Peritoneal Cavity Anatomy: Mapping the Peritoneal Spaces on CT

The peritoneum is a thin serous membrane that lines the abdominal wall (the parietal peritoneum) and covers the abdominal organs (the visceral peritoneum). A thin film of fluid (about 50 to 100 mL) separates these two layers and lubricates them. The potential space between them is the peritoneal cavity.

Peritoneal ligaments are double folds of peritoneum that support structures within the cavity; the omentum and mesentery are examples of these named peritoneal folds. Most abdominal ligaments arise from the ventral or dorsal mesentery. As these reflections attach to the posterior abdominal wall and to organs, they partition the peritoneal cavity into a set of interconnected peritoneal spaces. Understanding this map is fundamental to predicting how ascites, blood, pus, and metastatic disease collect and spread on cross-sectional imaging. Let us dive deeper into this.

In brief, the transverse mesocolon divides the cavity into a supramesocolic compartment (above) and an inframesocolic compartment (below). The inframesocolic compartment is further split by the small-bowel mesentery into right and left infracolic spaces, and flanked by the right and left paracolic gutters. Inferiorly, all of these drain into the dependent pelvic peritoneal spaces.

In this article

• Why peritoneal anatomy matters in radiology
• Summary table of the peritoneal spaces
• The primary divider – transverse mesocolon
• Inframesocolic space
• Supramesocolic space and the lesser sac
• Pelvic peritoneal spaces
• Natural flow of peritoneal fluid
• Clinical and radiological applications
• Frequently asked questions
• References

Why is this significant?

Since the space enclosed by the peritoneum is only a potential space, normal peritoneal folds, ligaments, and thus the spaces are not identifiable at imaging. They become conspicuous only when distended or involved by a pathological process. The peritoneal ligaments, mesenteries, and omenta may either serve as barriers for pathological processes or facilitate the spread of disease across them.

• Identifying etiology (infective, inflammatory, neoplastic, traumatic): slow fluid accumulation from a chronic systemic disease may be localized to one or few spaces, whereas a rapid build-up from trauma or acute pancreatitis may not respect natural boundaries and spills into multiple spaces.
• Localizing pathology to guide the clinician or surgeon (for example, guiding drain placement or deciding the surgical approach).
• Establishing the extent of peritoneal disease, ascites, intraperitoneal metastases, and collections.
• Establishing prognosis and anticipating the route of spread of a disease process.
• Comparison and follow-up across serial studies.

Summary of the peritoneal spaces

The Primary Divider – Transverse Mesocolon

The transverse mesocolon divides the peritoneum into the supramesocolic and inframesocolic spaces.

Inframesocolic space

Right and Left Inframesocolic Spaces

The ascending and descending colon separate the right and left inframesocolic compartments (RMC and LMC respectively) from the paracolic gutters. The obliquely oriented root of the small-bowel mesentery further subdivides the inframesocolic compartment into these right and left infracolic spaces.

The right inframesocolic space is smaller and is limited inferiorly by the attachment of the small-bowel mesentery to the cecum; for this reason, collections here generally do not extend into the pelvis. The left inframesocolic space is larger and has free communication with the pelvis, so left-sided collections readily descend into the pelvic peritoneal recesses.

Paracolic Spaces

The paracolic spaces (gutters) lie lateral to the peritoneal reflections of the left and right sides of the colon. The right paracolic gutter is larger than the left and communicates freely with the right subphrenic space, making it the principal channel by which fluid ascends from the pelvis to the upper abdomen. The connection between the left paracolic gutter and the left subphrenic space is partially limited by the phrenicocolic ligament. Both paracolic gutters communicate freely with the pelvic spaces.

Supramesocolic space

Right Supramesocolic Spaces

The right supramesocolic spaces include the right subphrenic space, Morison pouch (the subhepatic or hepatorenal space), and the lesser sac (omental bursa). The right subphrenic space is separated from the left perihepatic space by the falciform ligament, which varies in size and may not always serve as a barrier to the spread of disease. Morison pouch is the most dependent part of the upper abdomen in a supine patient, which is why free fluid and blood collect here early on trauma imaging.

Left Supramesocolic Spaces

The left supramesocolic space includes the perihepatic, left subphrenic, and perisplenic spaces. The phrenicocolic ligament is a partial restriction to the spread of pathologic processes from the left paracolic gutter to the left subphrenic space.

The Lesser Sac (Omental Bursa)

The lesser sac is the isolated peritoneal recess that lies behind the stomach and lesser omentum and in front of the pancreas. It communicates with the main (greater) peritoneal cavity only through the narrow epiploic foramen of Winslow. Because of this relative isolation, fluid confined to the lesser sac (for example, in acute pancreatitis or a perforated posterior gastric ulcer) can be localized here, and a large lesser-sac collection can be mistaken for a pancreatic pseudocyst or a cystic mass.

Pelvic Peritoneal Spaces

Inferiorly, the peritoneal cavity reflects over the pelvic organs to form the most dependent recesses of the whole cavity. In the midline, the peritoneum dips to form the rectouterine pouch (pouch of Douglas) in women and the rectovesical pouch in men. On either side of the bladder lie the paravesical spaces. Because these pouches are the lowest points of the peritoneal cavity in both the supine and erect positions, they are the first place free fluid, blood, pus, and gravity-dependent drop metastases tend to collect. Always scrutinise the pelvis for a small amount of dependent fluid when searching for early or subtle peritoneal disease.

Natural Flow of Peritoneal Fluid

Peritoneal fluid does not distribute randomly. Its movement is governed by gravity, by the negative pressure generated beneath the diaphragm during respiration, and by the boundaries formed by the mesenteries and ligaments. Fluid produced anywhere in the abdomen tends to pool first in the pelvis, then ascends preferentially along the larger right paracolic gutter to reach Morison pouch and the right subphrenic space. Ascent on the left is comparatively limited by the phrenicocolic ligament.

These dynamics explain the classic sites where intraperitoneal seeding and residual collections are most often found:

• The pouch of Douglas / rectovesical pouch (the most dependent recess).
• The lower recess of the small-bowel mesentery near the ileocecal junction.
• The superior aspect of the sigmoid mesocolon.
• The right paracolic gutter and Morison pouch.

Clinical and Radiological Applications

• Trauma (FAST and CT): Morison pouch and the pelvic recesses are the earliest sites for free intraperitoneal blood; a targeted look here increases sensitivity for haemoperitoneum.
• Peritoneal carcinomatosis: gravity-dependent and flow-dependent sites (pouch of Douglas, right paracolic gutter, greater omentum, and the ileocecal mesentery) are the highest-yield locations to search for soft-tissue implants and omental caking.
• Abscess and infection: the subphrenic spaces, Morison pouch, lesser sac, and pelvis are common locations for post-operative or post-perforation collections, and the anatomy guides percutaneous drain planning.
• Acute pancreatitis: the lesser sac and the anterior pararenal space are typical sites for peripancreatic fluid, and involvement of specific ligaments predicts the route of extension.

Frequently Asked Questions

What divides the peritoneal cavity into supramesocolic and inframesocolic compartments?

The transverse mesocolon is the primary divider. It separates the supramesocolic compartment (containing the subphrenic, subhepatic, and perisplenic spaces and the lesser sac) from the inframesocolic compartment (the right and left infracolic spaces and the paracolic gutters).

What is the most dependent part of the peritoneal cavity?

The pouch of Douglas (rectouterine pouch) in women and the rectovesical pouch in men are the most dependent recesses, so free fluid, pus, and drop metastases collect there first. In the upper abdomen, Morison pouch (the hepatorenal recess) is the most dependent space in a supine patient.

Why does peritoneal fluid tend to ascend on the right side?

The right paracolic gutter is larger and communicates freely with the right subphrenic space and Morison pouch, whereas the phrenicocolic ligament partly blocks upward flow along the smaller left paracolic gutter. Negative subphrenic pressure during respiration also draws fluid upward toward the right diaphragm.

What is the lesser sac and how does it communicate with the rest of the cavity?

The lesser sac (omental bursa) is a peritoneal recess behind the stomach and in front of the pancreas. It connects with the main peritoneal cavity only through the epiploic foramen of Winslow, which is why collections such as those from pancreatitis can become isolated within it.

References

1. Tirkes T, Sandrasegaran K, Patel AA, et al. Peritoneal and retroperitoneal anatomy and its relevance for cross-sectional imaging. RadioGraphics. 2012;32(2):437–451. doi:10.1148/rg.322115032
2. Healy JC, Reznek RH. The peritoneum, mesenteries and omenta: normal anatomy and pathological processes. European Radiology. 1998;8(6):886–900. doi:10.1007/s003300050485

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