Peritoneal Cavity - Anatomy & Physiology

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Introduction

The peritoneum is the serous membrane that lines the abdominal cavity. It lies directly beneath the abdominal musculature (rectus abdominis and transverse abdominis). It is a type of loose connective tissue and is covered by mesothelium. Extensions of the peritoneum form the mesentries, omenta and ligaments that support the abdominal contents. The peritoneum produces fluid to lubricate abdominal viscera. The peritoneum also enhances immune responses and walls off infection in the abdomen to prevent peritonitis.

Development

In the early embryo, the primitive gut tube is suspended by the dorsal and ventral mesogastria. The mesogastria divide the embryo into two cavities, called the left and right coelomic cavities. The ventral mesogastrium atrophies caudal to the pylorus of the stomach and cranial to the rectum. This gives the entire small intestine and most of the large intestine large scope for expansion and rotation. It also allows the left and right coelomic cavities coalesce. This forms one cavity; the peritoneal cavity. Peritoneal structures develop from the dorsal and ventral mesogastria.

Structure & Histology

Lining the abdomen is a thin layer of loose connective tissue covered by a single layer of mesothelial cells. The layer of mesothelial cells is referred to as the peritoneum. Collectively, the connective tissue and peritoneum are referred to as the serosa.
- Mesothelial cells are simple squamous and of mesodermal origin.
  - Have microvilli on their surface.
  - Very fragile but regenerate very quickly.
A small amount of fibroelastic tissue is present within the connective tissue layer to provide support.
There are two layers of peritoneum lining the abdomen.
- Lining the abdominal wall is the parietal layer.
- Lining the abdominal viscera is the visceral layer.
- The small space within these two layers is called the peritoneal cavity.
- NB: In reality there are no viscera located in the peritoneal cavity. However the cavity created by the serosa is also confusingly referred to as the peritoneal cavity, and contains most abdominal contents.
The peritoneum evaginates to extend into the inguinal canals.
Many species have lymphoid tissue aggregates and fixed phagocytes in the omentum that are not covered by mesothelial cells
The peritoneum is smooth and clear in the healthy animal.
NB: Post mortem change and autolysis result in increased volumes of red-brown fluid in the abdomen, not associated with other pathology, no roughening of surfaces of organs or peritoneum.

Peritoneal Fluid

Peritoneal fluid is produced by mesothelial cells.
The potential space formed by the two layers of peritoneum is filled with a thin film of fluid that allows the two layers to slide over each other freely.
Peritoneal fluid is also produced as a transudate which coats the serosal surface of viscera to facilitate frictionless movement e.g. during peristalsis.
It is in equilibrium with plasma but doesn't contain high molecular weight molecules like fibrinogen.

Function

Secretes a small volume of clear fluid for lubrication
- Allows the abdominal viscera to move around by reducing the friction between them.
  - High fibrinolytic activity to protect against the formation of adhesions.
Provides a route for entry of blood and nerve and lymphatics

Types of Peritoneum

Terminology: becomes double in some areas

Mesentry: attaches viscera to the dorsal abdominal wall.
Omentum: from the stomach to other viscera.
Ligament: from viscera not involved in digestion to the abdominal wall or to other viscera that are not involved in digestion (e.g.ligaments of the liver).

Intraperitoneal Organs

Organs are intraperitoneal if they are enclosed by a fold of visceral peritoneum.
Intraperitoneal organs include:
- Stomach
- Small Intestine
- Large Intestine
- Liver
- Pancreas
- Spleen

Retroperitoneal Organs

Organs are described as retroperitoneal if they are located behind the parietal peritoneum.
Retroperitoneal organs include:
- Kidneys
- Adrenal glands
- Rectum
- Urinary Bladder

Species Differences

Small animals
- Low volume of clear straw coloured fluid
- Specific gravity <1.015 to 1.018
- Total protein 25-39g/L
- Nucleated cell count <3x10e9/L
- Mesothelial cells/macrophages

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