The peritoneum is a thin, continuous membrane that lines the inside of the abdominal cavity and covers many of the organs within it. It helps protect abdominal organs, reduces friction as they move, supports blood vessels and nerves, and provides pathways for lymphatic drainage and immune defense. Although it is only a few layers of cells thick in many places, this membrane plays an important role in the normal function of the digestive system and other abdominal organs.
The peritoneum is more than a simple protective covering. Its folds help suspend organs, connect structures to the abdominal wall, and create spaces through which fluid can circulate. Understanding its anatomy also helps explain several medical conditions, including peritonitis, ascites, and certain abdominal hernias.
What is the peritoneum?
The peritoneum is a serous membrane, meaning it is a thin, smooth tissue that lines a body cavity and covers some of the organs inside it. Similar membranes surround the lungs and heart, where they help reduce friction during breathing and the heartbeat. In the abdomen, the peritoneum serves a comparable purpose while also helping organize the position of abdominal organs.
It consists of a layer of specialized cells called mesothelial cells, supported by underlying connective tissue. The mesothelial surface is normally smooth and moist, allowing adjacent surfaces to slide past one another with minimal resistance.
The peritoneum forms a continuous lining, but anatomically it is described in two main parts: the parietal peritoneum and the visceral peritoneum. These layers are connected and reflect from one surface to another, creating folds and supporting structures around many abdominal organs.
Not every organ in the abdomen is completely covered by the peritoneum. Some organs are suspended within the peritoneal cavity by folds of the membrane, while others lie behind it or are covered on only part of their surfaces. This distinction is important because an organ’s relationship to the peritoneum influences its anatomy, blood supply, mobility, and potential routes of disease spread.
The two layers of the peritoneum
The peritoneum has two principal layers, each with a distinct location and function.
Parietal peritoneum
The parietal peritoneum lines the inner surface of the abdominal and pelvic walls, including much of the underside of the diaphragm, the muscular sheet that separates the chest from the abdomen.
This layer is supplied by nerves that also serve the body wall. As a result, irritation of the parietal peritoneum often produces relatively sharp, well-localized pain. A person may be able to identify the area where the pain occurs because the sensory nerves convey information from a specific region.
For example, when inflammation spreads to the parietal peritoneum near an inflamed abdominal organ, movement, coughing, or pressure may intensify the pain. This sensitivity helps explain why clinicians consider tenderness and pain during movement when evaluating certain abdominal conditions.
Visceral peritoneum
The visceral peritoneum covers the outer surfaces of many abdominal organs, including much of the stomach, small intestine, and portions of the large intestine. It also covers the surfaces of several other organs in the abdomen.
Unlike the parietal layer, the visceral peritoneum receives much of its sensory supply through the autonomic nerves associated with the organs it covers. Pain arising from this layer or the organs beneath it is often less precisely localized. It may feel dull, cramping, or diffuse rather than sharply concentrated in one spot.
The visceral peritoneum also follows the contours of organs and contributes to the folds that connect them to nearby structures. These relationships allow many organs to move while maintaining connections to their blood vessels, lymphatic vessels, and nerves.
Although the two layers are continuous, their differences in nerve supply explain why irritation in different parts of the peritoneum can produce different patterns of pain.
What is the peritoneal cavity?
The peritoneal cavity is the potential space between the parietal and visceral peritoneum. It is called a potential space because the two surfaces normally lie close together rather than forming a large, open chamber.
A small amount of lubricating fluid is normally present between the layers. This fluid allows the surfaces to glide over one another as the intestines contract, the stomach changes shape, and the diaphragm moves during breathing.
The peritoneal cavity is not simply an empty space containing all the abdominal organs. Many organs are located within the abdomen but are not enclosed by the cavity. Instead, the cavity lies between the membrane’s two layers, which extend around and reflect across different structures.
In people with typical anatomy, the peritoneal cavity is closed in men. In women, it has an indirect connection to the exterior through the reproductive tract, including the openings of the fallopian tubes. This anatomical difference has implications for the possible routes by which infection can spread into the abdominal cavity.
The cavity also contains spaces and recesses created by the arrangement of the peritoneal folds. These spaces are relevant to the movement of fluid and the distribution of infection within the abdomen.
Peritoneal folds, mesentery, omentum, and ligaments
The peritoneum does not form a flat sheet across the abdomen. Instead, it folds around organs and connects different structures. These folds help hold organs in position while allowing an appropriate degree of movement.
Three important types of peritoneal structures are the mesentery, the omentum, and the peritoneal ligaments.
The mesentery
A mesentery is a double layer of peritoneum that connects an organ to the abdominal wall or another supporting structure. The mesentery of the small intestine, for example, attaches much of the small intestine to the back wall of the abdomen.
The mesentery provides a route for arteries, veins, lymphatic vessels, and nerves to reach the intestine. It also contains connective tissue and fat that support these structures.
Rather than rigidly fixing the intestine in place, the mesentery allows it to move while maintaining its essential connections. This flexibility is important because intestinal contents must travel through a long, coiled digestive tract that continually changes shape during digestion.
The omentum
The omentum consists of peritoneal folds associated with the stomach and nearby organs. The greater omentum hangs from the greater curvature of the stomach and extends over portions of the intestines like a flexible apron. The lesser omentum connects the liver to the lesser curvature of the stomach and the first part of the small intestine.
The greater omentum contains fat, blood vessels, and immune cells. It can move within the abdomen and sometimes collects near areas of inflammation, helping to limit the spread of infection or other inflammatory processes. For this reason, it is sometimes informally described as the abdomen’s protective apron.
The omentum also stores fat and contributes to local immune activity. Its functions extend beyond simply covering or cushioning organs.
Peritoneal ligaments
Peritoneal ligaments are folds of the membrane that connect organs to other organs or to the abdominal wall. Despite their name, they are not necessarily equivalent to the strong, fibrous ligaments that stabilize joints.
These folds help maintain anatomical relationships and may carry blood vessels or other structures. The falciform ligament, for example, connects the liver to the anterior abdominal wall and the underside of the diaphragm.
Together, mesenteries, omenta, and peritoneal ligaments create a supporting framework that helps organize the abdominal cavity without preventing the movement required for normal organ function.
Which organs are covered by the peritoneum?
Many abdominal organs are covered by the visceral peritoneum, but the extent of coverage varies.
The stomach, spleen, jejunum, ileum, and much of the liver are largely covered by peritoneum. Parts of the large intestine, including the transverse colon and sigmoid colon, are also covered and suspended by peritoneal folds.
Other organs lie behind the peritoneum. These are described as retroperitoneal, meaning they are located between the peritoneum and the back wall of the abdomen. The kidneys, adrenal glands, most of the pancreas, much of the duodenum, and portions of the ascending and descending colon are examples.
The urinary bladder and several pelvic organs have their own specific relationships to the peritoneum. The membrane covers portions of their surfaces rather than completely enclosing them.
The term intraperitoneal is commonly used for organs that are suspended within the peritoneal cavity by peritoneal folds. The term retroperitoneal describes structures located behind the peritoneum. These labels refer to anatomical relationships, not whether an organ is literally floating in an open cavity.
This distinction matters in medicine. Retroperitoneal organs can develop diseases or injuries without immediately affecting the peritoneal cavity. Conversely, conditions involving intraperitoneal organs may cause inflammation or fluid accumulation within that cavity.
The main functions of the peritoneum
The peritoneum performs several interconnected functions that support the normal operation of the abdominal organs.
Reducing friction between organs
One of its most immediate functions is lubrication. The mesothelial cells help maintain a smooth surface, and the small amount of peritoneal fluid allows organs and abdominal walls to move relative to one another.
This movement occurs continuously. The stomach expands and contracts, the intestines produce waves of muscular contraction called peristalsis, and the diaphragm moves during breathing. Without a lubricated surface, these movements would create greater friction and could damage tissues.
Supporting abdominal organs
Peritoneal folds help suspend organs and maintain their relationships with neighboring structures. Mesenteries, for example, keep blood vessels, nerves, and lymphatic vessels connected to the intestine while permitting movement.
This arrangement provides both stability and flexibility. The abdominal organs can shift as the body changes position or as the digestive tract fills and empties, yet their essential vascular and nervous connections remain intact.
Providing routes for blood vessels, nerves, and lymphatics
The peritoneum itself does not supply every organ it covers, but its folds contain pathways for structures that do. Arteries deliver oxygen and nutrients, veins carry blood away, and lymphatic vessels transport excess tissue fluid and immune cells.
These routes also help explain how infections and cancers can spread through the abdomen. Disease may extend along tissue planes or involve peritoneal surfaces, while tumor cells can sometimes detach from an organ and implant on the membrane.
Contributing to immune defense
The peritoneal cavity contains immune cells that help detect and respond to microorganisms and tissue injury. Mesothelial cells also participate in inflammatory signaling and interactions with immune cells.
The greater omentum can contribute to local defense by accumulating near inflamed areas. Together, these processes may help contain some infections and support tissue repair.
However, the peritoneum’s response to infection can also cause harm. Widespread inflammation may interfere with normal organ function and lead to serious illness.
Absorbing and transporting fluid
The peritoneal surface allows water and dissolved substances to move between the cavity and the circulation through a combination of transport and drainage processes. Lymphatic vessels, including specialized pathways associated with the diaphragm, help remove fluid and particles from the cavity.
This capacity has practical medical applications. In peritoneal dialysis, a treatment for kidney failure, dialysis fluid is placed in the peritoneal cavity. Waste products and excess electrolytes move from blood vessels in the peritoneal tissues into the fluid, while water moves according to osmotic gradients. The fluid is then drained, carrying the removed substances with it.
How the peritoneum develops and maintains its structure
The peritoneum develops from embryonic tissue that also gives rise to other serous membranes. During early development, the body’s internal cavities form and organs grow into specific positions within them. As the digestive tract develops and rotates, the peritoneum forms folds, attachments, and spaces around many of the organs.
Some of these attachments change as the organs grow. Certain portions of the digestive tract retain mesenteries, while other portions become closely attached to the posterior abdominal wall and are considered secondarily retroperitoneal. The resulting arrangement explains much of the adult anatomy.
In adulthood, the mesothelial cells that line the peritoneum help maintain a low-friction surface and regulate interactions between the cavity and surrounding tissues. The underlying connective tissue provides structural support, while blood vessels, lymphatic vessels, and immune cells contribute to the membrane’s broader functions.
When the peritoneum is injured or inflamed, it can participate in tissue repair. Yet healing may also produce adhesions, which are bands of scar tissue that connect surfaces that would normally slide freely past one another.
Common medical conditions involving the peritoneum
Because the peritoneum covers many organs and occupies a large internal surface area, it can be affected by infections, inflammatory disorders, fluid imbalances, injuries, and cancer.
Peritonitis
Peritonitis is inflammation of the peritoneum. It can develop when bacteria or other irritants enter the peritoneal cavity, including through a perforated organ, a ruptured appendix, or leakage from an injured section of the digestive tract.
The inflammation may cause abdominal pain, tenderness, swelling, nausea, and fever. When the parietal peritoneum is involved, pain may become sharp and worse with movement. Severe peritonitis can lead to widespread infection, circulatory problems, and organ failure.
Peritonitis requires prompt medical evaluation. Treatment depends on the cause and may involve antibiotics, drainage, or surgery to control the source of contamination.
In people receiving peritoneal dialysis, infection can occur if microorganisms enter the cavity through the dialysis catheter or during exchanges. Cloudy drainage fluid and abdominal discomfort are warning signs that require urgent assessment by the treatment team.
Ascites
Ascites is an abnormal accumulation of fluid in the peritoneal cavity. It is most commonly associated with advanced liver disease, particularly cirrhosis, but it can also result from certain cancers, heart failure, and other conditions.
The fluid accumulates when the balance among blood pressure, protein concentrations, fluid movement, and lymphatic drainage is disrupted. In cirrhosis, changes in pressure within the portal circulation and the body’s regulation of salt and water play important roles.
Ascites may cause abdominal enlargement, discomfort, early fullness during meals, and shortness of breath when the fluid accumulation is substantial. Treatment focuses on the underlying cause and may include dietary sodium restriction, medications that help the body remove excess fluid, or drainage of the fluid with a needle.
Peritoneal adhesions
Adhesions are bands of scar tissue that form between tissues or organs. They can develop after abdominal surgery, infection, or inflammation.
Some adhesions cause no symptoms. Others may restrict the movement of the intestines or create a point where the bowel becomes trapped or kinked, potentially leading to intestinal obstruction. Symptoms of obstruction can include cramping abdominal pain, vomiting, swelling, and difficulty passing stool or gas.
Adhesions can be difficult to prevent completely because they arise from the body’s healing response. Surgical treatment may be necessary when they cause significant complications, but surgery can also lead to new adhesions.
Peritoneal cancer involvement
Cancer can involve the peritoneum in more than one way. Some cancers arise from tissues associated with the peritoneum, while cancers that begin in other organs may spread to its surface. The spread of cancer cells across the peritoneal lining is often called peritoneal metastasis.
Tumor deposits may interfere with normal fluid movement, contribute to ascites, or affect the function of nearby organs. The symptoms depend on the extent and location of disease and may include abdominal swelling, discomfort, changes in appetite, or bowel symptoms.
Treatment varies according to the cancer’s origin, its distribution, and the person’s overall health. Options may include systemic drug treatments, surgery in selected cases, and specialized approaches for certain forms of peritoneal disease.
Why the peritoneum matters in abdominal health
The peritoneum is essential to the organization and movement of abdominal organs. Its two layers create a lubricated interface, while its folds support organs and carry the vessels and nerves that keep them functioning. Its immune activity and ability to transport fluid also make it important in both normal physiology and disease.
Its anatomy helps explain why abdominal inflammation can cause pain in characteristic patterns, why fluid can accumulate around organs, and why infections or cancers sometimes spread across the abdominal cavity. The same membrane that protects and supports the organs can become a source of serious complications when its surfaces are inflamed, scarred, or affected by disease.
Understanding the peritoneum therefore provides a foundation for understanding many conditions of the abdomen, as well as medical procedures such as peritoneal dialysis that rely directly on its unique structure and functions.