hemoglobin hēˈməglōˌbĭn [key], respiratory protein found in the red blood cells (erythrocytes) of all vertebrates and some invertebrates. A hemoglobin molecule is composed of a protein group, known as globin, and four heme groups, each associated with an iron atom.

In the lungs, each iron atom combines reversibly with a molecule of oxygen. Each hemoglobin molecule also has attached a single cysteine amino acid, which attracts nitric oxide from the lungs. The enriched hemoglobin circulates and is carried through the body to the tissues, where the nitric oxide dilates the small capillaries, allowing hemoglobin to deliver its oxygen to the tissues. Then the oxygen- and nitric oxide–free hemoglobin molecule picks up carbon dioxide and free nitric oxide and transports both back to the lungs, where they are exhaled as waste.

Hemoglobin is produced in bone marrow by erythrocytes and is circulated with them until their destruction. It is then broken down in the spleen, and some of its components, such as iron, are recycled to the bone marrow. Other components, such as the heme groups, are broken down into bilirubin, transported to the liver, and secreted with the bile into the intestine for eventual elimination from the body.

Hemoglobin deficiency may be a result of structural abnormality in the hemoglobin molecules themselves. In sickle cell disease, this structural abnormality creates malformed red blood cells which clog blood vessels, severely restricting the supply of blood flowing to body tissues.

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