Peritoneum

The peritoneum is a thin membrane that lines the abdominal cavity, the space inside the abdomen. Thin and smooth like plastic wrap, it reduces friction when abdominal organs touch one another or the inner abdominal wall.

This membrane is broadly divided into two parts according to its location.

• Parietal peritoneum — The part that lines the inner wall of the abdomen (the inner surface of the abdominal wall).
• Visceral peritoneum — The part that covers the outer surfaces of organs such as the stomach, small intestine, large intestine, and liver.

These are not two separate sheets of membrane, but one continuous structure. The membrane lining the wall folds inward toward the organs and wraps around them, so the names simply differ according to location. The narrow space between the two parts is called the peritoneal cavity, and it contains a small amount of watery fluid secreted by the membrane.

The peritoneum does more than surround organs: it also helps hold them in their proper positions within the abdomen. The Mesentery, formed by folds of the membrane, suspends the intestines from the back wall of the abdomen, with blood vessels, nerves, and lymphatic vessels running through it. The greater omentum, which hangs downward like an apron, is also a structure formed by a fold of the peritoneum.

The relationship with the peritoneum differs from organ to organ. Some organs, such as the stomach and most of the small intestine, are surrounded by the peritoneum and move relatively freely within the abdomen, while others, such as the kidneys, lie behind it.

The Abdominal wall consists of the outer layers of the abdomen, including skin, fascia, and muscles such as the rectus abdominis, obliques, and transversus abdominis. The peritoneum is the membrane at the innermost surface of that wall. Although they are close to each other, their roles differ: the abdominal wall provides muscular layers that move the trunk and support the organs, while the peritoneum is a membrane that helps the organs move smoothly.

You can read about how pressure inside the abdomen changes in Intra-abdominal pressure. During breathing, the Diaphragm, abdominal wall muscles, and muscles around the pelvis work together to regulate pressure within the abdominal cavity.

You are rarely aware of the peritoneum in everyday life. However, the parietal peritoneum lining the inside of the abdomen can sense the location of pain relatively precisely, so when it is irritated, you can point to the painful spot with a finger. By contrast, sensation from the visceral peritoneum around the organs is more diffuse. Discomfort around the organs therefore often feels like “a heavy feeling somewhere in my abdomen.”

• Move your abdomen with deep breathing. Letting your abdomen gently expand as you breathe in and draw inward as you breathe out moves the diaphragm, abdominal wall, and organs together.
• Try a gentle walk after meals. It helps the intestines move.
• Try stroking your abdomen clockwise. Lightly stroking it with your palm helps your abdomen feel more comfortable. You can read about approaches that work with the abdomen, such as visceral manipulation, in Visceral Manipulation.

If you develop sudden, severe abdominal pain and your abdomen feels hard and rigid to the touch, or if you have fever along with vomiting, seek prompt medical evaluation. Conditions such as peritonitis, an inflammation of the peritoneum, can present this way.