The iron in hemoglobin is divalent iron, which is the ferrous state, because only when the iron in hemoglobin is divalent iron, hemoglobin can reversibly combine with oxygen, and after combining with oxygen, transport oxygen to the parts of the tissues that need it, and then transport the carbon dioxide produced by the tissues to the lungs and expel it from the body. If the iron in hemoglobin is trivalent, the hemoglobin loses its ability to bind oxygen, and the body will suffer from ischemia and hypoxia. There is a clinical disease called enterogenic cyanosis, which is nitrite poisoning. As the divalent iron in the nitrite hemoglobin oxidizes to trivalent iron, the hemoglobin loses its ability to bind with oxygen, which causes tissue hypoxia, and the patient appears in a state of severe hypoxia, including obvious cyanosis of the lips and mouth. At this time, it is necessary to apply reducing agents to reduce trivalent iron to divalent iron, and the ability to carry oxygen is restored, and the commonly used drugs are methylene blue and vitamin C.