Peritoneum is the thin serous membrane that lines the walls of the abdominal and pelvic cavities and clothes the abdominal and pelvic viscera. It is the largest serous membrane of human body and has a rather complex arrangement. The peritoneum of living body possesses a certain degree of mobility on the extraperitoneal fat and can be stretched to certain degree without tearing.
Layers of peritoneum:
The peritoneum can be regarded as a balloon into which the organs are pressed from outside. Due to this, two layers are formed. One layer lines the walls of the abdominal and pelvic cavities and is called the parietal peritoneum. The other layer covers the organs and is known as the visceral peritoneum. The space between the parietal and visceral layers, which is the inside space of the balloon, is called the peritoneal cavity.
Between the parietal peritoneum and the fascial lining of the abdominal and pelvic walls, there is a layer of connective tissue called the extraperitoneal tissue. It varies in amount in different regions. In the area of the kidneys, it contains a large amount of fat, which provides support to them. In contrast to the parietal peritoneum, the visceral peritoneum is bound to the underlying viscera by only a small amount of connective tissue.
The peritoneal cavity is a potential space between the parietal peritoneum and visceral peritoneum. 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.
In males the peritoneal cavity is closed but in females there exists a communication with the exterior through the uterine tubes, the uterus and the vagina.
Divisions of Peritoneal Cavity:
The peritoneal cavity is the largest cavity of human body and the surface area of the parietal and visceral layers is enormous. The entire peritoneal cavity can be divided into two parts: Greater Sac and Lesser Sac.
- Greater Sac: It is the main compartment of the peritoneal cavity and extends from the diaphragm down into the pelvis.
- Lesser Sac: It is smaller in size and lies behind the stomach. It is in free communication with the greater sac through an oval window called the opening of the lesser sac, or “epiploic foramen”.
Classification of abdominal structures in relation to peritoneum:
The abdominal structures, on the basis of their relation with the peritoneum, are classified into three categories: Intraperitoneal, Retroperitoneal and Infraperitoneal.
Intraperitoneal organs: An organ is said to be Intraperitoneal when it is almost totally covered with visceral peritoneum. The Intraperitoneal organs of human body are:
- First part of duodenum
- Transverse colon
- Sigmoid colon
- Upper 1/3 of Rectum
- Uterus (Females)
- Fallopian tubes (Females)
- Ovaries (Females)
Retroperitoneal Organs: These are organs which lie behind the peritoneum and are only partially covered with visceral peritoneum. The retroperitoneal organs of human body are:
- Second and third parts of cuodenum
- Ascending colon
- Descending colon
- Middle 1/3 of Rectum
- Adrenal glands
- Proximal ureters
- Renal vessels
- Gonadal blood vessels
- Inferior vena cave
Infraperitoneal organs: These are organs which lie inferior to the peritoneum in the pelvis. These include:
- Lower 1/3 of rectum
- Urinary bladder
- Distal ureters
Important: Structures that are intraperitoneal are generally mobile, while those that are retroperitoneal are relatively fixed in their location.
It should be kept in mind that no organ actually lies within the peritoneal cavity. The Intraperitoneal organs are surrounded by peritoneal cavity but are covered with visceral peritoneum which separates them.
Peritoneal ligaments, omenta and mesenteries:
Ligaments: Peritoneal ligaments are two layered folds of peritoneum that connect the viscera to the walls of abdomen. Unlike other ligaments of body, they don’t possess the dense fibrous tissue and are therefore unique. A good example of peritoneal ligaments is found in liver, which is connected to the diaphragm by falciform ligament, coronary ligament, and right and left triangular ligaments.
Omenta: Omenta are two layered folds of peritoneum that connect the stomach to other viscera. There are three more obvious omenta in human body: Greater omentum, Lesser omentum and Gastrosplenic omentum.
- Greater omentum: It connects the greater curvature of the stomach to the transverse colon. It hangs down like an apron on the coils of the small intestine and is folded back on itself to be attached to the transverse colon.
- Lesser omentum: It suspends the lesser curvature of the stomach from the fissure of the ligamentum venosum (fibrous remnant of the ductus venosus of fetal circulation) and porta hepatis.
- Gastrosplenic omentum: As the name suggests, it connects the stomach to the hilum of spleen.
Mesenteries: Mesenteries are two layered folds of peritoneum, which connect the parts of the intestine to the posterior abdominal wall. For instance the mesentery of the small intestine connects the coils of jejunum and ileum to the posterior abdominal wall.
The ligaments, omenta and mesenteries permit blood vessels, lymphatics and nerves to reach the viscera without having to pierce the peritoneum.
Peritoneal pouches, recesses, and gutters:
The peritoneum is a highly folded membrane resulting in formation of lots of pouches, recesses and gutters. Some of the important of them are listed below:
- Lesser Sac
- Greater Sac
- Duodenal recesses
- Cecal recesses
- Intersigmoid recesses
- Subphrenic spaces
- Paracolic gutters
Blood supply and lymph drainage of peritoneum:
Parietal peritoneum is supplied by somatic blood vessels of the abdominal and pelvic walls. Its lymphatics join those in the body wall and drain to parietal lymph nodes. Visceral peritoneum is best considered as an integral part of the viscera which it overlies. It derives its blood supply from the viscera, and its lymphatics join the visceral vessels to drain to the regional lymph nodes.
Nerve supply to peritoneum:
Nerve supply to Parietal peritoneum:
Parietal peritonem is sensitive to pain, temperature, touch and pressure. It is innervated in accordance to the region where it exists, as explained below;
- The parietal peritoneum lining the anterior abdominal wall is supplied by lower six thoracic and first lumbar nerves. It should be kept in mind that these are the same nerves that supply the muscles and skin of the anterior abdominal wall.
- In the region of diaphragm, the nerve supply is different. The central part of the diaphragmatic peritoneum is innervated by phrenic nerves. Peripherally, the nerve supply comes through the lower six thoracic nerves.
- In the pelvic region, the parietal peritoneum is mainly supplied by the obturator never, which is a branch of the lumbar plexus.
Nerve supply to visceral peritoneum:
Unlike the parietal peritoneum, the visceral peritoneum is sensitive only to stretch and tearing. The sensations of touch, pressure or temperature are absent. The sensation of pain arises secondary to tearing and is very limited. It is of less severe nature and is referred to an area of abdominal wall according to the affected region of intestinal tract.
The nerve supply comes through the autonomic afferent nerves that supply the underlying viscera or the ones that travel in the mesenteries. Over distension of a viscus causes stretch of visceral peritoneum and leads to pain.
Functions of peritoneum:
Peritoneum performs a number of important functions, some of which are listed below:
- The peritoneal fluid, which is secreted by the peritoneum into the peritoneal cavity, ensures that the mobile viscera glide easily on one another. As a result of the movements of the diaphragm and abdominal muscles, together with the peristaltic movements of the intestinal tract, the peritoneal fluid is not static and has a direction of flow towards the subphrenic spaces.
- The peritoneal coverings of the intestine tend to stick together in infection. The greater omentum, which is commonly constantly on the move, may adhere to other peritoneal surfaces around a focus of infection. Thus many intraperitoneal infections are sealed off and remain localized.
- The peritoneal folds play important role in suspending various organs within the abdominal cavity. They also serve as a means of conveying blood vessels, lymphatics, and nerves to these organs.
- Large amounts of fat are stored in the peritoneal ligaments and mesenteries. Greater omentum displays especially large amount of fatty material, which has a number of potential benefits to human body.