Hemoglobin (or haemoglobin) is a protein in red blood cells which contains iron. It is used to transport oxygen around the human body. Hemoglobin is found in the red blood cells of all vertebrates apart from white-blooded fish. It also occurs in some invertebrates. Some other invertebrates use other chemicals such as hemocyanin
Red blood cells get their colour from hemoglobin, which is red. There are millions of hemoglobin molecules in each red blood cell and millions of red blood cells in the human body. When hemoglobin has oxygen attached, it is called oxyhemoglobin.
The most common type of hemoglobin in mammals contains four such subunits. Each subunit of hemoglobin is a globular protein (globin) with a heme group inside it. Each heme group has one iron atom. This binds one oxygen molecule. So the complete hemoglobin molecule has four globin chains, four heme molecules, and four iron atoms. When hemoglobin is in the lungs, it picks up oxygen in its hemes, and carries it to the rest of the body.