Peritoneum and peritoneal cavity

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CHAPTER 64 Peritoneum and peritoneal cavity

The peritoneum is the largest serous membrane in the body, and its arrangements are complex. In males it forms a closed sac, but in females it is open at the lateral ends of the uterine tubes. It consists of a single layer of flat mesothelial cells lying on a layer of loose connective tissue. The mesothelium usually forms a continuous surface, but in some areas may be fenestrated. Neighbouring cells are joined by junctional complexes, but probably permit the passage of macrophages. The submesothelial connective tissue may also contain macrophages, lymphocytes and adipocytes (in some regions). Mesothelial cells may transform into fibroblasts, which may play an important role in the formation of peritoneal adhesions after surgery or inflammation of the peritoneum.

The peritoneal cavity is a potential space between the parietal peritoneum, which lines the abdominal wall, and infoldings of visceral peritoneum, which suspend the abdominal viscera within the cavity. It contains a small amount of serous fluid, but is otherwise empty. The fluid lubricates the visceral peritoneum and allows the mobile viscera to glide freely on the abdominal wall and each other within the limits dictated by their attachments. It contains water, proteins, electrolytes and solutes derived from interstitial fluid in the adjacent tissues and from the plasma in the local blood vessels. It normally contains a few cells, including desquamated mesothelium, nomadic peritoneal macrophages, mast cells, fibroblasts, lymphocytes and other leukocytes. Some cells, particularly macrophages, migrate freely between the peritoneal cavity and the surrounding connective tissue. Lymphocytes provide both cellular and humoral immunological defence mechanisms within the peritoneal cavity. The intraperitoneal fluid is directed by gravity to dependent sites within the peritoneal cavity, and also flows in a cephalad direction as a consequence of the negative upper intra-abdominal pressures which are generated by respiration.

The peritoneal cavity never contains gas in normal circumstances, although the amount of fluid may be increased in inflammatory conditions of the viscera. In females blood or fluid may occasionally escape from the uterine tubes into the pelvic peritoneal cavity during menstruation.

Extraperitoneal connective tissue separates the parietal peritoneum from the muscular layers of the abdominal walls. The parietal peritoneum covering the anterior abdominal wall and pelvic walls is generally attached only loosely by this tissue, an arrangement which allows for considerable alteration in the size of the bladder and rectum. The extraperitoneal tissue on the inferior surface of the diaphragm and behind the linea alba is denser and more firmly adherent. The extraperitoneal tissue frequently contains large amounts of fat over the posterior abdominal wall, especially in obese males. The visceral peritoneum is firmly adherent to the underlying tissues and cannot be easily detached. Its connective tissue layer is often continuous with the fibrous matrix of the wall of the underlying viscera and rarely contains much loose connective or adipose tissue. The visceral peritoneum is often considered as part of the underlying viscus for clinical and pathological purposes such as the staging of carcinoma.

GENERAL ARRANGEMENT OF THE PERITONEUM

In utero, the alimentary tract develops as a single tube suspended in the coelomic cavity by ventral and dorsal mesenteries (see Ch. 73). Ultimately, the ventral mesentery is largely resorbed, although some parts persist in the upper abdomen and form structures such as the falciform ligament. The mesenteries of the intestines in the adult are the remnants of the dorsal mesentery. The migration and subsequent fixation of parts of the gastrointestinal tract produce the so-called ‘retroperitoneal’ segments of bowel (duodenum, ascending colon, descending colon, and rectum), and four separate intraperitoneal bowel loops suspended by mesenteries of variable lengths. These are all covered by visceral peritoneum which is continuous with the parietal peritoneum covering the posterior abdominal wall. The first intraperitoneal loop is formed by the intraperitoneal oesophagus, the stomach and first part of the duodenum. The second loop is made up of the duodenojejunal junction, jejunum, ileum and occasionally the caecum and proximal ascending colon. The third loop contains the transverse colon and the final loop contains the sigmoid colon and occasionally the distal descending colon.

Where the visceral peritoneum encloses or suspends organs within the peritoneal cavity, the peritoneum and related connective tissues are known as the peritoneal ligaments, omenta or mesenteries. All but the greater omentum are composed of two layers of visceral peritoneum separated by variable amounts of connective tissue. The greater omentum is folded back on itself and is therefore made up of four layers of closely applied visceral peritoneum, which are separated by variable amounts of adipose tissue. The mesenteries attach their respective viscera to the posterior abdominal wall: the attachment is referred to as the mesenteric root, and the peritoneum of the mesentery is continuous with that of the posterior abdominal wall in this area (Fig. 64.1).

Fig. 64.1 The posterior abdominal wall, showing the lines of peritoneal reflection, after removal of the liver, spleen, stomach, jejunum, ileum, caecum, transverse colon and sigmoid colon. The various sessile (retroperitoneal) organs are seen shining through the posterior parietal peritoneum. Note: the ascending and descending colon, duodenum, kidneys, suprarenals, pancreas and inferior vena cava. Line W–W represents the plane of Fig. 64.2. Line X–X represents the plane of Fig. 64.3. Line Y–Y represents the plane of Fig. 64.4. Line A–A represents the plane of Fig. 64.5A. Line B–B represents the plane of Fig. 64.5B. Line C–C represents the plane of Fig. 64.5C.

Although they are described as intraperitoneal, strictly speaking the suspended organs do not lie within the peritoneal cavity because they are covered by visceral peritoneum. They are continuous with the extraperitoneal tissues, including the retroperitoneum, via subperitoneal tissue lying between the folds of visceral peritoneum. The loose areolar connective tissues of the extraperitoneal and subperitoneal tissues are sometimes conceptualized as ‘spaces’ because fluid or blood collects relatively easily within them. The subperitoneal tissues contain the neurovascular bundles and lymphatic channels which supply the suspended organs. In obese individuals, extensive adipose tissue within the mesenteries and omenta may obscure the neurovascular bundles. In contrast, in the very young, the elderly or the malnourished, the mesentery may contain very little adipose tissue and the neurovascular bundles are usually obvious.

PERITONEUM OF THE UPPER ABDOMEN

The abdominal oesophagus, stomach, liver and spleen all lie within a double fold of visceral peritoneum which runs from the posterior to the anterior abdominal wall. This fold has no recognized name, but has been referred to as the mesogastrium by Coakley & Hricak 1999 because it is derived from the fetal mesogastrium. It has a complex attachment to the wall of the abdominal cavity and gives rise to the falciform ligament, coronary ligaments, lesser omentum (gastrohepatic and hepatoduodenal ligaments), greater omentum (including gastrocolic ligament), gastrosplenic ligament, splenorenal ligament, and phrenicocolic ligament (Figs 64.2, 64.3, 64.4).

Fig. 64.2 Sagittal section through the abdomen to the right of the epiploic foramen along one line of W–W in Fig. 64.1.

Fig. 64.3 Section through the upper part of the abdominal cavity, along the line X–X in Fig. 64.1. The boundaries of the epiploic foramen are shown and a small recess of the lesser sac is displayed in front of the head of the pancreas. Note that the transverse colon and its mesocolon are adherent to the posterior two layers of the greater omentum.

Fig. 64.4 Sagittal section through the abdomen, approximately in the median plane. Compare with Fig. 64.1. The section cuts the posterior abdominal wall along the line Y–Y in Fig. 64.1. The peritoneum is shown in blue except along its cut edges, which are left white.

The falciform ligament

The falciform ligament is a thin anteroposterior peritoneal fold which connects the liver to the posterior aspect of the anterior abdominal wall just to the right of the midline. It extends inferiorly to the level of the umbilicus, and is widest between the liver and umbilicus. The ligament narrows superiorly as the distance between the liver and anterior abdominal wall reduces and narrows to just a centimetre or so in height over the superior surface of the liver. Its two peritoneal layers divide to enclose the liver and are continuous with the visceral peritoneum that is adherent to the surface of the liver. Superiorly, they are reflected onto the inferior surface of the diaphragm and are continuous with the parietal peritoneum over the right dome. At the posterior limit, or apex, of the falciform ligament, the two layers are also reflected vertically left and right, and are continuous with the anterior layers of the left triangular ligament and the superior layer of the coronary ligament of the liver. The inferior aspect of the falciform ligament forms a free border where the two peritoneal layers become continuous with each other as they fold over to enclose the ligamentum teres. Because the peritoneum of the falciform ligament is continuous with that covering the posterior abdominal wall and the periumbilical anterior abdominal wall, blood arising from retroperitoneal haemorrhage (commonly acute haemorrhagic pancreatitis) may track between the folds of peritoneum and appear as haemorrhagic discolouration around the umbilicus (Cullen’s sign). Inflammatory change from the pancreas may spread via the gastrohepatic ligament (lesser omentum) and then via the falciform ligament to the umbilicus.

The peritoneal connections of the liver

The liver is almost completely covered in visceral peritoneum, and only the ‘bare area’ is in direct contact with the right dome of the diaphragm. Peritoneal folds, the ligaments of the liver, run from the liver to the surrounding viscera and to the abdominal wall and diaphragm (Fig. 64.5); they are described in detail in Chapter 68.

Fig. 64.5 Transverse sections through the abdomen. A, At the level of the line A–A in Fig. 64.1. The line passes through the bare area of the liver at the superior end of the lesser omentum. The parts of the left subphrenic space are clearly seen although they are continuous with each other. B, At the level of line B–B in Fig. 64.1, viewed from below. The peritoneal cavity is shown by a blue ‘wash’; the peritoneum and its cut edges in pale blue. C, Transverse section through the abdomen at the level of the line C–C in Fig. 64.1, viewed from below. Colours as in Fig. 64.1.

The coronary ligament is formed by the reflection of the peritoneum from the diaphragm onto the posterior surfaces of the right lobe of the liver. Between the two layers of this ligament, a large area of liver, the bare area, is devoid of peritoneal covering. At this point the liver is attached to the diaphragm by areolar tissue and is in continuity inferiorly with the uppermost part of the anterior pararenal space. The layers of the coronary ligament are continuous on the right with the right triangular ligament.

The upper layer of the coronary ligament is continuous superiorly with the peritoneum over the inferior surface of the diaphragm and inferiorly with the peritoneum over the right and superior surfaces of the liver. At the lower margin of the bare area the lower layer of the coronary ligament becomes continuous inferiorly with the peritoneum of the posterior abdominal wall over the right suprarenal gland and upper pole of the right kidney, and superiorly with the peritoneum over the inferior surface of the liver.

The left triangular ligament is a double layer of peritoneum which extends over the superior border of the left lobe of the liver to a variable length. Medially, its anterior leaf is continuous with the left layer of the falciform ligament. The posterior layer is continuous with the left layer of the lesser omentum. The left triangular ligament lies in front of the abdominal part of the oesophagus, the upper end of the lesser omentum and part of the fundus of the stomach. Intraoperative division of the left triangular ligament permits mobilization of the left lobe of the liver in order to expose the abdominal oesophagus and crura of the diaphragm.

The right triangular ligament is a short V-shaped fold formed by the approximation of the two layers of the coronary ligament at its right lateral end, and is continuous with the peritoneum of the right posterolateral abdominal wall. The coronary ligament is reflected inferiorly and is directly continuous with the peritoneum over the upper pole of the right kidney. This fold is sometimes referred to as the hepatorenal ligament. The recess formed between the peritoneum of the inferior surface of the liver, the hepatorenal ligament and the peritoneum over the right kidney is known as the hepatorenal pouch (of Morison). In the supine position this is the most dependent part of the peritoneal cavity in the upper abdomen, and is a common site of pathological fluid accumulation.

The peritoneum is reflected inferolaterally from the posterior layer of the left triangular ligament onto the posterior abdominal wall above the oesophageal opening of the diaphragm. It lines the inferior surface of the left dome of the diaphragm and continues backwards onto the posterior abdominal wall. Inferiorly, it is reflected behind the spleen onto the most lateral part of the mesentery of the transverse colon and the splenic flexure. It continues down lateral to the descending colon into the pelvis, and forms the left paracolic ‘gutter’. Medially, the peritoneum covering the left upper posterior abdominal wall is reflected anteriorly to form the left layer of the upper end of the lesser omentum, the peritoneum over the left aspect of the abdominal oesophagus and the left layer of the splenorenal ligament.

From the inferior layer of the coronary ligament the peritoneum descends over the anterior surface of the right kidney to the front of the first part of the duodenum and hepatic flexure of the colon. Medially it passes in front of a short segment of the inferior vena cava between the duodenum and liver, forming as it does so the posterior wall of the epiploic foramen. This narrow strip broadens out as the peritoneum continues across the midline onto the posterior wall of the lesser sac.

The peritoneum lines the posterior abdominal wall over the diaphragmatic crura, the upper abdominal aorta, the coeliac axis, nodes and plexus and the upper border of the pancreas. Inferiorly, below the liver, it continues down on the posterior abdominal wall to the right of the ascending colon, forming the right paracolic ‘gutter’ between the anterolateral abdominal wall and colon.