37. DESCENDING AORTA & COMMON ILIAC ARTERY

The descending aorta is the longest section of the aorta, running from the T4 vertebra (thoracic) to the T12 vertebra, where it becomes the abdominal aorta, descending to L4 before bifurcating into the right and left common iliac arteries. It supplies the torso, organs, and, through the iliac arteries, the pelvis and lower limbs. 

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Anatomy of the Descending Aorta

• Thoracic Portion (Descending Thoracic Aorta): Begins at the lower border of T4, descending on the left side of the vertebral column in the posterior mediastinum.
• Diaphragm Passage: Passes behind the heart and lungs, entering the abdomen through the aortic hiatus at the T12 vertebra.
• Branches (Thoracic): Intercostal arteries (supplying the ribs), bronchial arteries (lungs), and esophageal branches.
• Abdominal Portion (Abdominal Aorta): Continues from T12, descending anterior to the vertebral column until L4.

• Branches (Abdominal): Supplies abdominal organs via the celiac trunk, superior/inferior mesenteric arteries, renal arteries, and lumbar arteries. 

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Anatomy of the Common Iliac Artery

• Origin: The abdominal aorta terminates at the level of the L4 vertebra, branching into the right and left common iliac arteries.
• Course: These arteries pass downward and laterally, extending to the pelvic brim (level of the L5-S1 intervertebral disc).
• Termination: Each common iliac artery divides into two main branches:
  • Internal Iliac Artery: Supplies the pelvic walls, viscera, and gluteal region.

  • External Iliac Artery: Continues to become the femoral artery, providing the main blood supply to the lower limb. 

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Key Anatomical Relations

• Veins: The hemiazygos veins lie behind the thoracic aorta.
• Organs: The esophagus crosses anterior to the descending aorta.
• Muscles: The abdominal aorta runs between the crura of the diaphragm.
• Bifurcation: The common iliac arteries are located near the umbilicus and are in close proximity to the ureters.