The Peritoneum: The Membrane That Tells You Where It Hurts

One cavity, two linings

The best model is a fist pushed into a soft balloon: the fist never enters the balloon's air. The peritoneum is a serous membrane — a sheet of flat mesothelium resting on a thin layer of connective tissue — and it is arranged as one continuous closed bag. The layer that lines the inner surface of the abdominal and pelvic walls, the undersurface of the diaphragm and the front of the posterior wall is the PARIETAL peritoneum. The layer that is reflected onto the organs and clings to them like cling film is the VISCERAL peritoneum. Between the two lies the PERITONEAL CAVITY, and the single most misunderstood fact about it is that it is a POTENTIAL space: it contains no organs at all, only a film of straw-coloured serous fluid measured in millilitres, rich in water, electrolytes, protein and macrophages, which lets the two wet surfaces glide over one another as the gut writhes and the diaphragm rises and falls. Organs described as intraperitoneal are not INSIDE the cavity; they are invaginated into it from outside, exactly as the fist is invaginated into the balloon without ever entering the air within. In the male the bag is completely closed. In the female it is not: the free ends of the uterine tubes open directly into it, so the peritoneal cavity communicates with the exterior through tube, uterus and vagina — an anatomical doorway that lets a lower genital infection climb to the pelvic peritoneum, and lets a shed ovum find its way into the tube at all. It is why pelvic inflammatory disease is a peritoneal disease, and it is a route that simply does not exist in men.

Two layers, two nervous systems

This single section is the spine of the whole subject; everything clinical hangs from it. The two layers differ because they were built by different tissue. The parietal peritoneum develops from the SOMATOPLEURE — the somatic layer of lateral plate mesoderm that also builds the body wall — and it therefore takes its nerve supply from the SAME SOMATIC NERVES as the wall lying over it: the lower intercostal nerves (roughly T7–T11), the subcostal nerve (T12), and the iliohypogastric and ilioinguinal nerves (L1) described in the anterior abdominal wall; the peritoneum over the central diaphragm is supplied by the PHRENIC nerve (C3–C5), and much of the pelvic parietal peritoneum by the obturator nerve (L2–L4). Somatic nerves carry fast, precisely mapped information, so parietal pain is SHARP, SEVERE and EXACTLY LOCALISED — the patient points with one finger. Because those same spinal segments also supply the overlying muscles, irritating the parietal layer triggers a reflex arc that contracts them: first voluntary guarding, then involuntary rigidity, and finally the board-like abdomen that no amount of reassurance can relax. It is also why the pain is worse with movement, with coughing, with the jolt of a speed bump — every one of those slides an inflamed parietal surface across the organ beneath it.

The visceral peritoneum develops from the SPLANCHNOPLEURE — the splanchnic layer of the same mesoderm, the layer that builds the gut wall — and its afferents are AUTONOMIC, travelling back with the sympathetic fibres along the arteries to reach the spinal cord over many segments. That wiring is slow, sparse and poorly mapped, so visceral pain is DULL, ACHING, POORLY LOCALISED and referred to the MIDLINE, because the input arrives bilaterally and the brain cannot decide which side sent it. Where in the midline depends entirely on embryological origin, a rule set out in foregut, midgut and hindgut: foregut structures refer to the EPIGASTRIUM, midgut structures to the PERIUMBILICAL region, hindgut structures to the SUPRAPUBIC region. Just as important is what the visceral layer cannot feel. Cut it, crush it, burn it with diathermy and the conscious patient feels nothing — which is why bowel can be handled and resected under regional anaesthesia. What it feels intensely is STRETCH and ISCHAEMIA: distension of a hollow organ, spasm of smooth muscle against an obstruction, traction on a mesentery, or a gut segment losing its blood supply. Colic — pain that grips and releases in waves — is the sound of a tube trying to push past something, and it is always visceral.

The greater sac, the lesser sac and the door between them

The bag has a hidden back room, and it has exactly one entrance. The main part of the peritoneal cavity is the GREATER SAC — everything you meet on opening the abdomen. Behind the stomach lies a flattened diverticulum of it, the LESSER SAC or omental bursa, created during development when the stomach rotated and dragged its dorsal mesentery leftwards. Its anterior wall is the lesser omentum, the posterior surface of the stomach and the gastrocolic ligament; its posterior wall is the pancreas, the left kidney and left adrenal gland and the transverse mesocolon; above lies the caudate lobe of the liver and the diaphragm, and on the left the gastrosplenic and splenorenal ligaments — a topography developed in mesenteries, omenta and peritoneal ligaments. Its one communication with the greater sac is the EPIPLOIC FORAMEN, the foramen of Winslow, a vertical slit large enough to admit one or two fingers, and its four boundaries are worth knowing by heart because a surgeon's fingers find them by feel: ANTERIORLY the free edge of the lesser omentum, containing the PORTAL TRIAD — the bile duct in front and to the right, the hepatic artery in front and to the left, and the portal vein behind them both; POSTERIORLY the inferior vena cava; SUPERIORLY the caudate lobe of the liver; and INFERIORLY the first part of the duodenum. Because the sac is closed except at this one point, a leak into it is walled off from the rest of the abdomen: a pancreas that leaks enzymes after injury or severe pancreatitis fills the lesser sac and forms a pseudocyst that bulges the stomach forwards, while the rest of the cavity stays clean.

Inside the bag, or behind it

An organ is INTRAPERITONEAL when it is almost completely wrapped in visceral peritoneum and slung from the wall on a double fold — a mesentery — that carries its vessels, nerves and lymphatics to it. Such organs are mobile, can twist (volvulus), and can be delivered out of the wound by the surgeon: the stomach, the first part (cap) of the duodenum, the jejunum and ileum, the caecum and appendix, the transverse and sigmoid colon, the upper rectum, the liver, the spleen, the tail of the pancreas, and in the female the ovaries, uterine tubes and uterus. An organ is RETROPERITONEAL when it lies behind the peritoneum, plastered against the posterior abdominal wall with peritoneum on its front surface only; it is fixed, it has no mesentery, and it can be reached surgically from behind or from the side without ever opening the peritoneal cavity. The traditional list is remembered as SAD PUCKER: Suprarenal (adrenal) glands, Aorta and inferior vena cava, Duodenum (second, third and fourth parts), Pancreas (except the tail), Ureters, Colon (ascending and descending), Kidneys, oEsophagus (its abdominal part), and Rectum (its lower two thirds). Two consequences follow immediately. A retroperitoneal organ that ruptures bleeds into loose tissue rather than into the cavity, so the belly may stay soft and unimpressive while the patient exsanguinates; and a retroperitoneal structure irritates parietal peritoneum only late and from behind, which is why the pain of the kidney and ureter is felt in the loin rather than the front.

Not everything behind the peritoneum was always behind it. The list divides into two groups with different histories. PRIMARILY retroperitoneal organs never had a mesentery and developed behind the peritoneum from the start: the kidneys, ureters and adrenal glands, the aorta and inferior vena cava, the abdominal oesophagus and the lower rectum. SECONDARILY retroperitoneal organs began life intraperitoneal, slung on a mesentery, and were then pressed against the posterior wall as the gut rotated and grew, so that their mesentery fused with the parietal peritoneum and disappeared: the second to fourth parts of the duodenum, the head, neck and body of the pancreas, and the ascending and descending colon. That fusion leaves a bloodless plane of areolar tissue — the fascia of Toldt — and it is one of the most useful facts in abdominal surgery, because a surgeon who finds that old fusion line can lift the colon or the duodenum forwards off the posterior wall with almost no bleeding and no risk to the ureter or the great vessels behind. Mobilising the duodenum in this way is the Kocher manoeuvre; developing the same plane along the colon is the first step of every colectomy. Embryology, in other words, left a set of dotted lines saying cut here.

Where fluid goes when it is free

Blood, pus and bile are not creative; they simply run downhill and stop where the anatomy tells them to. Above the transverse colon the cavity is divided into SUPRACOLIC compartments, below it into INFRACOLIC compartments, and the two communicate along the sides through the PARACOLIC GUTTERS — troughs lying lateral to the ascending and descending colon. The subphrenic (subdiaphragmatic) spaces lie between the diaphragm and the liver, right and left, separated in front by the falciform ligament. Below the liver on the right is the HEPATORENAL RECESS, MORISON'S POUCH, bounded by the visceral surface of the liver in front and the right kidney and adrenal behind — and this is the LOWEST part of the peritoneal cavity when a person lies SUPINE. Anything free therefore drains into it, which is why a subphrenic or subhepatic abscess is a classic complication of a perforated appendix or a leaking anastomosis, and why the very first window of the FAST scan in a shocked trauma patient is placed over the right upper quadrant looking for a black stripe in Morison's pouch. The gutters do the transporting: the RIGHT paracolic gutter runs uninterrupted from the right iliac fossa up to the hepatorenal recess and the subphrenic space, so pus from an inflamed appendix can travel up to sit under the diaphragm, and conversely the contents of a perforated duodenal ulcer can run DOWN the same gutter and collect in the right iliac fossa — producing right-sided tenderness in a patient whose disease is in the epigastrium. The LEFT gutter is dammed superiorly by the phrenicocolic ligament, which is why left-sided collections spread less readily upwards. When the patient is UPRIGHT the lowest point is not Morison's pouch but the pelvis: the RECTOUTERINE pouch of Douglas in the female, the RECTOVESICAL pouch in the male. Fluid pooling there is felt as a bulge on rectal or vaginal examination, drained through the posterior vaginal fornix, and produces the pelvic symptoms — diarrhoea, urinary frequency, tenesmus — of an abscess sitting on rectum and bladder.

Four jobs for a sheet of cells

First, LUBRICATION: the mesothelium secretes a slippery serous fluid so that metres of gut can slide over one another and over the wall thousands of times a day without friction; when it is damaged by infection, blood, bile or surgical handling, fibrin is laid down instead and the surfaces stick — adhesions, the commonest cause of small bowel obstruction in the developed world, and the reason a second operation is always harder than the first. Second, IMMUNE DEFENCE: the peritoneal fluid is patrolled by macrophages, and the greater omentum carries dense aggregates of leucocytes called MILKY SPOTS that pour cells into the cavity at the first sign of infection. Third, FAT STORAGE: the omenta and mesenteries hold large depots of adipose tissue, insulating and cushioning, and it is this visceral fat — metabolically active and drained by the portal vein straight into the liver — that carries the cardiometabolic risk of central obesity. Fourth, and most remarkable, WALLING OFF: the greater omentum is mobile, and it migrates towards inflammation, wrapping itself around a hot appendix, a perforation or an anastomosis and sealing it off from the rest of the cavity. Rutherford Morison called it the abdominal policeman, and the name has stuck for a century because it is exactly right: a great many perforations never become generalised peritonitis because a sheet of fat got there first. Children, whose omentum is short and thin, are worse at this than adults — which is one reason appendicitis in a small child is more likely to end in free perforation.

The peritoneum in the clinic

PERITONITIS is inflammation of the membrane itself, and its signs are the parietal layer shouting: constant pain made worse by the least movement, a patient who lies absolutely still with the knees drawn up, guarding, rebound tenderness, percussion tenderness, and in a perforated peptic ulcer the classic BOARD-LIKE RIGIDITY, with gas under the diaphragm on an erect chest film and free acid tracking down the right paracolic gutter. It is one of the presentations gathered in the acute abdomen. ASCITES is the opposite problem — the potential space filling slowly and painlessly with litres of fluid, most often from portal hypertension in cirrhosis, and drained by PARACENTESIS through a needle placed in the flank, lateral to the rectus sheath so as to avoid the inferior epigastric artery. That the membrane behaves like a large semipermeable surface is exploited deliberately in PERITONEAL DIALYSIS, where dialysate run into the cavity through a catheter allows waste solutes to diffuse across roughly two square metres of capillary-rich mesothelium — dialysis without a machine, whose feared complication is bacterial peritonitis introduced along the catheter. LAPAROSCOPY inflates the same space with carbon dioxide to create a working cavity, and the gas that remains afterwards irritates the diaphragmatic peritoneum, giving the SHOULDER-TIP pain carried by the phrenic nerve to C4 that is explained in the diaphragm. Finally, a wet continuous surface is an ideal highway for cancer: TRANSCOELOMIC SPREAD lets ovarian and gastric carcinoma seed the whole cavity, producing omental cake, malignant ascites, bilateral ovarian Krukenberg tumours from a stomach primary, and a nodule at the umbilicus known as the Sister Mary Joseph nodule.