In higher vertebrates, the peritoneum is the serous membrane that forms the lining of the abdominal cavity — it covers most of the intra-abdominal organs. It is composed of a layer of mesothelium supported by a thin layer of connective tissue. The peritoneum both supports the abdominal organs and serves as a conduit for their blood and lymph vessels and nerves.

Structure

Layers

The abdominal cavity (the space bounded by the vertebrae, abdominal muscles, diaphragm and pelvic floor) should not be confused with the intraperitoneal space (located within the abdominal cavity, but wrapped in peritoneum). For example, a kidney is inside the abdominal cavity, but is retroperitoneal.

Although they ultimately form one continuous sheet, two types or layers of peritoneum and a potential space between them are referenced:

• The outer layer, called the parietal peritoneum, is attached to the abdominal wall.
• The inner layer, the visceral peritoneum, is wrapped around the internal organs that are located inside the intraperitoneal cavity.
• The potential space between these two layers is the peritoneal cavity; it is filled with a small amount (about 50 ml) of slippery serous fluid that allows the two layers to slide freely over each other.
• The term mesentery is often used to refer to a double layer of visceral peritoneum. There are often blood vessels, nerves, and other structures between these layers. It should be noted that the space between these two layers is technically outside of the peritoneal sac, and thus not in the peritoneal cavity.

Subdivisions

There are two main regions of the peritoneum, connected by the epiploic foramen:

• the greater sac (or general cavity of the abdomen), represented in red in the diagrams above.
• the lesser sac (or omental bursa), represented in blue. The lesser sac is divided into two "omenta":
• The lesser omentum (or gastrohepatic) is attached to the lesser curvature of the stomach and the liver.
• The greater omentum (or gastrocolic) hangs from the greater curve of the stomach and loops down in front of the intestines before curving back upwards to attach to the transverse colon. In effect it is draped in front of the intestines like an apron and may serve as an insulating or protective layer.

The mesentery is the part of the peritoneum through which most abdominal organs are attached to the abdominal wall and supplied with blood and lymph vessels and nerves.

Structures include:

SOURCES	STRUCTURE	FROM	TO	CONTAINS
	OMENTA
dorsal mesentery	* greater omentum	greater curvature of stomach (and spleen)
dorsal mesentery	** gastrosplenic ligament	stomach	spleen	short gastric artery, left gastro-omental artery
dorsal mesentery	** gastrophrenic ligament	stomach	diaphragm	-
dorsal mesentery	** gastrocolic ligament	stomach	transverse colon	-
dorsal mesentery	** splenorenal ligament	spleen	kidney	splenic artery, tail of pancreas
ventral mesentery	* lesser omentum	lesser curvature of the stomach (and duodenum)
ventral mesentery	** hepatogastric ligament	stomach	liver
ventral mesentery	** hepatoduodenal ligament	duodenum	liver	hepatic artery proper, hepatic portal vein, bile duct
	MESENTERIES
dorsal mesentery	* Mesentery proper	small intestine – jejunum and ileum	posterior abdominal wall	superior mesenteric artery
dorsal mesentery	* transverse mesocolon	transverse colon	posterior abdominal wall	middle colic
dorsal mesentery	* sigmoid mesocolon	sigmoid colon	pelvic wall	sigmoid arteries
dorsal mesentery	* mesoappendix	mesentery of ileum	appendix	appendicular artery
	OTHER LIGAMENTS AND FOLDS
ventral mesentery	* falciform ligament	liver	thoracic diaphragm, anterior abdominal wall	round ligament of liver, paraumbilical veins
left umbilical vein	* round ligament of liver	liver	umbilicus
ventral mesentery	* coronary ligament	liver	thoracic diaphragm
ductus venosus	* ligamentum venosum	liver	liver
	* phrenicocolic ligament	left colic flexure	thoracic diaphragm
ventral mesentery	* left triangular ligament, right triangular ligament	liver
	* umbilical folds	urinary bladder
	* ileocecal fold	ileum	cecum
	* broad ligament of the uterus	uterus	pelvic wall	mesovarium, mesosalpinx, mesometrium
	* ovarian ligament	uterus	inguinal canal
	* suspensory ligament of the ovary	ovary	pelvic wall	ovarian artery

In addition, in the pelvic cavity there are several structures that are usually named not for the peritoneum, but for the areas defined by the peritoneal folds:

Name	Location	Genders possessing structure
Rectovesical pouch	between rectum and urinary bladder	male only
Rectouterine pouch	between rectum and uterus	female only
Vesicouterine pouch	between urinary bladder and uterus	female only
Pararectal fossa	surrounding rectum	male and female
Paravesical fossa	surrounding urinary bladder	male and female

Development

The peritoneum develops ultimately from the mesoderm of the trilaminar embryo. As the mesoderm differentiates, one region known as the lateral plate mesoderm splits to form two layers separated by an intraembryonic coelom. These two layers develop later into the visceral and parietal layers found in all serous cavities, including the peritoneum.

As an embryo develops, the various abdominal organs grow into the abdominal cavity from structures in the abdominal wall. In this process they become enveloped in a layer of peritoneum. The growing organs "take their blood vessels with them" from the abdominal wall, and these blood vessels become covered by peritoneum, forming a mesentery.

Clinical aspects

Pathology

• Pneumoperitoneum is the presence of gas within the peritoneal cavity, as may occur when a perforation forms in the stomach or intestines, and heralds a perilous situation.
• Peritonitis is an inflammation of the peritoneal lining or cavity, which may occur with either a perforation or by spread of infection through the wall of one of the abdominal organs. This too is a serious condition, and often requires emergency surgery.
• Ascites is an accumulation of excess fluid within the peritoneal cavity.

Peritoneal dialysis

In one form of dialysis, the peritoneal dialysis, a glucose solution is run through a tube into the peritoneal cavity. The fluid is left there for a prescribed amount of time to absorb waste products, and then removed through the tube. The reason for this effect is the high number of arteries and veins in the peritoneal cavity. Through the mechanism of diffusion, waste products are removed from the blood.

Classification of abdominal structures

The structures in the abdomen are classified as intraperitoneal, retroperitoneal or infraperitoneal depending on whether they are covered with visceral peritoneum and have a mesentery or not.

Intraperitoneal	Retroperitoneal	Infraperitoneal / Subperitoneal
Stomach, First part of the duodenum [5 cm], jejunum, ileum	The rest of the duodenum
Cecum, appendix, transverse colon, sigmoid colon	Ascending colon, descending colon
Rectum, upper 1/3	Rectum, middle 1/3	Rectum, lower 1/3
Liver, Spleen	Pancreas
	Kidneys, suprarenal glands, ureters, renal vessels	Urinary bladder, ureters(end)
In women: Uterus, Fallopian tubes, ovaries	Gonadal blood vessels
	Inferior vena cava, Aorta

Structures that are intraperitoneal are generally mobile, while those that are retroperitoneal are relatively fixed in their location.

Some structures, such as the kidneys, are "primarily retroperitoneal", while others such as the majority of the duodenum, are "secondarily retroperitoneal", meaning that structure developed intraperitoneally but lost its mesentery and thus became retroperitoneal.

Etymology

Peritoneum is derived from Greek. Peri- means around, while -ton- refers to stretching. Thus, peritoneuum means stretched around or stretched over.

Additional images

References

• Tortora, Gerard J., Anagnostakos, Reginald Merryweather, Nicholas P. (1984) Principles of Anatomy and Physiology, Harper & Row Publishers, New York ISBN 0-06-046656-1

External links

• Overview and diagrams at colostate.edu