Red blood cells have hemoglobin proteins in them; 95% of RBC’s dry matter is hemoglobin. Hemoglobin is a tetramer protein with iron cofactors in it. The iron enables the RBC hemoglobin to carry oxygen. Oxygen oxidized the iron for Fe2+ to Fe3+ and is carried to the oxygen-deficient region in the body. The oxygen is released and the Fe3+ reverts to Fe2+. The hemoglobin changes conformity when it binds oxygen.