Scaling of respiratory areas in relation to oxygen consumption of vertebrates.

Resting oxygen consumption is generally related to the 0.75 power of the body mass. More recent studies on the morphometry of the lungs and gills of vertebrates indicate that the respiratory surfaces increase in area more closely to M1.0. A possible explanation is that the gas exchange surface is more to be related to the maximum oxygen uptake capacity, rather than to the resting condition. However, recent studies, especially in African mammals, suggest that Vo2max scales to a power similar to that for resting oxygen consumption and hence cannot be directly related to the respiratory surfaces. In fishes the situation seems to be different as Vo2max appears to increase more rapidly than standard Vo2. Consequently, the ratio between these two is greater in larger specimens and hence they have a greater scope for activity. A possible interpretation of this apparent difference may be related to differences in ventilation, as the tidal ventilation of lungs can lead to a reduction in the difference in oxygen pressure across the exchange surface, whereas in fish gills with increasing size and increasing water velocity there would be a reduction in the resistance to gas transfer. However, it is important to appreciate that such generalizations probably do not hold in all cases and the position of intermediate groups of vertebrates needs to be more closely assessed.