Exploring Anatomy: the Human Abdomen PERITONEUM AND PERITONEAL CAVITY PERITONEUM The peritoneum is a thin serous membrane that lines the abdominal cavity and covers, in variable amounts, the viscera within the abdominal cavity. It is a continuous membrane that can be described as being either parietal or visceral: • Parietal peritoneum lines the abdominal and pelvic walls. • Visceral peritoneum lines the viscera (organs). It is important to remember that the parietal and visceral peritoneum is one continuous layer. Where it lines the abdominal or pelvic walls it is referred to as being parietal peritoneum and where it is reflected from the abdominal wall to cover the viscera it is referred to as visceral peritoneum. The relationship between the peritoneum and viscera can be further described as either intra- or retro-peritoneal: • An intraperitoneal organ is completely surrounded by visceral peritoneum and is suspended within the peritoneal cavity by a mesentery. For examples, the jejunum and ileum are considered to be intraperitoneal organs. • A retroperitoneal organ is typically anchored to the posterior abdominal wall and is covered by a layer of parietal peritoneum. For example, the kidneys are considered to be retroperitoneal. Terms such as extraperitoneal and subperitoneal are also used to describe organs that are not enclosed with visceral peritoneum. Between the parietal peritoneum and the visceral peritoneum is a potential space – this is known as the peritoneal cavity or sac. It is a completely closed space, except in the female where there are two small openings for the uterine tubes. The peritoneal cavity only contains a thin film of peritoneal fluid that keeps the viscera moist to allow the smooth movement of viscera. No organs are found in the peritoneal cavity. Mesenteries, peritoneal ligaments and omenta The peritoneal cavity contains numerous structures that support or suspend the abdominal viscera. Mesenteries A mesentery is a double layer of peritoneum that suspends an organ from the posterior abdominal wall and marks the continuation of parietal and visceral peritoneum. Is it formed as a viscus invaginates and takes a covering of peritoneum (visceral). The peritoneum left behind approximates together and forms a double layer that is anchored to the posterior body wall. This double layer is important as it permits blood vessels, nerves and lymphatic vessels to pass through and reach the, now suspended, organ. Viscera that are anchored to the body wall by a mesentery are mobile with the degree of mobility being dependant on the length of the mesentery. There are four mesenteries: • The mesentery is associated with the small intestines (jejunum and ileum). • The transverse mesocolon is associated with the transverse colon. • The sigmoid mesocolon is associated with the sigmoid colon. • The mesoappendix is associated with the appendix. Peritoneal ligaments A peritoneal ligament is a double layer of peritoneum that connects an organ with another organ or the body wall. They are typically named after the organs they attach and are associated with the omenta. Omenta An omentum is a double layer of peritoneum that extends from the stomach to the colon, spleen or diaphragm. There are two omenta, known as the greater and lesser. The greater omentum is derived from the dorsal mesentery and the lesser omentum from the ventral mesentery. The greater omentum extends from the greater curvature of the stomach and first part of the duodenum and comprises three peritoneal ligaments: • Gastrocolic ligament – passes from the greater curvature to colon. This is the largest of the three peritoneal ligaments that form the greater omentum. It drapes inferiorly from the greater omentum to lie over the coils of small intestines. At its inferior limit it turns posteriorly and ascends to fuse with the visceral peritoneum of the transverse colon and the superior layer of the mesocolon. This creates a potential space between the anterior and posterior layers, known as the inferior recess of the omental bursa, in the adult these layers fuse. The gastrocolic ligament is a fatty apron like structure that moves around the peritoneal cavity preventing the visceral and parietal peritoneum from adhering. • Gastrosplenic ligament – passes from the greater curvature to the spleen. • Gastrophrenic ligament – passes from the greater curvature to diaphragm. The lesser omentum is another double-layered extension of peritoneum that passes from the lesser curvature of the stomach and first part of the duodenum to the liver. It is comprised of two peritoneal ligaments: • Hepatogastric ligament – passes from the lesser curvature of the stomach to the liver and is thin and membranous compared to the hepatoduodenal. • Hepatoduodenal ligament – passes from the first part of the duodenum to the liver. This ligament is considerably thicker than the hepatogastric and contains the portal triad (hepatic portal vein, hepatic artery and bile duct). The hepatoduodenal ligament marks the free edge of the lesser omentum; a finger passed posterior to this ligament will pass from the greater sac to the lesser sac via the epiploic foramen. SUBDIVISIONS OF PERITONEAL CAVITY In order to fully appreciate the subdivisions of the peritoneal cavity an understanding of the developmental process is helpful. The gastrointestinal tract is essentially a central tube that is suspended from the posterior and anterior abdominal walls. Suspending the gut tube from the posterior abdominal wall is the dorsal mesentary that extends along the entire length. The ventral mesentery suspends the gut tube from the anterior abdominal wall, but only extends along the proximal portions (stomach and proximal part of duodenum). This results in the peritoneal cavity being partitioned into right and left compartments as the dorsal and ventral mesenteries span between the posterior and anterior abdominal wall. As the gut tube at the level of the stomach rotates these two compartments will eventually go on to form the greater sac (left compartment) and lesser sac, which is also known as the omental bursa (right compartment). As the ventral mesentary does not extend to the more distal portions of the gut tube there is a free edge that passes between the proximal part of the duodenum and the anterior abdominal wall. During development this is vitally important as the umbilical vein runs within this free edge to deliver oxtgenated blood to the developing foetus from the placenta. In the adult a remnant of this mesentery is the lesser omentum that passes between the stomach, duodenum and liver. Greater and lesser sacs Once the stomach has rotated, the left compartment is now positioned anterior to the stomach and is known as the greater sac. This is the larger part of the peritoneal cavity and entering the abdomen through the anterior abdominal wall will open into this space. The lesser sac is located posterior to the stomach and lesser omentum and can be divided into two recesses, a superior recess and an inferior recess: • The superior recess is located posterior to the stomach. Superiorly it is bounded by the diaphragm, to the left is the spleen, to the right is the liver and posteriorly is the pancreas. The superior recess is continuous inferiorly with the inferior recess. • The inferior recess is a potential space located between the anterior and posterior layers of the gastrocolic ligament. These two sacs communicate via the epiploic foramen. This is located posterior to the free- edge of the lesser omentum, as it passes from the proximal part of the duodenum to the liver. The boundaries of the epiploic foramen are: • Anteriorly – hepatoduodenal ligament containing the portal triad. • Superiorly – caudate lobe of liver. • Posteriorly – inferior vena cava. • Inferiorly – proximal part of duodenum. Supracolic and infracolic compartments The greater sac can be further divided into two compartments, the supracolic and infracolic compartments, by the transverse mesocolon. The supracolic is located superior to the transverse colon and contains the stomach, liver and spleen. The infracolic compartment is located inferior to the transverse colon and can be divided into left and right infracolic spaces by the mesentery. It contains the small intestines and the ascending, descending and sigmoid colon. The supracolic and infracolic compartments can freely communicate by paracolic gutters. These are grooves found on the lateral side of the ascending and descending colons. The right paracolic gutter is continuous with the hepatorenal space, the lowest point of the abdomen when the patient is supine, and the left paracolic gutter is continuous with the pelvic cavity. Vasculature, nerves and lymphatic drainage As the parietal peritoneum lines the abdominopelvic cavity, it is served by the same blood vessels and nerves, and has the same lymphatic drainage as the body wall. It can precisely localise temperature, pressure and pain sensation. The visceral peritoneum is served by the same blood vessels and nerves as the organ it lines. The lymphatic drainage follows the same path as its corresponding organ. The visceral peritoneum is sensitive to stretch and chemical stimulation, but insensitive to temperature and pressure sensation. This results in poor localisation of pain sensation with it being referred to the dermatomes of the corresponding spinal cord segment (see Clinical Notes: referred pain). CLINICAL NOTES Peritonitis If the peritoneum becomes inflammed it is known as peritonitis. This can be due to