Blood gets its color from oxygen-carrying respiratory pigments, and there are a number of different types. The iron-containing pigments found in blood include haemoglobins (red), myoglobins (red), chlorocruorins (green) and haemerythrins (violet). Haemocyanin (blue), which is found in horseshoe crabs and other organisms, contains copper not iron.

Some haemocyanins have a higher affinity for oxygen than most haemoglobins and probably function more as an oxygen storage medium than as an oxygen transport medium.

To counteract this, organisms that live in environments in which the pH varies usually have a respiratory pigment that is less sensitive to pH change than the pigments found in animals which live in more stable environments. Thus a haemocyanin that is very sensitive to pH would be detrimental to an organism that lives in a pH-labile environment, even if in other respects it is a & quot;better" oxygen carrier. Conversely, a pH-stable haemoglobin might be a "better& quot; oxygen carrier, even though its saturation curve is less efficient than a particular haemocyanin at a certain pH.

Compiled from answers by: