Vessels of Posterior Abdominal Wall

The major neurovascular bundle of the inferior trunk, including the abdominal aorta, the inferior vena cava, and the aortic peri-arterial nerve plexus, courses in the midline of the posterior abdominal wall, anterior to the bodies of the lumbar vertebrae.

Abdominal Aorta

Most arteries supplying the posterior abdominal wall arise from the abdominal aorta. The subcostal arteries arise from the thoracic aorta and distribute inferior to the 12^th rib. The abdominal aorta is approximately 13 cm in length. It begins at the aortic hiatus in the diaphragm at the level of the T₁₂ vertebra and ends at the level of the L₄ vertebra by dividing into the right and left common iliac arteries. The abdominal aorta may be represented on the anterior abdominal wall by a band (approximately 2 cm wide) extending from a median point, approximately 2.5 cm superior to the transpyloric plane to a point slightly (2-3 cm) inferior to and to the left of the umbilicus at the level of the supracristal plane (plane of the highest points of the iliac crests). In children and lean adults, the lower abdominal aorta is sufficiently close to the anterior abdominal wall that its pulsations may be detected or apparent when the wall is relaxed.

The common iliac arteries diverge and run inferolaterally, following the medial border of the psoas muscles to the pelvic brim. Here each common iliac artery divides into the internal and external iliac arteries. The internal iliac artery enters the pelvis. The external iliac artery follows the iliopsoas muscle. Just before leaving the abdomen, the external iliac artery gives rise to the inferior epigastric and deep circumflex iliac arteries, which supply the anterolateral abdominal wall.

Relations of Abdominal Aorta

From superior to inferior, the important anterior relations of the abdominal aorta are the:

• Celiac plexus and ganglion.

• Body of the pancreas and splenic vein.

• Left renal vein.

• Horizontal part of the duodenum.

• Coils of small intestine.

The abdominal aorta descends anterior to the bodies of the T₁₂-L₄ vertebrae. The left lumbar veins pass posterior to the aorta to reach the IVC. On the right, the aorta is related to the azygos vein, cisterna chyli, thoracic duct, right crus of the diaphragm, and right celiac ganglion. On the left, the aorta is related to the left crus of the diaphragm and the left celiac ganglion.

Branches of the Abdominal Aorta

The branches of the descending (thoracic and abdominal) aorta may be described as arising and coursing in three "vascular planes" and can be classified as being visceral or parietal and paired or unpaired. Paired parietal branches of the aorta serve the diaphragm and posterior abdominal wall.

The median sacral artery, an unpaired parietal branch, may be said to occupy a fourth (posterior) plane because it arises from the posterior aspect of the aorta just proximal to its bifurcation. Although markedly smaller, it could also be considered a midline "continuation" of the aorta, in which case its lateral branches, the small lumbar arteries and lateral sacral branches, would also be included as part of the paired parietal branches.

Veins of Posterior Abdominal Wall

The veins of the posterior abdominal wall are tributaries of the IVC, except for the left testicular or ovarian vein, which enters the left renal vein instead of entering the IVC. The IVC, the largest vein in the body, has no valves except for a variable, non-functional one at its orifice in the right atrium of the heart. The IVC returns poorly oxygenated blood from the lower limbs, most of the back, the abdominal walls, and the abdominopelvic viscera. Blood from the abdominal viscera passes through the portal venous system and the liver before entering the IVC via the hepatic veins.

The inferior vena cava (IVC) begins anterior to the L₅ vertebra by the union of the common iliac veins. The union occurs approximately 2.5 cm to the right of the median plane, inferior to the aortic bifurcation and posterior to the proximal part of the right common iliac artery. The IVC ascends on the right side of the bodies of the L₃-L₅ vertebrae and on the right psoas major to the right of the aorta. The IVC leaves the abdomen by passing through the caval opening in the diaphragm and enters the thorax at the T₈ vertebral level. Because it is formed one vertebral level inferior to the aortic bifurcation, and traverses the diaphragm four vertebral levels superior to the aortic hiatus, the overall length of the IVC is 7 cm greater than that of the abdominal aorta, although most of the additional length is intrahepatic. The IVC collects poorly oxygenated blood from the lower limbs and non-portal blood from the abdomen and pelvis. Almost all the blood from the gastrointestinal tract is collected by the hepatic portal system and passes through the hepatic veins to the IVC.

The tributaries of the IVC correspond to the paired visceral and parietal branches of the abdominal aorta. The veins that correspond to the unpaired visceral branches of the aorta are instead tributaries of the hepatic portal vein. The blood they carry does ultimately enter the IVC via the hepatic veins, after traversing the liver.

The branches corresponding to the paired visceral branches of the abdominal aorta include the right suprarenal vein, the right and left renal veins, and the right gonadal (testicular or ovarian) vein. The left suprarenal and gonadal veins drain indirectly into the IVC because they are tributaries of the left renal vein.

Paired parietal branches of the IVC include the inferior phrenic veins, the 3^rd (L₃) and 4^th (L₄) lumbar veins, and the common iliac veins. The ascending lumbar and azygos veins connect the IVC and SVC, either directly or indirectly providing collateral pathways.

Lymphatic Vessels and Lymph Nodes of Posterior Abdominal Wall

Lymphatic vessels and lymph nodes lie along the aorta, IVC, and iliac vessels. The common iliac lymph nodes receive lymph from the external and internal iliac lymph nodes. Lymph from the common iliac lymph nodes passes to the right and left lumbar lymph nodes. Lymph from the alimentary tract, liver, spleen, and pancreas passes along the celiac and superior and inferior mesenteric arteries to the pre-aortic lymph nodes (celiac and superior and inferior mesenteric nodes) scattered around the origins of these arteries from the aorta. Efferent vessels from these nodes form the intestinal lymphatic trunks, which may be single or multiple, and participate in the confluence of lymphatic trunks that gives rise to the thoracic duct.

The right and left lumbar (caval and aortic) lymph nodes lie on both sides of the IVC and aorta. These nodes receive lymph directly from the posterior abdominal wall, kidneys, ureters, testes or ovaries, uterus, and uterine tubes. They also receive lymph from the descending colon, pelvis, and lower limbs through the inferior mesenteric and common iliac lymph nodes. Efferent lymphatic vessels from the large lumbar lymph nodes form the right and left lumbar lymphatic trunks.

The inferior end of the thoracic duct lies anterior to the bodies of the L₁ and L₂ vertebrae between the right crus of the diaphragm and the aorta. The thoracic duct begins with the convergence of the main lymphatic ducts of the abdomen, which in only a small proportion of individuals takes the form of the commonly depicted, thin-walled sac or dilation, the cisterna chyli (chyle cistern). Cisterna chyli vary greatly in size and shape. More often there is merely a simple or plexiform convergence at this level of the right and left lumbar lymphatic trunks, the intestinal lymph trunk(s), and a pair of descending thoracic lymphatic trunks, which carry lymph from the lower six intercostal spaces on each side. Consequently, essentially all the lymphatic drainage from the lower half of the body (deep lymphatic drainage inferior to the level of the diaphragm and all superficial drainage inferior to the level of the umbilicus) converges in the abdomen to enter the beginning of the thoracic duct.

The thoracic duct ascends through the aortic hiatus in the diaphragm into the posterior mediastinum, where it collects more parietal and visceral drainage, particularly from the left upper quadrant of the body. The duct ultimately ends by entering the venous system at the junction of the left subclavian and internal jugular veins (the left venous angle).