The Posterior Abdominal Wall
The posterior abdominal wall lies at the back of the abdomen, behind the posterior peritoneum.
With the anterior structures removed (the stomach, jejunum and ileum) the retroperitoneal parts
of the gut tube can be more easily identified.
The duodenum
The first part of the duodenum continues on from the pylorus of the stomach suspended in a
mesentery. As the duodenum changes direction it becomes retroperitoneal in its descending
second part and transverse third part. On the left the third part of the duodenum passes anteriorly
and cranially to form the fourth part of the duodenum, continuous with the jejunum. The fourth
part of the duodenum is suspended in a fold of pertitoneum, the ligament of Trietz. The first and
second parts of the duodenum receive their blood supply from the anterior and posterior
pancreaticoduodenal arteries which branch from the gastroduodenal artery. The third and fourth
parts of the duodenum receive their arterial supply from the anterior and posterior inferior
pancreaticoduodenal arteries. The pancreaticoduodenal arteries form an anastomoses between the
celiac trunk and superior mesenteric artery. The portal vein is formed behind the neck of the
pancreas and passes to the porta hepatis behind the first part of the duodenum. The superior
mesenteric artery and vein lie anterior to the third part of the duodenum.
The colon
The ascending colon is retroperitoneal. The ileum enters the cecum at the ileocolic valve. The
appendix is attached to the cecum and may lie in a variety of positions, including behind the
cecum and within the pelvis. At the hepatic flexure the cecum is suspended in a mesentery as the
transverse colon. The greater omentum is draped over the transverse colon. The descending
colon begins at the splenic flexure, becoming retroperitoneal. As the descending colon reaches
the iliac fossa it forms the sigmoid colon which has a short mesentery. The sigmoid colon moves
towards the midline to form the rectum in the pelvis. The ileocolic artery, a terminal branch of the
superior mesenteric artery, supplies the terminal ileum, caecum and appendix. A branch from the
ileocolic or superior mesenteric, the right gastric, supplies the ascending colon. An early branch
from the superior mesenteric, the middle colic, supplies the transverse colon. The inferior
mesenteric artery supplies the descending and sigmoid colon and the rectum. The ascending left
colic branch of the inferior mesenteric runs upwards to the splenic flexure and forms an
anastomosis with the middle colic artery through the formation of the marginal artery.
The abdominal aorta
The aorta passes into the abdomen from the thorax in the midline lying on the vertebral bodies.
The crura of the diaphragm form an opening so that the aorta passes behind the diaphragm under
the median arcuate ligament. The aorta gives off four pairs of lumbar arteries that supply the
abdominal wall (similar to the intercostal arteries of the thorax). Four other pairs are also given
off: The inferior phrenic arteries supplying the diaphragm; the middle suprarenal arteries; the renal
arteries; the gonadal arteries. There are three unpaired arteries which arise from the anterior
aorta: the celiac trunk; the superior mesenteric artery; the inferior mesenteric artery. At the lower
border of the L4 lumbar vertebra the aorta bifurcates into the common iliac arteries.
The inferior vena cava
The IVC lies on the right side of the aorta. The IVC is formed by the left and right common iliac
veins which lie behind the right common iliac artery. The vein ascends the abdomen on the right
side of the lumbar vertebrae and receives the right gonadal vein(s), the renal veins (the left
gonadal vein drains into the left renal vein), the adrenal veins, the inferior phrenic veins. The IVC
passes behind the liver being clasped in a groove at the back of the liver by the caudate lobe.
Within the groove the IVC receives the hepatic veins. The lumbar veins drain into a pair of
ascending lumbar veins which pass behind the diaphragm to become the azygos venous system in
the thorax. There are connections between the ascending lumbar veins and the IVC so that blood
from the azygos/ascending lumbar system can either drain through the azygos vein into the
superior vena cava or through the connections to the inferior vena cava. (Note: all of the
intra-abdominal gut drains through the hepatic portal vein.)
The adrenal glands
The adrenal glands lie on the superior pole of each kidney, with the left adrenal gland lying more
on the medial aspect. The glands are separate from the capsule of the kidney. Each gland
receives arterial supply from three sources: the renal artery; aorta; inferior mesenteric artery. As
these branches reach the gland they break up into many small vessels. Venous drainage of the
right adrenal is directly to the IVC. Venous drainage of the left adrenal is to the left renal vein.
Pancreas
The pancreas is for the most part retroperitoneal but becomes suspended in a mesentery (the
lienorenal ligament) as the tail reaches the hilum of the spleen. The uncinate process, head and
neck of the pancreas lie within the curvature of the duodenum. The pancreatic ducts drain into
the duodenum. The main pancreatic duct drains the tail, body, uncinate process and part of the
head. In the head the main pancreatic duct joins the bile duct to form the ampulla of Vater to
drain into the second part of the duodenum. The sphincter of Oddi controls flow into the
duodenum through the major duodenal papilla. The accessory pancreatic duct drains part of the
head, either joining the main pancreatic duct or entering the duodenum separately as the minor
duodenal papilla. The portal vein is formed behind the neck of the pancreas. The superior
mesenteric artery and vein lie anterior to the uncinate process. The splenic artery supplies the
body and tail of the pancreas. The neck and head of the pancreas are supplied by the anterior and
posterior superior pancreaticoduodenal arteries which branch from the gastroduodenal artery.
The uncinate process and part of the head are supplied by the anterior and posterior inferior
pancreaticoduodenal arteries which arise from the superior mesenteric artery. The pancreatic
veins drain into the portal vein.
The sympathetic system
The lumbar sympathetic trunk is a continuation of the sympathetic chain in the thorax.
The splanchnic nerves arise from the thoracic sympathetic chain as preganglionic fibres. The
greater splanchnic nerve arises from T5-T9 and passes into the abdomen through the crura of the
diaphragm to synapse in the celiac ganglia. The celiac ganglia lie on either side of the celiac trunk
and send postganglionic fibres with all of the branches of the celiac trunk. Similarly the lesser
splanchnic (T10,11) and the least splanchnic nerves (T12) arise as preganglionic fibres in the
thorax, pass to the abdomen where they synapse in preaortic ganglia (celiac, superior mesenteric,
inferior mesenteric ganglia) before being distributed with the arteries to the tissues. A preaortic
plexus of autonomic fibres is formed by sympathetic fibres from the preaortic ganglia and from the
lumbar sympathetic chain, and by parasympathetic fibres from the vagus and S2,3,4. The pelvic
organs receive their autonomic innervation from the superior and inferior hypogastric plexuses
formed by extension of the preaortic plexus into the pelvis.
The muscles
The muscles of the posterior abdominal wall are the psoas, quadratus lumborum and the iliacus.
The psoas muscle arises from the sides of the upper lumbar vertebrae and the intervertebral discs.
The muscle runs downwards into the pelvis and out again under the inguinal ligament. It inserts
into the lesser trochanter of the femur in common with the iliacus muscle. The psoas is innervated
by the L2,3,4 lumbar nerves. The muscle is enclosed within the psoas fascia, a compartment
which may limit the spread of a psoas abscess. The psoas muscle flexes the hip, or flexes the
lumbar spine. Several structures such as the kidney and ureter, gonadal vessels, appendix and
lumbar nerves have a close relationship to the muscle. Patients attempt to immobilize the psoas
muscle when there is pain from any of these structures. This is accomplished by drawing the
knees upwards passively.
The quadratus lumborum muscle arises from the medial half of the twelfth rib and inserts into the
iliac crest. It forms a bed for the kidney. It is innervated by the T12 and lumbar nerves. Its
action is to fix the twelfth rib during inspiration.
The iliacus muscle arises from the iliac fossa in the pelvis. It runs below the inguinal ligament to
insert together with psoas into the lesser trochanter. It is innervated by the femoral nerve.
The nerves
The nerves of the posterior abdominal wall are the subcostal (T12), the
ilioinguinal and iliohypogastric (L1), the lateral cutaneous nerve of the
thigh (L2,3) and the femoral (L2,3,4) emerging from the lateral side of
the psoas muscle; the obturator nerve (L2,3,4) emerging in the pelvis
from the medial side of the psoas muscle; the genitofemoral nerve(l1,2)
emerging through the psoas muscle onto its anterior surface.
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