248 



Comparative Animal Physiology 



Hiestand*"- demonstrated in dragonfly nymphs and crayfish that the t,. 

 value is depressed by carbon dioxide, by low pH, and by a combination of both, 

 acting as respiratory stimulants apparently through a control center. On the 

 other hand, in those invertebrates containing hemoglobin, carbon dioxide 

 increase may reduce the oxygen uptake, as Ewer'**" demonstrated in Chirono- 

 mus, and so tend to elevate the critical tension. Elimination of the action of 

 hemoglobin in the blood by use of carbon monoxide more effectively accom- 

 plishes the same thing. Oxygen consumption in the earthworm is normally 

 relatively independent of tension down to about 3 per cent (20 mm. Hg.), but 

 in the presence of carbon monoxide it is independent down to only 8 per cent 

 of an atmosphere (60 mm. Hg). 



Temperature, Tension and Oxygen Consumption. At low temperatures 

 the metabolic activity of poikilotherms is reduced and they are able to with- 

 stand oxygen tensions below those required at higher activity levels (Fig. 53). 



20 



15 10 5 



0, TENSION V. 



Fig. 56. Relation of oxygen tension to oxygen consumption at different temperatures 

 in white mice. At low tensions the rates of O^ uptake are the same despite temperature 

 differences. After Chevillard et al."® 



In hibernating mammals also the oxygen requirements may fall to 5 per cent 

 of that of non-hibernating animals. Homoiotherms are faced with another 

 problem at low temperatures, however, and any small change in oxygen tension 

 can have profound effects. Mice maintained at 30° C. can tolerate oxygen 

 depletion to 38 mm. Hg partial pressure, but those kept at 20° C. withstand 

 concentration reductions only down to 55-60 mm. Hg when the reduction 

 in oxygen occurs at the same rate.***^ However, if care is taken to reduce the 

 oxygen concentration slowly enough, those maintained at 20° C. can also 

 tolerate oxygen at 38 mm. Hg pressure. Although the oxygen consumption 



