Space
Where did the iron in your blood come from?
Every atom of iron in your blood was made inside stars that died before the Sun existed. Stars like the Sun cannot make iron at all.
Source: Burbidge, Burbidge, Fowler & Hoyle (1957), Reviews of Modern Physics.
Context
From stars that died before the Sun formed. Fusion in a star's core can build elements up to iron, but fusing iron absorbs energy instead of releasing it, so a star cannot make heavier elements that way, and only the most massive stars get as far as iron at all.
The Sun will never produce iron. The atoms in your haemoglobin were scattered into space by earlier generations of stars and later collected into the cloud that became the solar system. This is the picture laid out by Burbidge, Burbidge, Fowler and Hoyle in 1957.
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