The slow diffusion of O₂ molecules in water affects both air- and water-breathing animals. Eukaryotic cells carry out cellular respiration in their mitochondria, which are located in the cytoplasm—an aqueous medium. Cells are bathed in extracellular fluid—also an aqueous medium. In addition, all respiratory surfaces must be protected from drying out by a thin film of fluid through which O₂ must diffuse. Even in air-breathing animals, the slow rate of O₂ diffusion in water limits the efficiency of O₂ distribution from gas exchange surfaces to the sites of cellular respiration.

Diffusion of O₂ in water is so slow that even animal cells with low rates of metabolism cannot function more than a few millimeters away from a good source of environmental O₂. Therefore there are severe size and shape limits on the many species of invertebrates that lack internal systems for distributing O₂. Most of these species are very small, but some, such as marine flatworms, have grown larger by evolving a flat, thin body with a large external surface area (Figure 48.1A). Another strategy is seen in sponges, which have bodies made of cells that surround water channels through which the external medium flows (Figure 48.1B) (see also Figure 30.2B). A critical factor enabling larger, more complex animal bodies has been the evolution of specialized respiratory systems with large surface areas such as gills that are highly permeable to respiratory gases (Figure 48.1C).

O₂ can be obtained more easily from air than from water for several reasons:

You can appreciate how important these facts were for the evolutionary transition of life to the terrestrial environment, because they meant that there were fewer constraints on the evolution of higher metabolic rates.