Each hemoglobin protein is made up subunits called haemes, which are what give blood its red color. More specifically, the haemes can bind iron molecules, and these iron molecules bind oxygen. The blood cells are red because of the interaction between iron and oxygen. However, when carbon dioxide binds to iron molecule, the blood becomes cherry red in color. The haemoglobin in which iron is bound to carbon dioxide is called as carbaminohaemogloblin.