Respiratory pigments are metalloproteins that transport O<sub>2</sub>, the best known being the bright red/crimson coloured hemoglobin in human blood. The colour derives from Fe<sup>2+</sup> at the core of a tetraporphyrin ring. But less well known is <strong>blue blood</strong>, and here the colour derives from an oxyhemocyanin unit based on Cu<sup>1+</sup> (the de-oxy form is colourless) rather than iron.