Peritoneum: Your Core from the Inside

Our organs slip and slide over each other all the time. As we breathe, digest, and move, the liver and large intestine slide across each other, the stomach and spleen slip around each other, and the bladder and intestines glide over each other to accommodate movements in our everyday lives. A thin membrane serves as the lubricating, protective barrier that makes it possible. Introducing the peritoneum.

WHAT DOES IT LOOK LIKE & WHERE DOES IT LIVE?

The word peritoneum derives from the Greek for “stretched over,” evoking an irresistible comparison to plastic cling wrap. The peritoneum is a smooth, translucent membrane less than 1mm thick (IMAGE 1). It covers the inside of the abdominal and pelvic cavities, stretching over and clinging to each of the abdominal and pelvic organs. By encasing the organs, it allows them to slip and slide over one another with ease. 

A View from the Inside

We cannot directly palpate the peritoneum. It’s deep in our core, forming the backside of the abdominal wall and the deepest part of our backs. To visualize this better, let’s imagine we are inside the abdominal and pelvic space. We would be surrounded by organs that are less familiar to our touch, but looking past the organs and through the translucent peritoneum for a moment, we would see the deepest surfaces of some muscles we know quite well. 

Imagine you are standing in an upright form, looking up. Hovering overhead, you can see the dome of the diaphragm arching over you like a rotunda. Facing front, you see the posterior rectus sheath encircling you with the transversalis fascia and transversus abdominis behind it. Turning around, as you continue to peer through the peritoneal veil, you see the long psoas major and minor, running along both sides of the spine, with quadratus lumborum hugging them at their sides. Swooping around the ilia you see the soft cushion of iliacus lining the hips. Looking down into the deep pelvis you see the muscles of the pelvic floor, including levator ani, coccygeus, obturator internus, and even part of piriformis. Now that you’ve gotten a good look around, bring your focus back to the thin membrane that covers everything, including all the organs that have been crowding around you. 

Full Belly, Empty Sac

Inside the abdominal cavity, the peritoneum covers the walls and every last little nook and cranny between and around all the organs. The peritoneum has two names depending on the location:

• Parietal peritoneum: covers the abdominal and pelvic walls (the ones you were looking at during our imaging exercise above.)
• Visceral peritoneum: covers the abdominal organs, including the liver, gallbladder, spleen, pancreas, stomach, small intestines, colon, omenta, uterus, ovaries, and bladder.

Even though it has two names, it is the same continuous membrane; there is just one peritoneum per person. You will sometimes also see it referenced as the peritoneal sac or peritoneal cavity. 

The abdominal cavity is full of organs, but technically, the peritoneal sac itself is empty. Imagine pressing your finger into the side of a large, not fully inflated balloon, until it sinks a couple inches deep. Technically, your finger is both inside and outside the balloon at the same time. It’s the same with your abdominal and pelvic organs.

In this analogy, the balloon is the peritoneum, and your finger is an organ. As the gut develops embryologically, the organs grow by pushing into the peritoneal cavity, taking the peritoneum with them. They remain completely covered in peritoneum, just like your finger as it presses into the balloon. If you take a second finger and press it into the balloon from the opposite side, the two fingers could touch each other, mediated by the balloon’s inner surface. This is how organ-to-organ interfaces are created in the abdomen. Gliding is ensured because the slippery peritoneal surface is always sliding over another slippery peritoneal surface. 

WHAT IS IT MADE OF?

The peritoneum is a serosal membrane, cousin to the more familiar mucosal membranes that line the inside of the mouth and nose. While mucosal membranes interface more directly with the external environment, serosal membranes line internal cavities, and include the peritoneum as well as the pleura of the lungs and the heart’s pericardium. Serosal membranes secrete a lubricating fluid that makes the already smooth membrane surface even more slippery. The peritoneal cavity is often described as a fluid-filled space, however, there is only about 50mL (just over three tablespoons) of fluid spread out thin over all the peritoneal surfaces combined. So maybe “fluid-covered” space would be more accurate than “fluid-filled.”

WHAT DOES IT DO?

Immune function

Up until the past few decades, the peritoneum was understood to be just a physical barrier blocking pathogens. New understandings have revealed it as an active immune player, filtering peritoneal fluid in a dynamic relationship with the blood and lymph. The membrane selectively allows or prevents fluids, cells, and pathogens from crossing its borders, where immune cells and lymph channels lurk, ready to respond immediately to any invaders. Peritoneal secretions are full of various immune factors, keeping a constant watch out for pathogens and invaders.

Adhesions

The peritoneum also plays a role in adhesion prevention and formation. Peritoneal adhesions are a type of fibrotic scarring that causes otherwise sliding surfaces to adhere together: organ-to-organ or organ-to-abdominal wall (IMAGE 2). They can be asymptomatic, causing no issues at all, or they can result in pain, obstructed bowels, and even infertility. Adhesions develop from infection and other inflammatory processes, but surgeries are by far the main culprit.  

While not every abdominal surgery results in adhesions, the percentage is quite high. One study found adhesions in 93% of patients who had at least one abdominal surgery. Comparatively, patients with no prior surgery showed an incidence of adhesions in only 10.4%.[i] Can bodywork impact adhesions?[ii] More research is needed to understand how, when, and what kinds of work are most beneficial

WHY WE CARE

Organ-sliding and shapeshifting of our abdominal cavity are essential to the health of each one of us. It is likely that some, if not many, of our clients have had one or more abdominal surgeries, including the very common ones like gallbladder removal, cesarean birth, and hysterectomy, which impact peritoneal surfaces. Learning more about the organization of your core beyond the muscles is a wonderful addition to your anatomy understanding and can promote new ways of thinking about your hands-on practice.

Resources:

Bove, G. & Chapelle, S. L. (2011). Visceral mobilization can lyse and prevent peritoneal adhesions in a rat model. Journal of Bodywork and Movement Therapies, 16(1), 76–82. https://doi.org/10.1016/j.jbmt.2011.02.004

Isaza-Restrepo, A. et al. 2018. “The Peritoneum: Beyond the Tissue – a Review.” Frontiers in Physiology 9 (June). https://doi.org/10.3389/fphys.2018.00738.

Kastelein, A. et al. 2019. “Embryology, Anatomy, Physiology and Pathophysiology of the Peritoneum and the Peritoneal Vasculature.” Seminars in Cell & Developmental Biology 92 (August): 27–36. https://doi.org/10.1016/j.semcdb.2018.09.007.

Menzies, D, and H Ellis. 1990. “Intestinal Obstruction from Adhesions--How Big Is the Problem?” Annals of the Royal College of Surgeons of England 72 (1): 60–63.

[ii] Researchers have demonstrated that visceral manipulation therapy might minimize the development of postoperative peritoneal adhesions in rats. These findings are encouraging, but more research is needed before we can apply this work to humans (Bove 2011).

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