Carbon dioxide in water- and air-breathers

Because water-breathers breathe much more water than air-breathers breathe air, and because in the two media the CO2 capacitance coefficients are similar, the difference in PCO2 between inspired and expired medium is much smaller in aquatic than in aerial animals, being generally inferior to 1 Torr. However, as regards CO2, water is a very complex medium, since the aquatic CO2 system varies with salinity, temperature, carbonation, photosynthesis and eventually ionic exchange in the gills. In no two waters is the pattern of CO2 exchange the same. In respiratory studies of aquatic animals it is very important to keep the carbon dioxide system of the water as constant as possible, or at least to be aware of its possible variation with many factors.