin layer of fibrous tissue
  • Peritoneal layers:
    • Visceral peritoneum: tightly covers the abdominal visceral organs
    • Parietal peritoneum: lines the inside of the abdominopelvic body wall
  • Peritoneal folds (see Compartments of the Peritoneal Cavity for details): 
    • Mesenteries
    • Peritoneal ligaments
    • Omenta
  • Subdivisions of the peritoneal cavity (see Neurovasculature for details)
    • Greater sac, which contains:
      • Supracolic compartment
      • Infracolic compartment 
    • Lesser sac

Visualizing the peritoneal cavity

  • The peritoneal cavity is the space enclosed by the parietal peritoneum, which lines the inner abdominal wall.
  • The visceral organs push into this space and are lined with visceral peritoneum.
  • There are no organs (only a serous fluid) between the parietal peritoneum and the visceral peritoneum.
  • Imagine the peritoneum as a balloon filling the abdominal cavity: 
    • If you push your finger into the balloon, your finger takes up space and can move around within the balloon without popping or piercing the balloon itself. 
    • The visceral abdominal organs protrude “into” the peritoneal cavity (like your finger pushing into the balloon) without actually piercing the peritoneum itself.

Boundaries of the peritoneal cavity

  • Anterior: anterior abdominal muscles
  • Posterior: 
    • Vertebrae and ribs
    • Aorta and vena cava
    • Kidneys and adrenal glands
  • Superior: diaphragm
  • Inferior: pelvic floor

Intraperitoneal versus retroperitoneal organs

Organs can be classified according to their peritoneal coverage:

  • Intraperitoneal: completely contained within the parietal peritoneum
  • Retroperitoneal: positioned between the body wall and the peritoneum (behind the peritoneal cavity)
    • Primary retroperitoneal: organs that were never within the peritoneal cavity
    • Secondarily retroperitoneal: organs that begin as intraperitoneal during initial development but end up as retroperitoneal once fully developed
  • Subperitoneal: organ positioned beneath the peritoneum
Table: Intraperitoneal versus retroperitoneal organs
LocationOrgans
Intraperitoneal
  • Stomach
  • 1st part of the duodenum (superior part)
  • Jejunum
  • Ileum
  • Cecum
  • Appendix
  • Transverse colon
  • Sigmoid colon
  • Liver
  • Gallbladder
  • Tail of pancreas
  • Spleen
  • Female reproductive organs:
    • Uterus
    • Fallopian tubes
    • Ovaries
Secondarily retroperitoneal
  • 2nd–4th parts of the duodenum
  • Head, neck, and body of pancreas
  • Ascending colon
  • Descending colon
  • Upper rectum
Primary retroperitoneal
  • Kidneys
  • Ureters
  • Adrenal/suprarenal glands
  • Abdominal aorta
  • Inferior vena cava
Subperitoneal
  • Bladder
  • Lower rectum
  • Anal canal
  • Female reproductive organs: vagina
  • Male reproductive organs:
    • Prostate
    • Seminal vesicles
Development of secondarily retroperitoneal organs

Development of secondarily retroperitoneal organs:
These organs begin suspended by a mesentery. However, as the organ develops, the mesentery regresses until the organ is flush with the posterior wall and ends up as a retroperitoneal organ.

Image by Lecturio.

Peritoneal Folds

Mesentery

  • 2 layers of peritoneum
  • Suspends and holds the visceral organs to the posterior abdominal wall
  • Contains:
    • Arteries and veins
    • Lymphatics
    • Nerves
  • Specific mesenteries: 
    • Mesentery of the small intestine: 
      • Connects the jejunum and ileum to the posterior abdominal wall
      • Typically referred to as simply “the mesentery”
    • Transverse mesocolon: connects the transverse colon to the posterior abdominal wall
    • Sigmoid mesocolon: V-shaped peritoneal fold that attaches the sigmoid colon to the abdominal wall
Blood supply of the small intestine
Blood supply of the small and large intestines running within the mesenteries
Image by Lecturio.

Ligaments

  • 2 layers of peritoneum
  • Connect 2 organs to each other or attach an organ to the body wall 
  • Some ligaments form part of an omentum.
  • Generally named for the 2 connecting body parts
  • Named ligaments: 
    • Phrenicocolic: diaphragm ↔ transverse colon
    • Splenorenal: spleen ↔ left kidney
    • Gastrophrenic: stomach ↔ diaphragm
    • Gastrocolic: stomach ↔ transverse colon
    • Gastrosplenic: stomach ↔ spleen
    • Hepatogastric: liver ↔ stomach 
    • Hepatoduodenal: liver ↔ duodenum
      • Between the porta hepatis of the liver and the superior part of the duodenum
      • Contains the portal triad: hepatic artery, portal vein, and common bile duct
    • Falciform ligament: liver ↔ anterior abdominal wall

Omenta

The omenta are layered sheets of peritoneum. The greater omentum and the lesser omentum are the 2 omenta.

Greater omentum:

  • Structure: a 4-layered fold of peritoneum 
  • Location: 
    • Hanging from the greater curve of the stomach like an apron
    • Covers the transverse colon and much of the small intestines
  • Highly mobile structure
  • Function:
    • Prevents adhesion formation between visceral organs and the abdominal wall
    • Surrounds inflamed organs → seals them off to limit adhesions 
  • Made up of 3 primary ligaments:
    • Gastrocolic
    • Gastrosplenic
    • Gastrophrenic

Lesser omentum:

  • Structure: double layer of peritoneum
  • Location: extends from the liver to the lesser curvature of the stomach and the proximal portion of the duodenum
  • Made up of 2 primary ligaments:
    • Hepatogastric ligament
    • Hepatoduodenal ligament
Lesser omentum and Omental foramen in situ

Lesser omentum and omental foramen in situ

Image by Lecturio.

Compartments of the Peritoneal Cavity

Overview

The peritoneal cavity is divided into several different compartments by the omenta and transverse mesocolon:

  • Greater sac:
    • Anterior portion of the cavity
    • Further subdivided into:
      • Supracolic compartment
      • Infracolic compartment
  • Lesser sac:
    • Located in the upper posterior portion of the abdomen
    • Communicates with the greater sac via the epiploic foramen, located on the free edge of the lesser omentum
A sagittal section through the abdomen depicting the greater and lesser sacs

A sagittal section through the abdomen depicting the greater and lesser sacs:
The greater sac is outlined in green, and the lesser sac is outlined in blue.

Image by Lecturio.

Greater sac

The greater sac is divided into 2 compartments by the transverse mesocolon.

  • Supracolic compartment:
    • Lies above the transverse mesocolon
    • Contains:
      • Stomach
      • Liver
      • Spleen
  • Infracolic compartment:
    • Lies below the transverse mesocolon 
    • Contains:
      • Small intestine
      • Ascending colon
      • Descending colon
    • Can be further subdivided into right and left halves by the mesentery of the small intestine
  • Supracolic and infracolic compartments communicate freely via the paracolic gutters.

Lesser sac

The lesser sac is also known as the omental bursa. 

  • Anatomic margins: 
    • Anterior: 
      • Lesser omentum
      • Stomach
    • Posterior: 
      • Pancreas
      • Left kidney and adrenal gland
    • Inferior: greater omentum
    • Superior and medial: liver
    • Lateral: spleen
  • Connected to the greater sac through the epiploic foramen
  • Boundaries of the epiploic foramen: 
    • Hepatoduodenal ligament (containing the portal triad) anteriorly
    • Inferior vena cava posteriorly
    • Caudate lobe of the liver superiorly
    • 1st part of the duodenum inferiorly

Neurovasculature

Table: Neurovasculature of the peritoneum
Parietal peritoneumVisceral peritoneum
Arterial supply From the abdominal wall vasculature Superior and inferior mesenteric arteries
Venous drainage Veins drain into the inferior vena cava Veins drain into the portal vein
Innervation Somatic innervation from spinal nerves T10–L1 (pain can be localized) Autonomic innervation (pain is difficult to localize)

Clinical Relevance

  • Peritonitis: inflammation of the peritoneum. Peritonitis may be due to an intraabdominal abscess, rupture of an infected abdominal organ or hollow viscus (commonly the appendix or a diverticulum of the sigmoid colon), or infection from peritoneal dialysis, or it may be spontaneous (typically in individuals with cirrhosis and ascites). 
  • Peritoneal adhesions: abnormal connections between the visceral peritoneum of adjacent organs or between the parietal and visceral layers of the peritoneum. Peritoneal adhesions are usually the result of inflammation and/or injury to the peritoneum. These adhesions can lead to pain, bowel obstruction, and volvulus. Surgical lysis of adhesions may occasionally be necessary if the individual is symptomatic, but often adhesions are asymptomatic and treatment is unnecessary.
  • Ascites: pathologic accumulation of fluid within the peritoneal cavity. Ascites occurs because of an osmotic and/or hydrostatic pressure imbalance secondary to portal hypertension (cirrhosis, heart failure) or nonportal hypertension (hypoalbuminemia, malignancy, infection). Individuals often present with progressive abdominal distention and weight gain. Abdominal exam may reveal shifting dullness and a positive fluid wave. 
  • FAST exam: point-of-care ultrasound (POCUS) examination protocol of the abdominal and thoracic cavities performed in the ED as part of the secondary survey in advanced trauma life support (ATLS). The main goal of the FAST exam is to identify free intraperitoneal fluid (blood) and pericardial effusion from trauma that may require emergent surgical attention. An individual can completely exsanguinate into the peritoneal cavity.
  • Peritoneal neoplasias: Rare cancers may originate from the peritoneum and/or omentum itself. 

References

  1. Richard L.D., Wayne, A.V., Mitchell, A.W.M. (2020). Abdomen. In Richard L.D., et al. (Ed.), Gray’s Anatomy for Students (4th ed. pp. 300–306.) Churchill Livingstone/Elsevier,
  2. Isaza-Restrepo A., et al. (2018). The peritoneum: beyond the tissue—a review. Front Physiol 9:738. Retrieved August 8, 2021, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6014125/
  3. Kalra, A., Wehrle, C. (2021). Anatomy, abdomen and pelvis, peritoneum. StatPearls. Retrieved August 25, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/32341/

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