A respiratory pigment
is a molecule, such as hemoglobin
in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin
. Hemoglobin is bright red when oxygenated, and dark red when deoxygenated, oxygenated haemocyanin is blue in color, deoxygenated is almost colorless. Oxygenated chlorocruarin turns green where oxygenated haemeryhrtin is a violet to pink colour, and colorless when deoxygenated. All vertebrates use the hemoglobin respiratory pigment.