Agenesis of the inferior vena cava is associated with multiple congenital heart defects. Embryologically, it arises because the right subcardinal vein does not join the hepatic section of the inferior vena cava. Instead, it fuses with the right supracardinal vein. Hence, at birth the inferior vena cava flows into the azygous system and thence the superior vena cava.
The hepatic section of the inferior vena cava, derived from the embryonic vitelline veins, enters the right atrium in a similar fashion to a normal inferior vena cava.
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