[thedoc (OP)]

Robin Datta  asked the Naked Scientists:
   
Insects breathe through spiracles and tracheae. The movement of gases is through some mechanical movement and some diffusion.

Vertebrates move the gases around in a chemical carrier, haemoglobin, which is itself packaged in erythrocytes. Hence the gases can be transported much greater distances from the gas exchange surfaces. The giant dragonflies of ages past when the atmosphere had higher levels of oxygen were possible because the partial pressure of the oxygen was higher throughout the transportation channels, and so in spite of the continuously decreasing gradient, adequate oxygen delivery to the tissues was still possible.

In spite of chitinous exoskeletons, Crustacea have some species that are way out of the size range of Insecta: the Crustacea have oxygen-carrying molecules in their circulatory system that free them from the constraints of mechanical movement and diffusion.

Among Vertebrata, the largest marine animals ever are not Pisces, but Mammalia (Cetacea): fish depend on gills for gas exchange, where both media are aqueous, while in cetacean lungs one medium is air (gaseous). Also the partial pressure of atmospheric oxygen is generally much better than the partial pressure of dissolved oxygen in water. Both factors make for more efficient oxygen delivery to distant tissues.

What do you think?