Models for a comparative functional analysis of gas exchange organs in vertebrates.

The analysis of external gas exchange in the various respiratory organs of vertebrates is based on models with gas transport properties determined by ventilatory, diffusive, and perfusive conductances and by the geometric arrangement of medium and blood flows. The following factors are examined: water vs. air as external medium; gas transport properties of blood; diffusive gas transfer between medium and blood; problems in assessing diffusion limitation in fish gills, amphibian skin, avian lungs, and mammalian lungs. Finally the limitations to the analysis imposed by various physiological and anatomical complexities are discussed.