Abdomen: Conceptual overview
General description
The abdomen is a cylindrical chamber between the inferior margin of the thorax and the superior
margin of the pelvis. The inferior thoracic aperture is the superior opening to the abdomen, closed
by the diaphragm. Inferiorly, the abdominal wall is continuous with the pelvic wall at the pelvic inlet.
Superficially, the inferior limit of the abdominal wall is the lower limb.
The chamber enclosed by the abdominal wall contains a peritoneal cavity, which communicates with
the pelvic cavity.
Abdominal viscera are either suspended in the peritoneal cavity by mesenteries or positioned
between the cavity and musculoskeletal walls. Abdominal viscera include:
- major elements of the gastrointestinal system: the caudal end of the oesophagus, stomach,
intestines, liver, pancreas and gallbladder.
- components of the urinary system: kidneys and ureters.
- the spleen.
- the suprarenal glands.
- major neurovascular structures.
1
,Functions
Houses and protects major viscera
The abdomen houses major elements of the gastrointestinal system, the spleen, and parts of the
urinary system.
Much of the liver, gallbladder, stomach, spleen, and parts of the colon are under the domes of the
diaphragm, so they are protected by the thoracic wall. The superior poles of the kidneys are deep to
the lower ribs. Viscera not beneath the diaphragm are supported by the muscular walls of the
abdomen.
2
, Breathing
The abdominal wall assists in breathing.
- It relaxes during inspiration to accommodate expansion of the thoracic cavity and the inferior
displacement of the abdominal viscera during contraction of the diaphragm.
- During expiration, it contracts to assist in elevating the domes of the diaphragm.
Material can be expelled from the airway by forced expiration using the abdominal muscles, in
coughing and sneezing.
Intraabdominal pressure
Contraction of abdominal muscles
increases intraabdominal pressure
when the diaphragm is in a fixed
position. Air is retained in the lungs by
closing valves in the larynx. Increased
intraabdominal pressure assists in
voiding the contents of the bladder
and rectum and in giving birth.
Component parts
Wall
The abdominal wall consists of muscle and bone. The skeletal elements of the wall are:
- The five lumbar vertebrae and the intervening intervertebral disks.
- The superior expanded parts of the pelvic bones.
- Bony components of the inferior thoracic wall, including the costal margin, rib XII, the end of rib XI,
and the xiphoid process.
Muscles make up the rest of the abdominal wall:
- The quadratus lumborum, psoas major, and iliacus muscles reinforce the posterior wall. The distal
ends of the psoas major and iliacus muscles pass into the thigh and are major flexors of the hip joint.
- Lateral parts of the abdominal wall are formed by three layers of muscles: Transversus abdominis,
internal oblique, and external oblique.
- Anteriorly, a segmented muscle (the rectus abdominis) on each side spans the distance between
the thoracic wall and the pelvis.
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General description
The abdomen is a cylindrical chamber between the inferior margin of the thorax and the superior
margin of the pelvis. The inferior thoracic aperture is the superior opening to the abdomen, closed
by the diaphragm. Inferiorly, the abdominal wall is continuous with the pelvic wall at the pelvic inlet.
Superficially, the inferior limit of the abdominal wall is the lower limb.
The chamber enclosed by the abdominal wall contains a peritoneal cavity, which communicates with
the pelvic cavity.
Abdominal viscera are either suspended in the peritoneal cavity by mesenteries or positioned
between the cavity and musculoskeletal walls. Abdominal viscera include:
- major elements of the gastrointestinal system: the caudal end of the oesophagus, stomach,
intestines, liver, pancreas and gallbladder.
- components of the urinary system: kidneys and ureters.
- the spleen.
- the suprarenal glands.
- major neurovascular structures.
1
,Functions
Houses and protects major viscera
The abdomen houses major elements of the gastrointestinal system, the spleen, and parts of the
urinary system.
Much of the liver, gallbladder, stomach, spleen, and parts of the colon are under the domes of the
diaphragm, so they are protected by the thoracic wall. The superior poles of the kidneys are deep to
the lower ribs. Viscera not beneath the diaphragm are supported by the muscular walls of the
abdomen.
2
, Breathing
The abdominal wall assists in breathing.
- It relaxes during inspiration to accommodate expansion of the thoracic cavity and the inferior
displacement of the abdominal viscera during contraction of the diaphragm.
- During expiration, it contracts to assist in elevating the domes of the diaphragm.
Material can be expelled from the airway by forced expiration using the abdominal muscles, in
coughing and sneezing.
Intraabdominal pressure
Contraction of abdominal muscles
increases intraabdominal pressure
when the diaphragm is in a fixed
position. Air is retained in the lungs by
closing valves in the larynx. Increased
intraabdominal pressure assists in
voiding the contents of the bladder
and rectum and in giving birth.
Component parts
Wall
The abdominal wall consists of muscle and bone. The skeletal elements of the wall are:
- The five lumbar vertebrae and the intervening intervertebral disks.
- The superior expanded parts of the pelvic bones.
- Bony components of the inferior thoracic wall, including the costal margin, rib XII, the end of rib XI,
and the xiphoid process.
Muscles make up the rest of the abdominal wall:
- The quadratus lumborum, psoas major, and iliacus muscles reinforce the posterior wall. The distal
ends of the psoas major and iliacus muscles pass into the thigh and are major flexors of the hip joint.
- Lateral parts of the abdominal wall are formed by three layers of muscles: Transversus abdominis,
internal oblique, and external oblique.
- Anteriorly, a segmented muscle (the rectus abdominis) on each side spans the distance between
the thoracic wall and the pelvis.
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