What is "respiratory pigment"

n. (context biochemistry English) any of several coloured metal-containing protein pigments which combine reversibly with oxygen and which are found in the tissues of invertebrates and whose role is to transport oxygen

A respiratory pigment is a molecule, such as haemoglobin in humans and other vertebrates, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin. Hemoglobin is bright red when oxygenated, and dark red(purplish) when deoxygenated, oxygenated haemocyanin is blue in color, deoxygenated is almost colorless. Oxygenated chlorocruorin turns from green to red, whereas oxygenated haemerythrin is a violet to pink colour and colorless when deoxygenated. Any of various colored conjugated proteins, such as hemoglobin, that occur in living organisms and function in oxygen transfer in cellular respiration.