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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