Peritoneal Cavity - Anatomy & Physiology

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Introduction

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

Structure & Histology

• The peritoneum is smooth and clear in the healthy animal.
• It consists of a thin layer of loose connective tissue covered by a single layer of mesothelial cells. 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.
    • High fibrinolytic activity to protect against the formation of adhesions.
  • 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.
  • The layer in contact with the abdominal wall is called the parietal layer.
  • The layer covering the abdominal viscera is called the visceral layer.
  • The small space within these two layers is called the peritoneal cavity.
• Many species have lymphoid tissue aggregates and fixed phagocytes in the omentum that are not covered by mesothelial cells
• 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

• 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.

Types of Peritoneum

Terminology:

• 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).

Species Differences

• Peritoneum is normally smooth and clear,
• Small volume of clear fluid for lubrication - in equilibrium with plasma but doesn't contain high molecular weight molecules like fibrinogen
• 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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