Transport of Gases in Blood
We know that two types of gas exchanges are constantly occurring in the animal body--one at the inter phase of the respiratory membrane and external environment and the other in the rest of the animal tissues. The principle underlying is the same at both places passive diffusion along a pressure gradient. In many invertebrates oxygen and carbon dioxide are carried dissolved in blood or haemolymph. The amount of oxygen carried in simple solution is small, therefore, in highly organised animals (many invertebrates-and all vertebrates) oxygen is transported bound reversibly to proteins that are oxygen carriers. These proteins contain a metal, commonly iron or copper and are coloured. They are known as respiratory pigments. The common respiratory pigments are listed in Table along with their molecular weights and occurrence in the animal kingdom.
Table: common respiratory pigments, their properties and occurrence in the animal kingdom