<P> In the embryo, the inferior vena cava and right atrium are separated by the valve of the inferior vena cava, also known as the Eustachian valve . In the adult, this valve typically has totally regressed or remains as a small fold of endocardium . </P> <P> Rarely, the inferior vena cava may vary in its size and position . In transposition of the great arteries the inferior vena cava may lie on the left . It may be replaced by two vessels beneath the level of the renal veins . </P> <P> The inferior vena cava is a vein . It carries deoxygenated blood from the lower half of the body to the right atrium of the heart . </P> <P> The corresponding vein that carries deoxygenated blood from the upper half of the body is the superior vena cava . </P>

Where does the blood in the vena cava come from
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