The iron in your body is not a tiny nail hidden in your blood.

An adult’s body typically contains a few grams of iron. The NIH indicates an order of magnitude of three to four grams, most of which is found in hemoglobin, the red blood cell protein that participates in oxygen transport.

This iron is integrated into biological structures. It does not circulate in the form of filings or small metal fragments. The same chemical element can have very different properties and functions depending on how it is associated with other atoms.

The image of the nail contained in a body makes it possible to think of a mass, but becomes misleading if we take it at the foot of the letter. The real surprise is finer: an element that we associate with tools and constructions also intervenes, on a molecular scale, in the daily functioning of a living organism.