In the lungs, where oxygen concentration is high, each hemoglobin molecule can bind with one molecule of oxygen. The erythrocyte containing the hemoglobin then travels through the bloodstream to the body's cells, where oxygen concentration is low, and the hemoglobin releases the oxygen for use by local tissue. It also picks up carbon dioxide, and this waste product is transported back to the lungs, where it can be released and exhaled.
Hemoglobin is made up of heme and globin. Heme is an iron-containing pigment that binds to oxygen. Globin, which holds the heme and influences how easily it stores and releases oxygen, is a protein consisting of two pairs of polypeptide chains. Globin can contain several different types of polypeptide chains, termed alpha, beta, and gamma. Each is coded for by a separate gene. The genes are evolutionarily related, and their differences are the result of ancient mutation events in an ancestral form that gave rise to each modern type.
The type of hemoglobin found in healthy adults contains two alpha chains and two beta chains. This form of hemoglobin is called HbA (hemoglobin A). As discussed below, sickle cell disease is due to mutations in the beta chains in HbA. A fetus or newborn baby does not produce HbA. Instead, it produces fetal hemoglobin, or HbF. Like HbA, fetal hemoglobin contains a pair of alpha chains. But in place of the beta chains, it contains a pair of gamma chains. As infants grow older, their bodies produce less and less HbF and more and more HbA.
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