SUPERCOOLING AND FRF.KZING— ANDJUS, LOVELOCK and SMITH 131 



0, UPTAKE 









CFOT 

 (mm.Hg) 



80 

 b 60- 



40- 



® 



® 



V/, 



@ 



h 



1 



^ 



I 



1- COMPLEMENTARY 

 O2 UPTAKE 



2- BASAL O2 UPTAKE 



3- TOTAL O2 UPTAKE 



4- CRITICAL FALL OF 

 O2 TENSION 



Fig. 1. — Critical fall of oxygen tension (black bars) in rats with different metabolic rates as 

 measured by oxygen uptake (white and shaded blocks). A, a, C, c, E, e, G, and g are hypo- 

 physectomized rats; H and h are young rats weighing about 35 grams. I, II, and IV are results 

 obtained at an ambient temperature of about 20° C. ; III are results obtained at 10° C. The values 

 for "Basal Oxygen Uptake" are obtained in preliminary measurements at 30-32° C. of ambient 

 temperature. The fall of oxygen tension is calculated by taking 159 mm. Hg as the starting 

 normal value. Each block or bar represents the average of 5 to 10 experiments. 



for thyroid-fed rats are not very different from those found in the controls when 

 the experiment is carried out at a high amhient temperature (30-32° C), ahhough 

 it is precisely at this temperature that the difference in rates of oxygen consumption 

 between the two groups of animals becomes most marked. On the other hand, 

 when the rates of oxygen consumption of the two groups are almost identical, as it 

 happens when they are both placed at a low environmental temperature (10° C), 

 the values of the lethal fall of oxygen tension differ greatly between the two groups. 

 While these results will not be discussed in detail, the significance of that portion 

 of oxygen uptake that is involved in thermoregulation should be emphasized. The 

 greater the proportion of the complementary heat production, the more easily will 

 a small drop in oxygen tension be reflected in decreased consumption of oxygen and 

 in the induction of hypothermia. When the share of heat which cannot be altered 

 by the mechanisms of thermoregulation represents the major portion of the total 

 heat production, it is then dangerous to attempt to decrease it by lowering the 

 oxygen tension. When the latter type of animal is exposed to a gradual fall of 

 oxygen tension, its heat production remains unaltered almost to the point of death. 

 Its body temperature does not fall fast enough to permit hypothermia to exert its 

 protective effect against oxygen lack. By contrast, when the major portion of the 

 total heat production is a result of chemical thermoregulation, it can be readilv de- 



