Nestled at the center of each of the proteins is another chemical: heme. And at the center of heme sits an atom of iron. It is a doll-within-a-doll-within-a-doll scheme. Red blood cells contain hemoglobin molecules that contain heme that, in turn, clasps iron atoms. It’s the iron that binds and unbinds the oxygen. The elaborate apparatus built around those four iron atoms in the hemoglobin molecule has a distinct molecular purpose. Red blood cells cannot simply bind oxygen and hold on to it; they have to release it. The red cells pick their payload—oxygen—from the capillaries of the lung and
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