HemoglobinHemoglobin, iron-protein compound in red blood cells that gives blood its red color and transports oxygen, carbon dioxide, and nitric oxide. Hemoglobin is present in all but the least complex of animals. It carries oxygen from the lungs or gills, where blood is oxygenated, to body cells. When saturated with oxygen, hemoglobin is called oxyhemoglobin. After releasing oxygen to the body tissues, hemoglobin reverses its function and picks up carbon dioxide, the waste product of cellular respiration, for transport to the lungs, where it is expired. When saturated with carbon dioxide, hemoglobin is known as carboxyhemoglobin.

In 1996 scientists discovered that, in addition to oxygen and carbon dioxide, hemoglobin takes up and releases a third gas, nitric oxide. Nitric oxide plays an important role in regulating blood pressure by relaxing the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regulating the amount of nitric oxide to which the vessels are exposed.

Hemoglobin is contained entirely in the red blood cells, amounting to perhaps 35 percent of their weight. To combine properly with oxygen, red blood cells must contain adequate hemoglobin. Hemoglobin, in turn, is dependent on iron for its formation. A deficiency of hemoglobin caused by a lack of iron in the body leads to anemia.

Hemoglobin carries more than 20 times its volume of oxygen. Some chemicals, such as carbon monoxide, combine so firmly with hemoglobin that it can no longer combine with oxygen and asphyxiation results.

After a life of perhaps 120 days, red blood cells are destroyed in the spleen, or in the course of circulation, their hemoglobin is broken into its constituents, including iron, which enters new blood cells formed in the bone marrow.

When blood vessels rupture, as in an injury, the red cells are released and escape into tissue, where they are broken down. The hemoglobin is converted into bile pigments, the color of which is responsible for the appearance of bruises.

Alterations in the structure of hemoglobin can lead to life-threatening illnesses. The most important of these conditions is sickle-cell anemia, which involves a hereditary change in one of the amino acids that make up hemoglobin. The thalassemias are a group of hereditary diseases of similar origin.

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