The azygos vein drains into the superior vena cava by arching over the root of the right lung. Its clinical importance lies in its role as a collateral pathway. If the SVC is obstructed above the azygos vein entry, which statement is TRUE?

• A Azygos vein cannot serve as a bypass in SVC obstruction
• B Azygos vein becomes dilated and diverts blood from the lower half of the body to the SVC below the obstruction — SVC syndrome may be bypassed ✓
• C SVC obstruction above azygos entry causes severe facial and upper limb edema, but azygos provides no decompression
• D Azygos vein carries blood only from the right thoracic wall

Explanation

The azygos vein enters the SVC just above the pericardial reflection. In SVC obstruction above the azygos entry, the azygos and hemiazygos system provides a collateral pathway: blood from the upper body drains into the azygos vein (via thoracic wall and oesophageal veins), then into the SVC below the obstruction, bypassing the block. If the obstruction is below azygos entry, this bypass is unavailable and SVC syndrome is more severe. The azygos also communicates with the lumber azygos, inferior vena cava, and vertebral venous plexus.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.