No. 8 1. Peritoneum

Chapter 6 The Peritoneum

Ⅰ. Introduction of Peritoneum The peritoneum is the largest and most complexly arranged serous membrane in the body. It consists of a single layer of flattened mesothelial cells which covers a layer of loose connective tissue. Its free surface is extremely smooth and slippery.

Parietal peritoneum and visceral peritoneum: The peritoneum is situated in the abdominal cavity and partly also in the pelvic cavity, a part of which lines the wall and is known as the parietal peritoneum, while the remainder is reflected over the contained viscera and is termed the visceral peritoneum.

Peritoneal cavity: The parietal and visceral layers of the peritoneum are in actual contact; the potential space between them is the peritoneal cavity which contains nothing but a little lubricating fluid. In the male, the peritoneal cavity is a closed sac, in the female, it communicates with the exterior indirectly through the uterine tubes, uterus, and vagina.

Functions: The peritoneum, especially the parietal peritoneum, has an absorptive capacity. After abdominal and pelvic operations, it has been customary to prop up the patient in bed so that any inflammatory intraperitoneal effusion will gravitate into the pelvis. One presumed reason for adopting this position was that the peritoneum in the subphrenic region has a greater absorptive capacity than the other regions; hence inflammatory products, if they gained access to this region, would more rapidly pass into the general circulation. The visceral peritoneum is firmly united to the viscera which it covers, and cannot be readily stripped off them.

Ⅱ. The Relationship Between Viscera and Peritoneum According to the variable extent of peritoneal investment, the abdominopelvic viscera may be categorized into following three groups:

Ⅰ) The Intraperitoneal Viscera Viscera completely surrounded by peritoneum that forms an integral part of serous layer belong to this group. Such organs are stomach, jejunum, ileum, cecum, vermiform appendix, transverse colon, sigmoid colon, spleen, ovaries, uterine tubes and superior part of the duodenum.

Ⅱ) The Interperitoneal Viscera Such organs are not completely wrapped by peritoneum with one surface attached to the abdominal walls or other organs. Liver, gallbladder, urinary bladder, uterus, upper part of the rectum, ascending and descending colon are the organs of this group.

Ⅲ) The Retroperitoneal Viscera Some of the abdominal and pelvic visera, such as the kidneys, ureters, suprarenal glands, pancreas, the middle part of rectum, the second and inferior parts of duodenum are retroperitoneal in position lying on the posterior abdominal wall and covered by peritoneum only on their anterior aspect.

Ⅲ. The Peritoneal Reflections Certain terms, often arbitrary, are commonly used for the peritoneal reflections. A peritoneal reflection that connects the intestine and body wall is usually named according to the part of the gut to which it is attached. For example, the reflection to jejunum and ileum is termed the mesentery, that to the transverse colon is the transverse mesocolon. Some peritoneal reflections between organs or between the body wall and organs, are termed ligaments or folds. Most of such ligaments or folds contain blood vessels. Broad peritoneal sheets associated with stomach are termed omenta.

Ⅰ) The Omentum There are two omenta, the lesser and the greater

1. The lesser omentum It is the fold of peritoneum which extends to the liver from the lesser curvature of the stomach and the commencement of the duodenum. It is continuous with the two layers which cover the anterorsuperior and posteroinferior surfaces of the stomach and about the first 2 cm of the duodenum. The hepatogastric ligament and the hepatoduodenal ligament: The portion of the lesser omentum extending between the liver and stomach is named the hepatogastric ligament, and that between the liver and duodenum the hepatoduodenal ligament.

Three key structures in the hepatoduodenal ligament: At the right border of the hepatoduodenal ligament the two layers are continuous, and form a free margin which is the anterior wall of the epiploic foramen. In the free margin the two layers of the hepatoduodenal ligament enclose the proper hepatic artery, hepatic portal vein and common bile duct (These three structures are called three key structures in the hepatoduodenal ligament), a few lymph nodes and lymph vessels, and the hepatic plexus of nerves. The common bile duct is situated anteriorly, the hepatic portal vein posteriorly and the proper hepatic artery is at the left of the common bile duct. The inferior vena cava lies behind the epiploic foramen.

2. The greater omentum It is the largest peritoneal fold. It consists of a double sheet, folded on itself so that it is made up of four layers. The two layers which descend from the stomach and commencement of the duodenum pass downwards in front of the small intestine for a variable distance; they then turn up on the back of itself, and ascend to the transverse colon, where the two layers are separated to cover the anterior and posterior surfaces of transverse colon. Then they form the transverse mesocolon.

The upper part of the greater omentum which extends between the stomach and the transverse colon is termed the gastrocolic ligament. In adult, the four layers of greater omentum are frequently adhered together, and are found wrapped about the organs in the upper part of the abdomen; only occasionally are they evenly dependent anterior to the intestines.

Functions: ① protective function: The greater omentum contains numerous fixed macrophages, which performs an important protective function. ② storehouse for fat: The greater omentum is usually thin, and presents a cribriform apperarance, but always contains some adipose tissue, which in fatty people is present in considerable quantity. ③ migration and limation: The greater omentum may limit spread of infection in the peritoneal cavity. Because it will migrate to the site of any inflammation in the peritoneal cavity and wrap itself around such a site, the greater omentum is commonly referred to as the “policeman” of the peritoneal cavity.

Ⅱ) The Mesenteries and Mesocolons 1. The mesentery (of the small intestine) It is a broad, fan-shaped fold of peritoneum connecting the coils of jejunum and ileum to the posterior abdominal wall. The portion attached to the posterior wall of the abdomen is called the radix (root ) of mesentery; it is about 15 cm long and is directed obliquely downwards from the duodenojejunal flexure (at the left side of the second lumbar vertebra) to the upper part of the right sacroiliac joint.

The intestinal border of the mesentery is about 6 m and is thrown into numerous pleats and frills. The mesentery consists of two layers of the peritoneum between which lie the jejunal and ileal branches of the superior mesenteric artery with their accompanying veins, nerve plexuses and lymph vessels, the mesenteric lymph nodes, connective tissue and fat.

2. The mesoappendix It is a triangular mesentery of the vermiform appendix, and is attached to the back of the lower end of the mesentery of ileum, close to the ileocecal junction. Its layers enclose the blood vessels, nerves and lymph vessels of the vermiform appendix, together with a lymph node.

3. The transverse mesocolon It is a broad fold which connects the transverse colon to the anterior border of the pancreas. Between the layers of the transverse mesocolon are the blood vessels, nerves and lymphatics of the transverse colon.

4. The sigmoid mesocolon It is a fold of peritoneum which attaches the sigmoid colon to the pelvic wall. The sigmoid and superior rectal vessels run between the layers of the sigmoid mesocolon, and the left ureter descends into the pelvis behind its apex.

Ⅲ) The Ligaments 1. The ligaments of the liver ① The falciform ligament of liver ② The ligamentum teres hepatis ③ The coronary ligament ④ The right triangular ligament ⑤ The left triangular ligament ⑥ The hepatogastric ligament ⑦ The hepatoduonedenal ligament

2. The ligaments of the spleen ① The splenorenal ligament: The splenic vessels pass between its two layers. ② The gastrosplenic ligament: The short gastric and left gastroepiploic branches of the splenic vessels run between its two layers.

3. The ligaments of the stomach ① The gastrohepatic ligament ② The gastrocolic ligament ③ The gastrosplenic ligament

4. The suspensory ligament of duodenum Sometimes named Treitz ligament. 5. The phrenicocolic ligament It is a fold of peritoneum which is continued from the left colic flexure to the diaphragm opposite the tenth and eleventh ribs.

Ⅳ. The Peritoneal Recesses In certain parts of the abdomen, peritoneal fold may bound recesses or fossae of the peritoneal cavity. These recesses are of surgical importance since they may become the site of internal herniae, that is, a piece of intestine may enter a recess and may be constricted (strangulated) by the peritoneal fold granding the entrance to the recess. From a surgical point of view the omental bursa can be considered to belong to this category, with its opening at the epiploic foramen, bounded in front by the free border of the lesser omentum.

Ⅰ) The Omental Bursa It is a part of the peritoneal sac and almost closed off from the remainder. Location: It lies behind the stomach and lesser omentum and has a small opening, the epiploic foramen, just above the first part of the duodenum.

Formation: (1) The anterior wall of the omental bursa is formed by: ① the posterior layer of lesser omentum, ② the peritoneum which covers the posteroinferior aspect of the stomach and about the first 2 cm of the duodenum, ③ the posterior surface of the anterior two layers of the greater omentum.

(2) The posterior wall is formed by: ① the anterior one of the posterior two layers of the greater omentum ② the peritoneum which lines the transverse colon, the transverse mesocolon, the pancreas, the left kidney and the left suprarenal gland.

The upper recess of the omental burse is roofed over by the diaphragm. The inferior recess of the bursa is limited by the lower border of the greater omentum, but, as a rule, partial fusion of the constituent layers of the greater omentum occurs after birth, so that the cavity of the omental bursa in the adult does not usually extend much below the transverse colon and its mesocolon.

(3) The omental bursa is limited to the left by the spleen and its peritoneal reflections, the splenorenal and the gastrosplenic ligaments. (4) On the right side, the omental bursa communicates with the greater peritoneal sac through the epiploic foramen.

Ⅱ) The Omental (Epiploic) Foramen (foramen of Winslow) Location: It lies between the liver and duodenum, just above the first part of the duodenum, is a short, vertically flattened passage, about 3 cm long, which leads out from the upper part of the right border of the omental bursa into the greater peritoneal sac.

Formation: (1) Its anterior wall is formed by the right margin of the lesser omentum, which contains between its two layers in this situation the common bile duct, the hepatic portal vein and the proper hepatic artery. (2) The roof of the epiploic foramen is the peritoneum covering the inferior of the caudate lobe of the liver. (3) Its posterior wall is the peritoneum descending in front of the inferior vena cava. (4) The floor of the epiploic foramen is limited by the upper border of the superior part of the duodenum.

Ⅲ) The Other Recesses (of much smaller size) They are sometimes found in relation to the duodenum, cecum and sigmoid colon.

1. Duodenal Recesses The superior duodenal recess The inferior duodenal recess The paraduodenal recess The duodenojejunal recess 2. Cecal recesses The superior ileocecal, The inferior ileocecal The retrocecal recesses The rectocolic recess 3. The intersigmoid recess

Ⅳ) Pouches In the lesser pelvis, the peritoneum dips downwards forming a larger fossa, named pouch.

1. In the male The rectovesical pouch lies between rectum and urinary bladder (or the seminal vesicles and ampullae ductus deferentes). The rectovesical pouch is the lowest part of the peritoneal cavity in anatomical position in male.

2. In the female The uterus and its broad ligaments divide the rectovesical pouch into two pouches, the rectouterine pouch (of Douglas) and the vesicouterine pouch. (1) The rectouterine pouch is formed between the anterior surface of the rectum and the posteriosurface of the uterus and the upper part of vagina. (2) The vesicouterine pouch is formed between the anteroinferior surface of the uterus and the posterior surface of the urinary bladder.

The rectouterine pouch is the lowest part of the peritoneal cavity in anatomical position in the female. With a patient propped in the semi-sitting posture, the fluid in the peritoneal cavity may descend either to the relatively accessible rectovesical pouch or to the rectouterine pouch which may be approached surgically through the rectum or vagina.

Ⅴ. Subdivision of Peritoneal Cavity The greater sac is subdivided by the greater omentum, transverse colon, and transverse mesocolon into a superoanterior part, the supramesocolic compartment, and an inferoposterior part, the inframesocolic compartment. These compartments form channels or recesses that determine how or where peritoneal fluid gravitates or spreads.

The inframesocolic compartment is further divided by mesentery of small intestine into right and left parts. The latter drains into the pelvis. The paracolic grooves are longitudinal depressions lateral to the ascending and descending colon. The supramesocolic compartment is subdivided by the liver into subphrenic and subhepatic spaces.