SCALING OF HYPERCAPNIC VENTILATORY RESPONSIVENESS IN BIRDS AND MAMMALS

The possible relationship between CO2 responsiveness and body mass in birds was explored using newly acquired ventilatory data from the barn swallow, Hirundo rustica, and the pigeon, Columbia livia, and that fr om the literature on four other species. Ventilatory responsiveness (% Delta V) of birds to 5% inspired CO2 is scaled to body mass to the 0.1 45 power (%Delta V proportional to Mb(0.145)). A similar allometric re lationship exists for data on 7 species of eutherian mammals taken fro m the literature (%Delta V proportional to Mb(0.130)). The reduced res ponsiveness to CO2 in small birds and mammals may be related to an ele vated hypoxic ventilatory sensitivity, as demonstrated in mammals (Bog gs and Tenney, Respir. Physiol. 58: 245-251, 1984). These scaling rela tionships may reflect a mechanism for minimizing the inhibition of ven tilation resulting from excessive loss of CO2 which thereby permits a higher hypoxic ventilatory response in small species. Other mechanisms , however, could include size related differences in mechanics or alve olar ventilation.