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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
See more Encyclopedia articles on: Anatomy and Physiology
Browse by Subject
- Earth and the Environment +-
- History +-
- Literature and the Arts +-
- Medicine +-
- People +-
- Philosophy and Religion +-
- Places +-
- Australia and Oceania
- Britain, Ireland, France, and the Low Countries
- Commonwealth of Independent States and the Baltic Nations
- Germany, Scandinavia, and Central Europe
- Latin America and the Caribbean
- Oceans, Continents, and Polar Regions
- Spain, Portugal, Italy, Greece, and the Balkans
- United States, Canada, and Greenland
- Plants and Animals +-
- Science and Technology +-
- Social Sciences and the Law +-
- Sports and Everyday Life +-