The heme-bound iron can be either reduced as Fe(II) or oxidized as Fe(III). When exposed to air, like when the blood passes through the lungs, or in cut meat,

The heme-bound iron can be either reduced as Fe(II) or oxidized as Fe(III). When exposed to air, like when the blood passes through the lungs, or in cut meat, it will be oxidized to Fe(III), in which case its color will be a pleasant vivid red. When the iron is reduced, the heme has a much duller color. The iron that can be absorbed or interact in any way with the intestine is either Fe(II), which is soluble in water or Fe(III) that is bound to proteins like hemoglobin or to other organic molecules. Otherwise, the Fe(III) ions form insoluble precipitates that pass through the intestine without effect.