Why does the human body need iron?
Iron in haemoglobin is what actually binds to oxygen and drops it off, bringing back carbon dioxide.
Haemoglobin is the protein found in blood that carries blood around the body. But it's not a protein.
Technically it's a proteid, which is a molecule that is mostly made out of polypeptide, but has some impurities. Like iron.
The iron in haemoglobin is what actually bonds to oxygen and drops it off when it is needed. The iron group is actually called haem, and gives the proteid the first syllable of its name.