- 1 Structure of the Peritoneum
- 2 Intraperitoneal & Retroperitoneal Organs
- 3 Peritoneal Reflections
- 4 Clinical Relevance
The peritoneum is a continuous transparent membrane which lines the abdominal cavity and covers the abdominal organs (or viscera).
It acts to support the viscera, and provides a pathway for blood vessels and lymph. In this article, we shall look at the structure of the peritoneum, the organs that are covered by it, and its clinical correlations.
Structure of the Peritoneum
The peritoneum consists of two layers which are continuous with each other; the parietal peritoneum and the visceral peritoneum. They both consist of a layer of simple squamous epithelial cells, called mesothelium.
The parietal peritoneum lines the internal surface of the abdominopelvic wall.
It is derived from somatic mesoderm in the embryo.
It receives the same somatic nerve supply as the region of the abdominal wall that it lines, therefore pain from the parietal peritoneum is well localised and it is sensitive to pressure, pain, laceration and temperature.
The visceral peritoneum invaginates to cover the majority of the abdominal viscera.
It is derived from splanchnic mesoderm in the embryo.
The visceral peritoneum has the same nerve supply as the viscera it invests. Unlike the parietal peritoneum, pain from the visceral peritoneum is poorly localised and is only sensitive to stretch and chemical irritation.
Pain from the visceral peritoneum is referred to areas of skin (dermatomes) which are supplied by the same sensory ganglia and spinal cord segments as the nerve fibres innervating the viscera.
The peritoneal cavity is a potential space between the parietal and visceral peritoneum. It contains a small amount of lubricating fluid.
Intraperitoneal & Retroperitoneal Organs
The abdominal viscera can be divided anatomically by their relationship with the peritoneum. There are two main groups, intraperitoneal and retroperitoneal organs.
These organs are only covered in peritoneum on their anterior surface. They can be subdivided into two groups:
- Primarily retroperitoneal organs develop and remain outside of the parietal peritoneum. The oesophagus, rectum and kidneys are all primarily retroperitoneal
- Secondarily retroperitoneal organs were initially intraperitoneal, suspended by mesentery. Through the course of embryogenesis they became retroperitoneal, with their mesentery fusing with the posterior abdominal wall. Thus, peritoneum only covers the anterior surface. Examples of secondarily retroperitoneal organs include is the ascending and descending colon.
A useful mnemonic to help in recalling which abdominal viscera are retroperitoneal is SAD PUCKER:
- S = Suprarenal (adrenal) Glands
- A = Aorta/IVC
- D =Duodenum (except the duodenal cap- first 2cm)
- P = Pancreas (except the tail)
- U = Ureters
- C = Colon (ascending and descending parts)
- K = Kidneys
- E = (O)esophagus
- R = Rectum
The peritoneum covers a multitude of viscera within the gut and conveys neurovascular structures from the body wall to the viscera. In order to adequately fulfil its functions, the peritoneum develops into a highly folded, complex structure and a number of terms are used to describe the folds and spaces that are part of the peritoneum.
A mesentery is double layer of visceral peritoneum. It connects an intraperitoneal organ to the (usually) posterior abdominal wall. It provides a pathway for nerves, blood vessels and lymphatics from the body wall to the viscera.
The mesentery of the small intestine is simply called ‘the mesentery’. Mesentery related to other parts of the gastrointestinal system is named according to the viscera it connects to, for example the transverse and sigmoid mesocolons, the mesoappendix.
The omentum is a double layer of peritoneum that extends from the stomach and proximal part of the duodenum to other abdominal organs.
The greater omentum consists of four layers of peritoneum. It descends from the greater curvature of the stomach and proximal part of the duodenum, then folds back up and attaches to the anterior surface of the transverse colon.
It has a role in immunity and is sometimes referred to as the ‘abdominal policeman’ because it can migrate to infected viscera.
The lesser omentum is considerably smaller and attaches from the lesser curvature of the stomach and the proximal part of the duodenum to the liver. It consists of two parts: the hepatogastric ligament and the hepatoduodenal ligament.
A peritoneal ligament is a double fold of peritoneum that connects viscera together or connects viscera to the abdominal wall, for example the hepatogastric ligament which connects the liver to the stomach.
Pain from the viscera is poorly localised. As described earlier, it is referred to areas of skin (dermatomes) which are supplied by the same sensory ganglia and spinal cord segments as the nerve fibres innervating the viscera.
Pain is referred according to the embryological origin of the organ; thus pain from foregut structures are referred to the epigastric region, midgut structures are to the umbilical region and hindgut structures to the pubic region of the abdomen.
- The foregut includes the oesophagus, stomach, pancreas, liver, gallbladder and the duodenum (proximal to the entrance of the common bile duct).
- The midgut extends from the duodenum (distal to the entrance of the common bile duct) to the junction of the proximal two thirds of the transverse colon with the distal third.
- The hindgut extends from the distal one third of the transverse colon to the upper part of the anal canal.
Pain in retroperitoneal organs (e.g. kidney, pancreas) may present as back pain.
Irritation of the diaphragm (e.g. as a result of inflammation of the liver, gallbladder or duodenum) may result in shoulder tip pain.
Referred Pain in Appendicitis
Initially pain from the appendix (midgut structure) and visceral peritoneum is referred to the umbilical region. As the appendix becomes inflamed and irritates the parietal peritoneum the pain becomes localised to the right lower quadrant.
Damage to the peritoneum can occur as a result of infection, surgery or injury.
The resulting inflammation and repair may cause the formation of fibrous scar tissue. This can result in abnormal attachments between the visceral peritoneum of adjacent organs or between visceral and parietal peritoneum.
Such adhesions can result in pain and complications such as volvulus, when the intestine becomes twisted around an adhesion resulting in a bowel obstruction.