system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - how doesn’t iron burst through blood vessels during an MRI?

There are metals in the body such as iron. Even tiny bits of iron ripping through vessels and organs I would think would do major damage to the body. How does an MRI not cause this to happen? Or does it?
gpt
What makes iron magnetic is that when iron is in a crystalline form (what you think of as iron metal), the electrons in the individual iron atoms align themselves in a way that interacts with magnetism. (Basically electrons (and many other subatomic particles) have a property called spin, when a bunch of electrons are spinning in the same direction, magnetism happens.)

Iron in blood is not in a metallic form - it takes the form of a molecule called hemoglobin. Hemoglobin is essentially a tangled chain of other stuff, with pockets holding single iron atoms throughout. Because those iron atoms are isolated and tangled up in this hemoglobin structure, they can't align like they do in metallic iron. As a result, hemoglobin isn't magnetic at all, and as such is not pulled around by MRI magnets.