We know that the main component of our red blood cells is hemoglobin, and each red blood cell hemoglobin has 4 iron ions, which are all Fe2+ , called ferrous ions. Our body’s red blood cells are only able to bind oxygen when the iron ions are Fe2+ , that is to say, it is able to carry oxygen to the tissues and oxygenate the tissues, which is the function of our hemoglobin. The so-called methemoglobin means that in some cases the ferrous ion in hemoglobin, Fe2+, is oxidized to Fe3+, and this time it is called methemoglobin. What is the harm of methemoglobin? Fe3+ cannot carry oxygen to supply our tissues with the oxygen they need, so the oxygen-carrying function of methemoglobin is gone. Clinical common cause of methemoglobin disease, such as nitrite poisoning, we all know that nitrite is a strong oxidizing agent, can oxidize Fe2+ in hemoglobin into Fe3+, so that hemoglobin loses the ability to carry oxygen and lead to the human body’s toxic reaction.