respiratory pigment in human blood is haemoglobin in the red blood cells . it combine with oxygen to form an unstable oxyhaemoglobin n thu transporet oxygen to all body parts n onreleasing oxygen it again becomes haemoglobin it collects carbon dioxide from body parts n combine with it to form unstable carboxy haemoglobin n carry it in that state to lungs where it release carbondioxide into exhaled air nagain becomes haemoglobin