Posterior Abdominal Wall and Retroperitoneal Organs

• The posterior abdominal wall is formed by the structures which lie behind the peritoneal cavity. The posterior abdominal wall is lined with parietal peritoneum, and beneath the parietal peritoneum is a layer of fat known as retro-peritoneal/extra-peritoneal fat.
• From the posterior abdominal wall, the peritoneum is folded forwards to form ligaments and mesenteries. The ligaments are the gastrophrenic, gastrohepatic and the splenorenal ligaments. The mesenteries are the transverse mesocolon, the mesentery and the sigmoid mesocolon.
• The posterior abdominal wall contains the retroperitoneal organs:

  • duodenum (although the 1st part of the duodenum is intraperitoneal).
  • pancreas
  • ascending and descending colon
  • sometimes caecum (although sometimes it's wrapped in visceral peritoneum and hence, intraperitoneal).
  • rectum
  • kidneys and suprarenal glands.
• The diaphragm forms the roof of the abdominal cavity, arching over the posterior abdominal wall muscles, and forming the medial and lateral arcuate ligaments.
• The space between the lumbar fascia and vertebral column is occupied by the posterior abdominal wall muscles: quadratus lumborum and psoas major (and minor).
• The most lateral of the posterior abdominal wall muscles is the quadratus lumborum.

  Quadratus lumborum arises inferiorly from the posterior third of the iliac crest and the lumbar transverse processes. It inserts superiorly onto the 12th rib.

  It is important in the depression of the 12th rib during breathing, which maximises the contraction of the diaphragm.

  It is innervated by the anterior primary rami of T12-L3.
• The medial muscle is the psoas major which flexes the thigh on the trunk, or trunk on thigh.

  It arises from the lumbar transverse processes and lumbar vertebral bodies. It attaches onto the lesser trochanter of the femur alongside the iliacus muscle (located on the iliac fossa).

  Psoas major is supplied by the anterior primary rami of L1-L3. Occasionally there is a psoas minor present.
• Endo-abdominal fascia lies anterior to quadratus lumborum and psoas major.

  The psoas major muscle lies within a fascial sheath in close approximation to many abdominal viscera. As a result, should any of these visceral organs become inflamed or infected, psoas major may become irritated.

  Enlargement of psoas major can also compress the nerves (from the lumbar plexus) that pass around and through the muscle.
• Infections of the posterior abdominal wall can track along the psoas fascia. The pus from such infections collects inside the psoas sheath and this can be seen as a swelling at the top of the thigh, mimicking a femoral hernia. Patients with lumbar tuberculosis (Pott’s disease) may not only develop a swelling at the distal end of the psoas sheath, but if enough pus collects there, also demonstrate a swelling in the lower abdomen. This pus will irritate the psoas muscle and cause it to go into a spasm, producing flexion of the hip and giving an excruciating pain.
• The lumbar plexus lies in the substance of psoas major and is formed from the anterior rami of L1-L4, but also usually from T12. Branches of the lumbar plexus emerge through or around psoas major.
• The branches of the lumbar plexus are:

  • iliohypogastric and ilioinguinal branches from L1
  • genitofemoral branch from L1 or L2
  • lateral femoral cutaneous branch from L2 and L3
  • obturator branch from L2-L4
  • lumbosacral trunk from L4 and L5 (this branch is also considered to be part of the sacral plexus).

  In addition to these branches, there may also be an accessory obturator nerve from L3 or L4, and other branches to the adjacent psoas and quadratus lumborum from L1-L3.
• The iliohypogastric branch is cutaneous to the region of the upper lateral gluteal region and mons pubis. The ilioinguinal nerve is sensory to the anterior skin of the external genitalia and upper medial thigh.

  Both these branches usually arise from L1, although this is variable, and both are motor to the muscles of the lower part of the anterior abdominal wall, but this too, is variable.
• The genitofemoral nerve arises from L1 and L2 and this nerve pierces the psoas muscle before dividing into a femoral branch containing the L1 fibres and a genital branch containing the L2 fibres.

  The femoral branch supplies a small region of skin over the femoral triangle. The genital branch supplies the cremaster muscle and dartos muscle in the spermatic cord in males, and the skin of the labia majora in females.
• The abdominal aorta lies mostly in the midline. It enters the abdomen under the median arcuate ligament at the level of T12 and ends at the level of L4 (roughly at the level of the umbilicus) by dividing into the common iliac arteries.

  The inferior vena cava begins in front of the body at L5 and ascends to the right of the aorta, to pierce the central tendon of the diaphragm.
• As the aorta descends through the diaphragm, it becomes known as the abdominal aorta and gives off both visceral and parietal branches. The visceral branches supply the organs whilst the parietal branches supply the body wall.

  The abdominal aorta terminates at the level of the umbilicus into the left and right common iliac arteries. These arteries in turn divide into the internal iliac artery (which supplies the pelvis and perineum) and an external iliac artery (which supplies the lower limb).

  The abdominal aorta can be visualized using contrast angiography, either using X-rays or CT.
• The parietal branches of the abdominal aorta are:

  • left and right inferior phrenic arteries (these run underneath the diaphragm).
  • 4 pairs of lumbar arteries
  • median sacral arteries (this is a single artery without a pair)
• The visceral branches of the abdominal aorta are:

  • middle suprarenal arteries (emerge at the level of L1)
  • renal arteries (emerge at the level of L1 or L2, and supply the kidneys)
  • gonadal arteries (emerge at the level of L2, and supply the testes and ovaries in males and females respectively).
  • coeliac trunk (emerges at the level of T12 & supplies the foregut)
  • superior mesenteric artery (emerges at the level of L1 & supplies the midgut)
  • inferior mesenteric artery (emerges at the level of L3 & supplies the hindgut)
• The middle suprarenal arteries, renal arteries, and gonadal arteries are all paired visceral branches.

  The coeliac trunk, superior mesenteric artery, and inferior mesenteric artery are all unpaired visceral branches. These arteries are also known as pre-aortic branches because they arise from the anterior surface of the abdominal aorta.
• The foregut is drained principally by the splenic vein, the midgut by the superior mesenteric vein and the hindgut by the inferior mesenteric vein. Ultimately, these three veins come together to form the hepatic portal vein, which takes the blood (rich in nutrients) to the liver. The liver then sends the blood back to the systemic circulation via the hepatic veins, which then drain into the inferior vena cava.
• As the inferior vena cava ascends through the abdomen, it receives tributaries that correspond to the branches of the abdominal aorta.

  However, the left suprarenal vein and left gonadal vein join the inferior vena cava by first joining the left renal vein. (Clinically, this means that a clot in the left renal vein will cause venous pressure to increase not just in the left renal vein, but also in the left suprarenal and left gonadal veins).
• Venous drainage of the digestive tract, pancreas and spleen is first through the hepatic portal vein, then through the capillaries/sinusoids of the liver, and then through the hepatic veins. The hepatic veins then join the inferior vena cava.
• The 4 pairs of lumbar veins are tributaries of the inferior vena cava, but they also communicate with each other just lateral to the vertebral column. As each of the communicating veins link up, they form the ascending lumbar vein on each side of the vertebral column.

  The ascending lumbar veins are continuous with the azygos and hemiazygos veins in the posterior mediastinum. The azygos and hemiazygos veins begin at the junction of the ascending lumbar vein and the subcostal vein, and pass deep to the left and right crus of the diaphragm to enter the posterior mediastinum.
• The ascending lumbar veins provide a channel that connects the inferior vena cava to the superior vena cava, as the azygos system of veins drains into the SVC.

  This connection is important in cases of caval blockage. If the IVC becomes blocked, then blood can reach the SVC by using this shunt, and eventually enter the heart. Similarly, if the SVC become blocked, blood can flow down this connection towards the IVC. This is permitted, since there are no valves in these veins.
• The lymphatic drainage of the abdomen follows the pattern of the arteries. There are groups of lymph nodes arranged around the origin of the arteries of the gut, and these are known as pre-aortic nodes. The pre-aortic nodes are the coeliac nodes, superior mesenteric nodes, and inferior mesenteric nodes.

  The paired organs have lymph which drains to nodes alongside the abdominal aorta, and these are known as para-aortic nodes, as they lie alongside the aorta. The para-aortic nodes are also referred to as the left and right lumbar nodes.
• The lymphatic vessels from the para-aortic lymph nodes coalesce to form the left and left lumbar trunks, and these terminate in the cysterna chyli, which is a large lymph sac near the aortic opening in the diaphragm.

  The cysterna chyli also receives the intestinal trunk from the pre-aortic nodes.

  From the cysterna chyli, the lymph travels through the aortic opening via the thoracic duct.
• The kidneys are paired organs, 10-13cm in length, 6cm deep, and 3cm wide. The left kidney is slightly larger than the right kidney.

  The kidneys take 25% of the cardiac output and filter the blood plasma, removing about 1% of it as urine.
• Functions of the kidney include:

  • urinary secretion
  • fluid, electrolyte, and acid-base balance
  • vitamin D metabolism
  • renin and erythropoietin production
• The urinary system develops from a ridge composed of intermediate mesoderm. This extends along the posterior wall of the abdominal cavity and grows in a cranial to caudal direction. The distal end eventually enters a cavity called the cloaca. There are 3 kidney systems that develop with one disappearing as another begins.
• Between day 20 to 25 of intrauterine life there is the formation of the pronephros. On day 25, as the pronephric duct proceeds in cranialcaudal direction it induces intermediate mesoderm in the thoracolumbar area to become mesonephric tubules. From day 35 the metanephros develops. This becomes the final functional renal system.
• The metanephric diverticulum arises close to the distal end of the mesonephric duct before it enters the cloaca. This diverticulum is then surrounded by the metanephric cap. The metanephric diverticulum gives rise to the ureter, renal pelvis, major and minor calyces, and the collecting tubules.
• The final metanephric kidneys begin their development within the pelvis. With foetal growth, the kidneys migrate upwards within the abdomen in week 6 to week 10, and the mesonephros is regressing from its cranial end. Meanwhile the metanephric diverticulum splits from the mesonephric duct to enter into the cloaca separately. The arterial supply to the kidney starts in pelvis, fed by branches of the common iliac arteries.
• During migration of the kidneys from the pelvis to the abdomen, the aorta forms new branches at an appropriate level, and these replace the more caudal branches. The mesonephros has now completely disappeared, but the mesonephric duct has been utilised by the gonad. Migration halts on reaching the suprarenal glands. The mesonephric artery at that point remains as the renal artery.
• Renal anomalies include:

  • duplex kidney (the kidney has two hila)
  • polycystic kidney (one or both kidneys are enlarged and bubbly in appearance due to the presence of cysts).
  • horseshoe kidney (the lower lobes of the kidneys are fused together, creating a bridge known as the isthmus).
• The kidneys lie in deep furrows at the side of the vertebral column, adjacent to the T12 to L3 vertebrae. The kidneys are tilted forward because they are connected to vessels; the hilum is more anteriorly placed than the body of the kidney.
• Each kidney has:

  • an anterior and posterior surface
  • a lateral margin
  • a medial hilum
  • a superior and inferior pole, with the superior pole being more medially positioned than the inferior pole
• The upper parts of the kidneys are protected by the ribs. However, the left kidney is higher than the right kidney due to the liver. As a result, the left kidney can reach the 11th rib, but the right kidney only reaches the 12th rib.

  The right kidney has its lower end opposite a plane drawn through the spine of the L3 vertebra. The ureters descend on the posterior wall anterior to the psoas major muscle.

  On contrast X-ray, the ureters can be seen close to the tips of the transverse processes of the lumbar vertebrae.
• The centres of the hila of the two kidneys lie about 5 cm from the median plane, the left and right respectively above and below the transpyloric plane (this plane passes through the tips of the 9th costal cartilage at the costal margin).
• Relations of the right kidney:

  • superior pole is in contact with the right suprarenal gland.
  • the anterior boarder of the superior pole is in contact with the bare area of the liver
  • the medial boarder is in contact with the descending duodenum
  • the inferior pole is in contact with the right colic flexure/hepatic flexure of the colon
• Relations of the left kidney:

  • superior pole is in contact with the left suprarenal gland
  • the anterior border is in contact with the stomach and spleen
  • medial border in contact with the tail of the pancreas
  • the inferior pole in contact with the left colic flexure/splenic flexure of the colon
• Muscular relations of the kidney:

  • superior poles of the kidneys are in contact with the diaphragm
  • laterally, the kidneys lie on transversus abdominis
  • medially, the kidneys, renal pelvis, and ureter are associated with psoas major.
  • the middle part of the kidney sits on quadratus lumborum