SUPERCOOLING AND FRRF.ZIXO ANDJUS, LOVELOCK and SMITH 133 



coinparisun to that recorded Ijefore the exposure, this cannot Ije interpreted as the 

 result of an "oxygen deht." Rather, the increased oxygen uptake in the recovery 

 phase is the result of the hypothermia itself. It is well known that hypothermia, if 

 it does not reach a critical level, acts as "internal cold" and stimulates the metaholic 

 rate to levels ahove that found in ndrniothermic animals exjKjsed to the same en- 

 vironmental temperature. In our experiments it is of significance that the consump- 

 tion of oxygen during recovery showed no unusual increases in those cases where 

 hypothermia alone was incapable of inducing a significant increase in heat produc- 

 tion. This occurs in very young rats with deficient thermoregulation, or in thyroid- 

 fed animals whose metabolic rates have already increased to a maximum. As might 

 be expected, the stimulative effect of hypothermia on the production of heat is sup- 

 pressed by oxygen lack. The appearance of hypothermia under hypoxic conditions 

 is not accompanied by the rise of oxygen uptake which normally occurs in animals 

 that are not deeply anesthetized. 



The role of oxygen and carbon dioxide. During cooling by the closed con- 

 tainer technique the animal is exposed not only to a gradual fall of oxygen tension, 

 but also to the accumulation of CO^. The effect of carbon dioxide can be illustrated 

 by the experimental data summarized in table I. It can be seen that, at a high en- 

 vironmental temperature (30-32° C.) the accumulation of carbon dioxide is detri- 

 mental to the survival of the animal enclosed in the vessel. Contrariwise, at low 

 ambient temperatures the rats survive better when carbon dioxide is allowed to 

 accumulate. This effect can be partly explained by the fact that the accumulation of 

 carbon dioxide slows down the oxygen uptake (fig. 3). This has two important 

 effects on survival in the closed vessel. First, the production of heat is retarded 

 (more than by hypoxia alone), and this promotes the appearance of hypothermia. 

 Second, the fall of the partial pressure of oxygen is slowed down so that hypo- 

 thermia has the time to reach deep levels before the oxygen tension in the tissues has 



TABLE I 



Effect of CO2 Accumulation on Survival in Closed Vessels Kept at Different 



Ambient Temperatures 



(At 30-33° C, rats survived when CO2 was continuously absorbed on KOH, but died when 



the experiment was repeated without absorbing the expired CO2. In the lower 



temperature range the reverse was observed.) 



