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The left superior intercostal vein drains complimentary regions as the right superior intercostal vein. The left superior intercostal vein joins either the accessory hemiazygos vein or the left brachiocephalic vein. For additional clarification, consult your reference textbook. The azygos, hemiazygos and accessory hemiazygos veins form a longitudinal venous system which can provide an alternate route for blood return to the heart if the inferior vena cava is blocked.

Links and References:
Grant's: 1.86, 1.87
Netter: 226
Rohen/Yokochi: 261