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.
• A small evagination extends into the thorax along the right side of the oesophagus.
• The peritoneum evaginates to extend into the inguinal canals.
• Fat is often stored beneath the peritoneum.
• 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.
• The peritoneal cavity is complete in the male, but a potential communication with the exterior exists in the female at the opening of the uterine tubes.

Peritoneal Fluid

• A small quantity of peritoneal fluid is produced by mesothelial cells.
• It fills the potential space formed by the two layers of peritoneum and 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.
• The fluid is constantly being produced and resorbed through the large surface area of the peritoneum.
  • For this reason drugs are sometimes administered by intraperitoneal injection.
  • Bacterial toxins are also absorbed readily and can cause inflammation of the peritoneum; peritonitis

Function

• Secretes a small volume of clear fluid for lubrication.
• Provides a route for entry of blood and nerve and lymphatics.
• High fibrinolytic activity to protect against the formation of adhesions.
  • Inflammed portions of the peritoneum adhere to each other and may become organised and permanent. This may help to wall of infections and bring leuckocytes to the site of infection.
  • This trait is taken advantage of in surgery when serosal surfaces are often turned in when closing an incision.

Types of Peritoneum

The peritoneum doubles up to form the following suspensory structures:

• Mesentery: from 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).

Mesenteries

• The entire alimetary tract is attached to the dorsal body wall by mesentery, a derivative of the dorsal mesogastrium.
  • Mesoduodenum from the duodenum to the abdominal roof. Contains the right lobe of the pancreas.
  • Great Mesentery from the jejunum and ileum to the abdominal roof. Contains the cranial abdominal artery and vein, mesenteric lymph nodes and mesenteric plexus.
  • Mesocolon from the colon to the abdominal roof.
  • Mesorectum from the rectum to the abdominal roof.
• The female reproductive tract is also suspended by peritoneal derivatives.
  • Broad Ligament consisting of mesometrium, mesosalpinx and mesovarium. From ovary, oviduct, uterus, cervix and cranial vagina to the dorsal body wall. Contains ovarian artery, uterine branch of the ovarian artery, uterine artery, ovarian vein, uterine vein and uterine lymph node (mare).

Omenta

• During development in the region of the stomach, a fold of the dorsal mesogastrium becomes extended into a curtain - like structure called the greater omentum.
  • Originates from the greater curvature of the stomach and attaches to the proximal part of the duodenum.
  • Potential space between the two sheets is called the omental bursa.
  • Covers the jejunal mass on the ventral abdominal wall.
  • Opening to the omental bursa is called the epiploic foramen.
• The lesser omentum runs from the lesser curvature of the stomach to the liver.

Ligaments

• Gastrophrenic Ligament - from the greater curvature of the stomach to the crura of the diaphragm.
• Gastrosplenic Ligament - part of the greater omentum. Connects the spleen to the stomach.
• Nephrosplenic Ligament (renosplenic ligament) - In the horse - from spleen to left kidney.
• Round Ligament - part of the broad ligament. From the ovary to the inguinal ring.
• Suspensory Ligament - from the ovary to the abdominal wall.
• Proper Ligament of the ovary - from the ovary to the oviduct.
• 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
  • Gall Bladder
  • Pancreas
  • Spleen

Retroperitoneal Organs

• Organs are described as retroperitoneal if they are located behind the parietal peritoneum.
• Retroperitoneal organs include:
  • Kidneys
  • Adrenal glands
  • Ureter
  • Urinary Bladder
  • Part of Oesophagus
  • Rectum
  • Ovaries
  • Uterus
  • Aorta
  • Caudal Vena Cava

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