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Hemoglobin (also spelled haemoglobin) is a protein in the blood which contains iron and 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. Because hemoglobin is red, red blood cells are 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 in it, its called oxyhemoglobin.
Hemoglobin is made from three parts: a heme molecule, a globin chain, and an iron atom. Heme is an organic molecule. This means it contains carbon. Each hemoglobin molecule has four globin chains, four heme molecules, and four iron atoms. Globin is a protein that is in the body. Each globin chain surrounds a heme. The iron atom goes in the middle of the heme molecule. When hemoglobin is in the lungs, it picks up oxygen in its heme, and carries it to the rest of the body.