The oxygen-carrying pigment and predominant protein in the red blood cells. Hemoglobin forms an unstable, reversible bond with oxygen. In its oxygenated state it is called oxyhemoglobin and is bright red. In the reduced state it is called deoxyhemoglobin and is purple-blue.

Each hemoglobin molecule is made up of four heme groups surrounding a globin group. Heme contains iron and gives a red color to the molecule. Globin consists of two linked pairs of polypeptide chains. The development of each chain is controlled at a separate genetic locus. Changes in the amino acid sequence of these chains results in abnormal hemoglobins. For example, hemoglobin S is found in sickle-cell disease, a severe type of anemia in which the red cells become sickle-shaped when oxygen is in short supply.

When red blood cells die, the hemoglobin within them is released and broken up: the iron in hemoglobin is salvaged, transported to the bone marrow by a protein called transferrin and used again in the production of new red blood cells; the remainder of the hemoglobin becomes a chemical called bilirubin that is excreted into the bile which is secreted into the intestine, where it gives the feces their characteristic yellow-brown color.