Organ relations to the peritoneum
Depending on the degree of coverage by the visceral layer, abdominal organs are divided into three groups:
- Intraperitoneal — covered on all sides and have a mesentery. These include: the stomach, jejunum, ileum, transverse and sigmoid colon, spleen, and the upper third of the rectum.
- Mesoperitoneal — covered on three sides. These are the liver, ascending and descending colon, and the middle third of the rectum. This also includes the initial segment of the superior part of the duodenum (for a length of 2.5–5.0 cm).
- Extraperitoneal — covered on only one side or entirely devoid of peritoneal coverage. This group includes the pancreas, kidneys, adrenal glands, urinary bladder, and the lower third of the rectum. The descending and inferior parts of the duodenum lie retroperitoneally and are covered by adventitia.
Peritoneal derivatives
Peritoneal derivatives — anatomical structures formed by duplicatures (folds) of the peritoneal layers. These include ligaments, mesenteries, omenta, and folds.
Their main function is the fixation of internal organs and the passage of neurovascular bundles. In addition, transitioning from the abdominal walls to the organs, the peritoneum forms a complex relief: numerous recesses, sinuses, and pouches (recessus).
Supramesocolic compartment
For convenience in studying topographic anatomy, the cavity is divided into two compartments. The boundary is formed by the transverse colon (colon transversum) and its mesentery (mesocolon transversum), located at the level of the second lumbar vertebra.
The upper compartment is bounded superiorly by the diaphragm and inferiorly by the transverse mesocolon. It contains the liver (hepar), stomach (ventriculus/gaster), spleen, pancreas (pancreas), and the superior part of the duodenum.
Key spaces of the upper compartment:
- Subphrenic recesses (recessus subphrenici) — clefts between the diaphragm and the superior surface of the liver.
- Subhepatic recess (recessus subhepaticus) — between the inferior surface of the liver and the transverse colon.
- Omental bursa (bursa omentalis) — a closed space posterior to the stomach and lesser omentum.
Lower floor of the abdominal cavity
The lower compartment extends from the transverse mesocolon to the pelvic cavity. It contains the small intestine (starting from the inferior part of the duodenum) and the large intestine. The relief of the lower compartment includes two paracolic gutters, five recesses, and two mesenteric sinuses.
- The right mesenteric sinus (sinus mesentericus dexter) is bounded superiorly by the transverse mesocolon, on the right by the ascending colon (colon ascendens), and on the left and inferiorly by the mesentery of the small intestine (mesenterium) and the terminal ileum. Anteriorly, it is often covered by the greater omentum. An important feature: this sinus is isolated from the lesser pelvis.
- The left mesenteric sinus (sinus mesentericus sinister) in its upper part is also often covered by the greater omentum, but, unlike the right one, it communicates with the pelvic cavity.
Communications between compartments
The compartments of the abdominal cavity are not hermetically isolated from each other. The main pathways of communication are the paracolic gutters running laterally to the ascending and descending colon.
- The right paracolic gutter connects the lower compartment with the right subphrenic space and subhepatic recess. This is the most significant pathway through which pathological contents (pus, blood, gastric juice) can spread between compartments.
- The left paracolic gutter is partially blocked by the phrenicocolic ligament (lig. phrenicocolicum), which prevents the free flow of fluid into the left subphrenic space.
The transverse mesocolon itself is the main barrier, but infection can penetrate through it along the course of blood vessels or at attachment sites.
To easily remember the peritoneal relations of the rectum from top to bottom, use the "I-M-E" rule: Intraperitoneal (upper third), Mesoperitoneal (middle third), Extraperitoneal (lower third).
Class and exam questions
How does the omental bursa communicate with the rest of the peritoneal cavity?
It communicates exclusively via the omental foramen (foramen epiploicum). If adhesions form due to inflammation, the foramen closes, and the pathological process (e.g., an abscess) becomes completely isolated within the bursa.
Which of the mesenteric sinuses is more dangerous in peritonitis and why?
Left mesenteric sinus (sinus mesentericus sinister). Unlike the right one, it anatomically communicates with the pelvic cavity, which facilitates the rapid spread of pathological effusions there.
How do surgeons gain access to the omental bursa?
Access can be achieved through the transverse mesocolon (mesocolon transversum). The incision is made in an avascular zone below Riolan's arch, which is used, for example, in posterior gastrojejunostomy by Hacker–Petersen.
What else to study on this topic
- Boundaries and walls of the omental bursa (bursa omentalis)
- Topography and ligamentous apparatus of the stomach
- Ligamentous apparatus of the liver and hepatic bursae
- Anatomy of the greater and lesser omenta
- Recesses of the lower abdominal compartment
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