Hemoglobin is an iron-containing protein used in the transport of oxygen. It is carried in the blood of most animals. In its oxygenated state, it is bright red and is responsible for the color of blood.
Hemoglobin bonds with free oxygen in the lungs or gills of an animal. This bond is easily reversible, so hemoglobin gives up its oxygen molecules to the oxygen-depleted cells by diffusion through the walls of the capillaries. Deoxygenated hemoglobin is then carried back to the lungs or gills to be used again. When red blood cells die, hemoglobin is broken down. The iron is transported to the bone marrow to be used again in the formation of new hemoglobin molecules.