The aorta enters the abdomen behind the median arcuate ligament of the diaphragm at the level of the twelfth thoracic vertebra. It descends behind the peritoneum, inclining slightly to the left of the midline to its bifurcation in front of the fourth lumbar vertebra (Fig. 4.89). Throughout its course, the abdominal aorta is accompanied by lymph vessels and nodes and is surrounded by a plexus of autonomic nerves (pp 196–199).
Posterior to the aorta lie the left lumbar veins, the anterior longitudinal ligament and the lumbar vertebral bodies. Anterior relations, from above downwards, include the body of the pancreas, the splenic and left renal veins, the horizontal duodenum, the root of the mesentery and coils of small intestine.
To the right of the aorta lie the right crus of the diaphragm and the inferior vena cava (Fig. 4.89). On the left are the left diaphragmatic crus, suprarenal gland and kidney.
Degenerative arterial disease often affects the abdominal aorta, which may become narrowed or may dilate (abdominal aortic aneurysm) and rupture causing fatal internal haemorrhage.
Three arteries arise from the anterior aspect of the aorta to supply the alimentary organs, while three pairs of lateral branches pass to the suprarenals, kidneys and gonads, respectively.
The coeliac and superior mesenteric arteries arise at the levels of the twelfth thoracic and first lumbar vertebrae, respectively, while the smaller inferior mesenteric artery takes origin at the level of the third lumbar vertebra (Fig. 4.89).
Details of the course and distribution of each of these arteries are given on pp 161, 177, 182.
The suprarenal branches of the aorta (Fig. 4.89) are small and variable and are often supplemented by branches from the inferior phrenic and renal arteries (Figs 4.86 & 4.92). The renal arteries usually arise at the level of the first lumbar vertebra but variations are common (p. 189).
The long and slender gonadal (testicular or ovarian) arteries usually arise from the aorta just below the renal arteries (Fig. 4.89). The gonadal artery on the right crosses in front of the inferior vena cava, and the arteries of both sides incline downwards and laterally through the retroperitoneal tissues on the psoas major muscles (Fig. 4.88). Each artery is accompanied by one or more gonadal veins and crosses anterior to the ureters but posterior to blood vessels supplying the intestines. Inferiorly, the testicular artery follows the psoas major round the pelvic brim and enters the deep inguinal ring (Fig. 4.23). The ovarian artery crosses the external iliac vessels and pelvic brim to reach the ovary within the pelvis (p. 221).
These vessels supply the diaphragm and the posterior walls of the abdomen and pelvis. The paired inferior phrenic arteries are the first branches of the aorta within the abdomen (Figs 4.86, 4.89 & 4.92) and often furnish small branches to the suprarenal glands before arching upwards and laterally on the abdominal surface of the diaphragm.
Four pairs of lumbar arteries usually arise from the posterolateral aspect of the aorta and supply the posterior abdominal wall (Figs 4.88, 4.90 & 4.103). The arteries on the right cross behind the inferior vena cava to curve round the side of the corresponding vertebral body deep to psoas major.
A single small vessel, the median sacral artery, arises from the back of the aortic bifurcation and descends behind the left common iliac vein to reach the anterior surface of the sacrum (Fig. 4.90).