1 A coloured compound that is capable of reversibly binding with oxygen at high oxygen concentrations and releasing it at low oxygen concentrations. Most such pigments are present in the blood (blood pigments), transporting oxygen within the circulatory system from the respiratory organs to the tissues of the body; an exception is myoglobin, which occurs in the muscles. In vertebrates the respiratory pigment is haemoglobin, contained in the erythrocytes (red blood cells). See also chlorocruorin; haemerythrin; haemocyanin.
2 Any of the proteins involved in cellular respiration as components of an electron transport chain. They include the cytochromes.