Peritoneum.pdf

Gastrointestinal System Peritoneum We will now begin a lecture on the Peritoneum.  Learning Outcomes After this lecture you should be able to: ▪ Define the term “mesentery” and “ligament” within the context of the abdomen and be a...

Gastrointestinal System Peritoneum We will now begin a lecture on the Peritoneum.  Learning Outcomes After this lecture you should be able to: ▪ Define the term “mesentery” and “ligament” within the context of the abdomen and be able to list them ▪ Be able to define the terms ‘intra-peritoneal’ and ‘retro-peritoneal’ ▪ Give an account of the greater and lesser omenta including its attachments and functions ▪ Describe the abdominal cavity ‘sacs’ and locate the connection between them ▪ Give an account of infection spread within the peritoneal cavity, and name the spaces where fluid collects ▪ Describe the reflections of peritoneum over the pelvic organs, and know the names of the spaces between their folds Here are the learning outcomes for the entire lecture.  Gastrointestinal System Peritoneum 1. Mesenteries and Ligaments 2. Greater and Lesser Omenta 3. Peritoneal Spaces This lecture is divided into 3 parts.  Gastrointestinal System Peritoneum Part 1: Mesenteries and Ligaments In this first part we will define the terms mesenteries and ligaments, and see examples of both.  Learning Outcomes After this lecture you should be able to: ▪ Define the term “mesentery” and “ligament” within the context of the abdomen and be able to list them ▪ Be able to define the terms ‘intra-peritoneal’ and ‘retro-peritoneal’ ▪ Give an account of the greater and lesser omenta including its attachments and functions ▪ Describe the abdominal cavity ‘sacs’ and locate the connection between them ▪ Give an account of infection spread within the peritoneal cavity, and name the spaces where fluid collects ▪ Describe the reflections of peritoneum over the pelvic organs, and know the names of the spaces between their folds The learning outcomes for this part of the lecture are that you should be able to; Define the term “mesentery” and “ligament” within the context of the abdomen and be able to list them Be able to define the terms ‘intra-peritoneal’ and ‘retro-peritoneal’ The remaining outcomes will be given in the subsequent parts of the lecture.  Mesenteries In the embryo, the gut tube is suspended in the middle of the abdominal cavity by a double-fold of membrane; a mesentery. There is a dorsal mesentery attaching the gut to the posterior wall, and a ventral one attaching the foregut only to the anterior wall. A mesentery is a double-fold of membrane supporting the gut. It usually contains fat, nerves and blood vessels. Mesenteries is plural. The term is a little strange as literally it means ‘middle of the intestines’ in Greek. Meso is middle and enteros is the gut. It doesn’t really mean middle of the gut though. I think this rather comes from the fact the gut sits in the middle of the abdomen in early development, supported by these membranes. The fate of the mesenteries is complicated, because the gastrointestinal tract elongates and rotates and ultimately occupies most of the abdominal cavity. Subdivisions of the Alimentary Tract Stomodeum Proctodeum As we have seen previously, the embryological division of the alimentary tract is into three segments; the foregut, midgut and hindgut. The foregut includes the oesophagus and pharynx. The proximal end of the gut is connected to an embryological region called the stomodeum but separated from it by the oropharyngeal membrane. The distal end of the gut tube is connected to a region called the proctodeum but separated from it from an anal membrane. Before birth, these blind-ends normally break down, but the distal one occasionally does not, leading to an imperforate anus. Both of these embryological regions has their own blood supply and nerve supply, which is distinctly different to that of the gut. Blood Supply of the Alimentary Tract The foregut receives the coeliac trunk, the midgut the superior mesenteric artery and the hindgut the inferior mesenteric artery.  Intra- and Retro- Peritoneal An “intra-peritoneal” organ (completely surrounded by peritoneum) A “retro-peritoneal” organ (covered partly by peritoneum) The dorsal mesenteries initially line the entire alimentary tract, but as the tract enlarges and twists, some of the mesenteries blend into the parietal peritoneum of the posterior abdominal wall and are therefore lost. Organs that lack a mesentery are said to be retro-peritoneal organs. Those with a mesentery are the intra-peritoneal organs. It should be noted however, that retroperitoneal organs may still retain a partial covering of peritoneum. Retroperitoneal organs are relatively immobile, whilst the intraperitoneal organs are freer to move, and that is the most significant thing to note clinically.  Primary Retroperitoneal Organs Some organs, such as the kidneys and suprarenal glands and the major blood vessels such as the aorta and inferior vena cava, develop behind the peritoneal cavity and never have a mesentery. These are what can be called Primary Retroperitoneal organs.  Secondary Retroperitoneal Organs An “intra-peritoneal” organ which is pushed to the posterior abdominal wall may become “retro- peritoneal” by resorption of peritoneum. This occurs as a result of elongation and rotation of the gut during development. Others however, begin as intraperitoneal organs but their mesenteries are reabsorbed. These include the pancreas, duodenum and the ascending and descending colons. The growth of the liver caused the pancreas and duodenum to be pushed against the back wall, causing their mesentery to fuse with the posterior body wall. Gut rotation has caused the ascending/descending colon to be cemented along the posterior body wall, and the mesentery reabsorbed.  Development of the Mesogastria Kidneys Dorsal Spleen mesogastrium Stomach Ventral Liver mesogastrium In the upper abdominal cavity the key part of the gut tube is the stomach. Its mesenteries are called the ventral and dorsal mesogastrium, since “gaster” is the Latin word for stomach. In the ventral mesogastrium is the developing liver and behind the spleen and pancreas. The pancreas becomes retroperitoneal. On the posterior wall itself are the two kidneys.  Mesogastrial Ligaments Kidneys Spleno-(lieno)-renal lig. Spleen Gastrosplenic lig. Stomach Gastrohepatic lig. (Lesser omentum) Liver Falciform lig. The parts of the mesenteries that connect organs are called ligaments. In front, connecting the liver to the parietal perineum is the falciform ligament. Other ligaments are named according to the organs they are connecting. Hence, the one between the liver and stomach is the gastrohepatic ligament. This is more commonly called the lesser omentum. Between the stomach and spleen is the gastrosplenic ligament. And the one between the spleen and kidneys is the splenorenal ligament. In each case, the name arises from either the Latin or Greek word for each organ. Splen is Greek, Lien is Latin, hence it can either be called the splenorenal or lienorenal ligament. Both are correct. Development of the Peritoneal Sacs 1 As development progresses, the liver grows at an enormous rate, and grows more to the right than the left. The liver is still connected to the anterior wall, via the falciform ligament. Development of the Peritoneal Sacs 2 This expansion of the liver forces the other organs (stomach, spleen) to the left. However, each of these organs remains connected to others by their respective ligaments. Development of the Peritoneal Sacs 3 As the liver finally adopts its position on the right side of the abdominal cavity, the right hand side of the peritoneal cavity is pushed backwards behind the stomach and lesser omentum. Development of the Peritoneal Sacs 4 Bare area of liver This is now called the lesser sac. There is more space for the left side of the peritoneal cavity, hence this expands and becomes the greater sac. There is however, a connection between the two sacs under the lower border of the lesser omentum, and this is indicated by the arrow.  Gastrointestinal System Peritoneum Part 2: Greater and Lesser Omenta Welcome to part 2 of this lecture on the Peritoneum. In this section, we will consider the Greater and Lesser Omenta.  Learning Outcomes After this lecture you should be able to: ▪ Define the term “mesentery” and “ligament” within the context of the abdomen and be able to list them ▪ Be able to define the terms ‘intra-peritoneal’ and ‘retro-peritoneal’ ▪ Give an account of the greater and lesser omenta including their attachments and functions ▪ Describe the abdominal cavity ‘sacs’ and locate the connection between them ▪ Give an account of infection spread within the peritoneal cavity, and name the spaces where fluid collects ▪ Describe the reflections of peritoneum over the pelvic organs, and know the names of the spaces between their folds The single Learning Outcome for this part of the lecture is to give an account of the greater and lesser omenta, including their attachments and functions. The remaining outcomes will be discussed in the final part of the lecture next time.  Growth of Stomach Dorsal mesentery Right vagus nerve Left vagus nerve Ventral mesentery In the previous lecture on the development of the gut, we discussed the rotation of the midgut, driven by the expansion of the liver. This expansion also caused rotation of the foregut, such that the stomach was not only pushed to the left, but it was rotated at the same time. Before rotation begins, the stomach has it ventral and dorsal mesogastria at the front and back respectively. On either side is the vagus nerves.  Rotation of Stomach The stomach evolves by two rotations along a longitudinal and antero- posterior axis. The longitudinal rotation of the stomach involves a 90° clockwise rotation, resulting in the right side of the stomach becoming posteriorly oriented, and the left side of the stomach facing anteriorly. This explains why the left vagus nerve innervates the anterior wall of the stomach, and the right vagus nerve innervates the posterior wall of the stomach in the adult. During this rotation, one side of the stomach grows faster than the other, forming the greater and lesser curvatures of the stomach. These curvatures are the locations of the omenta. The lesser omentum is attached to the lesser curvature and the greater omentum is attached to greater curvature.  Greater and Lesser Omentum The word omentum is derived from Latin, but its origins are obscure, other than it is related to the fact that the structure is filled with fat. This it truly is. The Greek word for it is Epiploon, which is associated with the phrase ‘to float upon’ - as the omentum floats upon the abdominal contents. Indeed, the greater omentum hangs down in front of the intestines like an apron. On route it fuses with the transverse colon, to form a gastrocolic ligament. As you can see, it is full of fat and also contains many omental blood vessels coursing through it. Here we can see a dissection image of the greater omentum hanging down from the greater curvature of the stomach. Highlighted in green above and attached to the lesser curvature is the lesser omentum.  Greater Sac & Greater Omentum Lesser Sac Greater Sac Here we can see a sagittal section of the abdominopelvic cavity, showing the position of the omenta above and below the stomach. The organ above the stomach is the liver, and that below is the transverse colon, which is attached to the posterior wall by the transverse mesocolon. The peritoneal cavity is divided into a lesser sac located behind the stomach, and a greater sac (everywhere else).  Epiploic Foramen There is a connection between the two sacs just under the right (inferior) free edge of the lesser omentum, and this is known as the epiploic foramen. This can be seen by lifting the inferior margin of the liver upwards as shown in this illustration.  Greater and Lesser Sacs Here it is again, this time in cross-section. The liver is on the right of the subject, but on your left, as we are looking at this from below as is standard practice on cross-sections. So the lesser sac is behind the stomach. A finger pushed in through the epiploic foramen can explore the lesser sac. The liver can be felt on the right, the lesser omentum and stomach in front, and the spleen to the left (if you have long enough fingers)! Now to turn our attention back to the greater omentum which hangs down in the greater sac ….. Development of the Greater Omentum Stomach Lesser Sac Omental bursa Pancreas Transverse mesocolon Transverse colon Let’s consider the development of this structure. It is originally formed from 4 layers, two of which are associated with the mesenteries of the stomach and two are associated with those of the pancreas. The inner two layers form the peritoneum lining the lesser sac. To form the greater omentum, these layers are going to fuse inferiorly at the location of the omental bursa, but also expand. Once these membranes fuse, the space disappears and hence so does the bursa. However, the term “omental bursa” is also often used synonymously, to mean the lesser sac (erroneously in my view). At the same time as this is going on the expanding membranes fuse with the membranes of the transverse colon – the transverse mesocolon. Confused? OK let’s watch a short animation of it. Development of the Greater Omentum This is an animation – please see lecture presentation to view. Development of the Greater Omentum Lesser Sac Transverse mesocolon Gastrocolic ligament Once the fusion of the layers has completed, it forms the gastrocolic ligament. Surgeons can gain access to the lesser sac by lifting up the greater omentum and cutting through the gastrocolic ligament. The greater omentum has some very important functions; it localises infections by sticking to any infected region. It traps the infection, and prevents it from spreading through the abdominal cavity. More than this, the omentum is highly vascular, and can via this route bring lymphocytes to the region to help tackle the infection. This trapping and imprisonment of infections, has brought it to be referred to as the ”abdominal policeman”.  Gastrointestinal System Peritoneum Part 3: Peritoneal Spaces We have now reached the last section on the peritoneum, where we will consider some of the spaces created by the greater sac.  Learning Outcomes After this lecture you should be able to: ▪ Define the term “mesentery” and “ligament” within the context of the abdomen and be able to list them ▪ Be able to define the terms ‘intra-peritoneal’ and ‘retro-peritoneal’ ▪ Give an account of the greater and lesser omenta including its attachments and functions ▪ Describe the abdominal cavity ‘sacs’ and locate the connection between them ▪ Give an account of infection spread within the peritoneal cavity, and name the spaces where fluid collects ▪ Describe the reflections of peritoneum over the pelvic organs, and know the names of the spaces between their folds The learning outcomes for this section of the lecture are that you should be able to; Describe the abdominal cavity ‘sacs’ and locate the connection between them Give an account of infection spread within the peritoneal cavity, and name the peritoneal cavities where fluid collects Describe the reflections of peritoneum over the pelvic organs, and know the names of the spaces between their folds  Peritoneum and Peritoneal Sacs Visceral Greater Sac Peritoneum Parietal Peritoneum Lesser Sac The parietal peritoneum lines the body wall. Visceral peritoneum lines the organs, and is continuous with the parietal one at the front and back. Not only does it wrap around the organs but it connects them. These connections are called ligaments. We normally think of ligaments being associated with joints, but the term ligament in Latin simply means ‘that which binds’. Hence the connection at the front is called the falciform ligament. At the back of the liver, there is a ligament that connects the liver to the stomach. This we can call the gastrohepatic ligament (gastrum for stomach and hepatus for liver). However, we also call this ligament the lesser omentum. It attaches to the lesser curvature of the stomach, matching the greater omentum which arises from the greater curvature. On the left hand side of the stomach we have a ligament which joins the spleen, and this is therefore the gastrosplenic ligament. Splen is Greek, so it can also be referred to by the Latin Lien, hence Gastrolienal ligament. The final ligament reaches the posterior abdominal wall where the kidneys are, and hence this is the Lieno-renal ligament (ren is Latin for kidney. We have previously seen how the peritoneal cavity is divided into a greater and a lesser sac. These sacs sit between the parietal and visceral peritoneal membranes. The greater sac and lesser sac.  Rotation of Gut and Mesenteries Lesser omentum Greater omentum Transverse mesocolon Sigmoid THE mesentery mesocolon In the embryo, the dorsal mesentery runs all the way down the posterior wall of the abdomen, attaching to the entire length of gut. As the gut tube expands, twists and folds, some parts of the gut tube are pushed backwards towards the posterior wall, and this means that there is no longer a requirement for a sling of mesentery. Those parts of the mesenteries therefore disappear. Let’s label what is left. Of course we still have an anterior mesentery in the form of the lesser omentum. The greater omentum of course is an expansion of part of the dorsal mesentery, but has moved to a more anterior position, so again doesn’t get lost. The mesentery of the jejunum and ileum remains and is called “THE mesentery”! This is a rather strange name, as it would suggest it is the only one. It isn’t! It attaches to the posterior wall obliquely from top left to bottom right. The mesenteries of the ascending colon (part of the midgut) and the descending colon (part of the hindgut) are lost, and those organs therefore become retroperitoneal. The caecum is variable in its covering of peritoneum, but usually loses its mesentery, and is therefore considered retroperitoneal. It is a highly variable organ, and perhaps the reality is that is isn’t easy to classify. The transverse colon remains suspended by the transverse mesocolon, which later becomes fused with the greater omentum as we saw in the previous section. The sigmoid colon retains a mesentery, the sigmoid mesocolon.  Peritoneal Spaces Bare area of Liver Position of Transverse Asc. & Desc. mesocolon Colon (Retro- Peritoneal The mesentery Organs) Sigmoid mesocolon Fluid movements within the greater sac of the peritoneal cavity will be limited by the attachments of the mesenteries. This will also limit the spread of infections within the sac. This is a diagram of the posterior wall of the abdomen, and the lines indicate the cut edge of the peritoneal membranes. Where the lines close together these indicate mesenteries, and where the lines are further apart, these indicate the positions of the retroperitoneal organs (though strangely the liver isn’t considered to be retroperitoneal (but that’s another story). The greater sac is divided into two halves by the transverse mesocolon, and we talk about supra-colic and infra-colic compartments respectively.  Fluid Movement The infra-colic space is continuous inferiorly with the pelvic cavity, hence infra-colic infections can travel to and from the pelvis. If the patient lies down, the fluid can travel in the right paracolic gutter leading upwards to the space between the liver and kidney – the hepatorenal pouch or to the right subphrenic space under the diaphragm. On the left, this can connect to the space around the stomach and spleen, and also with the subphrenic space on the left.  Pelvic Peritoneal Folds The pelvic organs are partially covered by the peritoneum above, which drapes over them. There are 3 pelvic organs in both sexes. In females the 3 organs are the bladder, uterus (and upper part of the vagina) and the rectum. The peritoneum then drapes over those 3 structures, creating folds between the organs. The anterior fold is the vesicouterine pouch, since the vesicum is bladder in Latin. Behind, is the rectouterine pouch or pouch of Douglas. The rectouterine pouch in females pouch is the lowest point of the peritoneal cavity, and this often where pelvic infections will track down to. In the male, the peritoneum forms a single fold between the bladder and rectum forming the rectovesical pouch. The 3rd pelvic organ in the male is the prostate gland, but this lies below the bladder and isn’t therefore covered by peritoneum.  Ascites (this is not included in the lecture presentation) Disease of some of the abdominal viscera can lead to an increase in the amount of peritoneal fluid. Ascites is the pathologic accumulation of fluid in the peritoneal cavity, most often caused by cirrhosis of the liver. However, it also may be due to cancer, heart failure, tuberculosis, pancreatic disease, dialysis, and many other causes. The development of ascites heralds a progressive deterioration in a patient's clinical condition, and only 50% of affected patients survive 2 years after onset. Patients with ascites have a swollen abdomen. The fluid that is contained within the peritoneal cavity can be percussed. As the abdomen is compressed on one side, the fluid moves over to the opposite side like a wave, and this can be detected by the examining clinician. The fluid can be drained off via a needle inserted into the peritoneal cavity (paracentesis), or drugs can be given that raise the volume of fluid removed from the blood stream to be excreted in the urine.  Peritoneal Dialysis (this is not included in the lecture presentation) There is normally less than 50ml of fluid in the peritoneal cavity. This fluid is able to be exchanged via the blood vessels within the mesenteries. Indeed, peritoneal fluid is actually formed as a filtrate of the blood, and it contains many blood derived cells. This fact can be put to use in patients with defective kidneys. In such patients, the kidneys are not filtering the blood effectively. Since the peritoneum is a semi- permeable membrane, it may be used in such cases as a filter. To do this, a dextrose-containing salt solution is introduced into the peritoneal cavity via a catheter. By diffusion and ultra-filtration, toxic materials move from the blood and tissues to the peritoneal cavity. This is called peritoneal dialysis. Solute and water absorption also occur via movement into blood circulation and lymphatics in the peritoneal cavity.  Gastrointestinal System Peritoneum And that then brings us to the end of this section, and indeed the end of the lecture on the Peritoneum. In the next lecture, we will begin to explore the abdominal organs starting with the stomach.