CALL FOR OXYGEN 7 



snakes, but recent investigations (M. Nielsen 1936) have 

 shown that even in warm-blooded man muscular activity is 

 correlated with a definite increase in temperature which 

 normally reaches 39.5°C. (103°F.) when the metabolism is at 

 the maximum of about 20 times the resting value. 



It is important to note that it is the rate of activity that 

 can be sustained over a certain length of time which deter- 

 mines the demands upon the respiratory organs. The oxida- 

 tive processes are in the main processes of restitution which 

 can be postponed for a certain length of time while the organ- 

 ism is incurring an "oxygen debt," to be paid off after the 

 burst of activity. When activity is sustained a "steady state" 

 is reached — in man after about 3-5 minutes — in which the 

 oxygen intake has ceased to rise and remains more or less 

 constant, corresponding to the rate of activity. In man and 

 also in the horse (Brody and Cunningham, 1936) the maxi- 

 mum oxygen uptake to be provided by the lungs in such 

 steady states of muscular activity is of the order of 10 to 15 or 

 even in some individuals 20 times the resting metabolism. 



In flying animals the maximum rate of oxygen uptake is 

 much higher, as indicated by the figure given for a flying 

 butterfly in Table 1. 



The influence of adverse conditions. In the warm-blooded 

 animals complete rest in a starving condition is characterized 

 by a definite level of metabolism called the "standard" or 

 "basal." It is important to note that in many cold-blooded 

 animals there is no such well-defined basal level. If food is 

 withheld the metabolism goes on decreasing, sometimes for 

 very long periods, until death supervenes (Tetens Nielsen, 

 1935, and unpublished observations by the writer on several 

 insects). The same takes place in other adverse conditions, 

 and there can be no doubt that also lack of oxygen is a factor 

 which can in many forms reduce the metabolism without 

 appreciable damage to the organism. When, for instance, 

 respiratory organs are removed by operation in May-fly larvae 

 (Morgan and Grierson, 1932) the respiratory exchange is 



