The iron-containing compound in hemoglobin that carries oxygen in red blood cells, spelled 'heme' in American English.
From Greek 'haima' meaning 'blood.' The 'haem' spelling is British; American 'heme' is a shortened form. The prefix appears in words like hematology.
Your blood is red specifically because of haem's iron content—swap that iron for magnesium and you'd have green blood, which some marine worms actually use instead!
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