Big Picture

The diaphragm forms the superior and much of the posterior border of the posterior abdominal wall. In addition, the psoas major, iliacus, and quadratus lumborum muscles form the posterior abdominal wall. These muscles function in respiration (diaphragm) as well as trunk and lower limb motion.

Muscles and Fascia

The muscles that form much of the structure of the posterosuperior abdominal wall are as follows (Figure 11-1):

• Diaphragm. A dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. The origin of the muscles of the diaphragm is along the internal circumference of the ribcage, sternum, and lumbar vertebrae. This portion of muscle consists of the esophageal hiatus, at the T10 vertebral level, and the aortic hiatus, at the T12 vertebral level. The muscle fibers are directed to the center of the diaphragm, to the central tendon. The central tendon consists of collagen tissue and the venal caval hiatus, which is at the T8 vertebral level. When the muscle is stimulated to contract (phrenic nerve, C1–C4), the muscle fibers shorten, causing the central tendon to move inferiorly and flatten. This action results in inspiration. The structural components of the diaphragm are as follows:
  
  
  
  • Right crus. Forms part of the aortic hiatus. The right crus also loops around the esophagus to form the esophageal hiatus and contributes to the suspensory ligament of the duodenum (ligament of Treitz).
    
  • Left crus. Forms part of the aortic hiatus.
  
  
• Quadratus lumborum muscle. Attaches to the iliac crest, lumbar transverse processes, and the 12th rib. The quadratus lumborum muscle is the bed on which the kidneys lie. The muscle laterally flexes the vertebral column and stabilizes the 12th rib during breathing. The subcostal and lumbar intercostal nerves provide innervation.
  
• Psoas major muscle. Attaches to the lumbar vertebrae superiorly and to the lesser trochanter of the femur inferiorly. Between these attachments, the psoas major muscle courses deep to the inguinal ligament and lateral to the femoral nerve. The psoas major muscle flexes the hip joint (when the vertebrae are stabilized) and the lumbar vertebrae. The psoas major muscle is innervated by the L1–L3 spinal nerves.

The appendix is in close relationship with the parietal peritoneum, including that covering the right psoas muscle. When the appendix is inflamed, the inflammation irritates the parietal peritoneum. The parietal peritoneum is innervated by somatic nerves, including pain fibers, and thus inflammation of the parietal peritoneum results in a “shooting” abdominal pain. To diagnose acute appendicitis, the physician will push on McBurney’s point when the patient is in a supine position. If the patient suffers from acute appendicitis, the pain reflex will flex the hip joint.

• Psoas minor muscle. Attaches to the L1 vertebra and pubic crest. The psoas minor muscle helps to tilt the pelvis and is innervated by the L1–L2 spinal nerves. This muscle is not present in everyone.
  
• Iliacus muscle. Attaches within the iliac fossa and lesser trochanter of the femur. Between its attachments, the iliacus muscle courses deep to the inguinal ligament and joins with the psoas major muscle to attach to the lesser trochanter of the femur. The combination of these two muscles in the thigh is often referred to as the iliopsoas muscle. The iliacus muscle flexes the hip. This muscle is innervated by the femoral nerve (L2–L3 spinal nerves).

Big Picture

The abdominal aorta and inferior vena cava course vertically in the retroperitoneal space, providing the vascular supply for the abdomen, pelvis, and perineum.

Abdominal Aorta

The aorta enters the abdomen from the thorax by traversing the aortic hiatus of the diaphragm at the T12 vertebral level. The aorta courses along the midline, on the anterior surface of vertebral bodies to the left of the inferior vena cava. The abdominal aorta has the following branches, from superior to inferior (Figure 11-2):

• Inferior phrenic arteries. The first paired branches of the aorta in the abdominal cavity. The inferior phrenic arteries supply the inferior surface of the diaphragm.
  
• Middle suprarenal arteries. One of three pairs of arteries supplying the adrenal glands.
  
• Gonadal arteries. Paired arteries that supply the gonads.
  
• Lumbar arteries. Usually, four pairs of arteries that supply the abdominal wall, similar to the intercostal arteries of the thorax.
  
• Celiac trunk. Unpaired artery that is located approximately at the L1 vertebral level. Supplies the foregut and organs associated with the foregut.
  
• Superior mesenteric artery. Unpaired artery that is located immediately below the celiac trunk. The superior mesenteric artery supplies the midgut.
  
• Inferior mesenteric artery. Unpaired artery that is located 4 to 5 cm superior to the bifurcation of the abdominal aorta into the common iliac arteries. The inferior mesenteric artery supplies the hindgut.
  
• Common iliac arteries. At the L4 vertebral level, the abdominal aorta bifurcates into the left and right common iliac arteries.

The aorta is the largest artery in the body and as such channels blood under high pressure. An abdominal aortic aneurysm (clinically referred to as an AAA) is a condition in which a section of the abdominal aorta expands or bulges, much like a balloon, in response to weakening of the vessel wall. An AAA can occur anywhere within the thoracic and abdominal aorta, but most occur inferior to the renal arteries. An AAA is a serious health condition because rupture of the abdominal aorta results in severe abdominal bleeding and is fatal within minutes. To prevent rupture, the weakened part of the aorta is often replaced with a tube-like replacement (aortic graft).

Inferior Vena Cava

The inferior vena cava is located to the right of the abdominal aorta. The union of the left and right common iliac veins forms the inferior vena cava. The inferior vena cava ascends along the right side of the vertebral bodies. Before entering the thoracic cavity, the inferior vena cava courses within a groove on the posterior surface of the liver. This portion of the inferior vena cava receives the hepatic veins. Along its course in the abdomen, the inferior vena cava receives the following tributaries:

• Right gonadal vein. Drains the right testis or ovary by entering the inferior vena cava, inferior to the right renal vein.
  
• Renal veins. Drain the kidneys. The gonadal veins are not bilaterally symmetrical. The left gonadal vein drains into the left renal vein, in contrast to the right gonadal vein.
  
  
  
  • Adrenal veins. Drain the adrenal glands, typically by entering the left and right renal veins.
  
  
• Inferior phrenic veins. Drain the inferior surface of the diaphragm.

The lumbar veins drain into a pair of ascending lumbar veins, which ascend posterior to the diaphragm to empty into the azygos system of veins in the thoracic cavity. Connections exist between the ascending lumbar veins and the inferior vena cava. Therefore, blood in the posterior abdominal wall may drain through the azygos vein or through the inferior vena cava. Remember, blood from the abdominal viscera drains through the hepatic portal system.

Blood from the lower limbs and the retroperitoneal organs drains into the inferior vena cava and ascends through the abdomen and into the thorax before entering the right atrium. In chronic thrombosis of the inferior vena cava, a blood clot decreases or obstructs blood flow. As a result, venous blood must flow via a different route on its return to the heart.