Boundaries of the abdominal cavity
External (superficial) and internal landmarks of this anatomical space do not coincide, which is important to consider in topographical anatomy.
External boundaries:
- Superior: xiphoid process and costal margin edges.
- Inferior: pubic symphysis, inguinal ligaments, and iliac crests.
- Lateral: conventional vertical lines dropped from the anterior ends of the 11th ribs to the iliac crests (they separate the abdomen from the lumbar region).
Internal boundaries:
- Superior: diaphragmatic dome. Due to its convex shape, the abdominal cavity extends deeply into the thoracic cage.
- Inferior: pelvic brim (linea terminalis), which serves as the watershed separating the abdomen from the pelvic cavity.
Peritoneum and peritoneal cavity
Peritoneum (peritoneum) is a thin serous membrane with a smooth and shiny surface. It is formed by the lamina propria (lamina propria) and lined with a simple squamous epithelium — mesothelium.
Two layers of the peritoneum are distinguished:
- Parietal layer (peritoneum parietale) — the parietal sheet lining the interior of the abdominal walls.
- Visceral layer (peritoneum viscerale) — the visceral sheet enveloping the organs.
Transitioning from the walls to the organs or from one organ to another, the serous membrane forms mesenteries (mesenteria), ligaments (ligamenta), and folds (plicae).
Between the parietal and visceral layers, a slit-like peritoneal cavity (cavitas peritonei) is formed. In a healthy body, it contains a minimal amount of serous fluid (liquor peritonei), which prevents organ friction. In pathological processes, pus, blood, or exudate can accumulate here, significantly increasing the volume of the space.
Organ relations to the peritoneum
Depending on the area covered by the serous membrane, all viscera are divided into three topographical groups.
| Group | Characteristic | Examples of organs |
|---|---|---|
| Intraperitoneal | Covered on all sides, have a mesentery | Stomach, small intestine, cecum, transverse colon, sigmoid colon, appendix (mesoappendix) |
| Mesoperitoneal | Covered on three sides | Liver, ascending and descending colon, filled urinary bladder |
| Extraperitoneal | Covered on one side or lie outside the peritoneum | Kidneys, adrenal glands, pancreas, major part of the duodenum |
Intra- and mesoperitoneal structures form the group of abdominal organs, while extraperitoneal (retroperitoneal) ones belong to the retroperitoneal organs.
Retroperitoneal space
Retroperitoneal space (spatium retroperitoneale) is located between the parietal peritoneum of the posterior abdominal wall anteriorly and the endoabdominal fascia (fascia endoabdominalis) posteriorly and laterally. Superiorly, it is bounded by the costal and lumbar parts of the diaphragm, and inferiorly it conventionally reaches the pelvic brim (linea terminalis).
This zone contains massive retroperitoneal adipose tissue, major vessels (aorta, inferior vena cava), nerves, as well as retroperitoneal organs: kidneys with ureters, adrenal glands, pancreas, and a fragment of the duodenum. Extraperitoneal spaces also include spatium extraperitoneale and spatium retropubicum.
Prefix rule: "Intra-" (inside) — the organ is fully enveloped by the peritoneum and has a mesentery. "Meso-" (between/middle) — covered on three sides. "Extra-" or "Retro-" (outside/behind) — lies behind the peritoneum, covered on only one side.
Class and exam questions
What is the difference between the abdominal cavity and the peritoneal cavity?
The abdominal cavity is the entire space bounded by the trunk walls. The peritoneal cavity is only a part of it, representing a slit-like space between the parietal and visceral layers of the serous membrane.
Why do internal and external abdominal boundaries not coincide?
Because of the diaphragmatic dome, which extends deeply into the thoracic cage. Therefore, the upper part of the abdominal cavity is actually protected by the lower ribs, above the outer boundary of the costal margins.
Does the peritoneal cavity communicate with the external environment?
In males, it is completely hermetic. In females, the cavity has a physiological communication with the external environment through the openings of the uterine tubes, uterine cavity, and vagina.
What else to study on this topic
- Division of the abdominal cavity into floors (upper, lower, pelvic)
- Complex course of the peritoneum: formation of omental bursae, recesses, and compartments
- Topography and fascial spaces of the retroperitoneal region
- Structure and functions of the endoabdominal fascia (fascia endoabdominalis)
- Anatomy of the seminal vesicles (vesiculae seminales) and their relation to the peritoneum
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