Respiration and Metabolism 



241 



underlying metabolic chemical reactions, and, judging from the constancy of 

 the values obtained, represents fundamental relatioftships between different 

 biological systems.^''' '^" By this formula the temperature characteristic— energy 

 of activation or "thermal increment"— of the given system may be found. This 

 value may vary, but in a wide variety of respiratory processes is constant at 

 11,500 or 16,500 calories. For practical purposes when the log of the reaction 

 rate is plotted against reciprocal of absolute temperature the thermal increment 

 is given by the slope of the curve. The inference that such thermodynamic 

 constancy between metabolic processes depends on so-called "master reactions" 

 or that comparable enzyme systems are at work remains to be demonstrated. 



High basal metabolic rate is a.basic condition for homoiothermism and heat 

 regulation. Heat production can be measured by direct calorimetry or may 

 be calculated from CO^ production and O^ consumption. In man with an 

 R.Q. of 0.82, heat is produced to the extent of 4.8 cal. per liter of O2 con- 



— . 280 - 



20 40 60 80 100 

 OXYGEN TENSION - mm. Hg 



Fig. 53. The interrelation between "maximum steady state" of oxygen consumption, 

 oxygen tension, and temperature in the goldfish, Carassius. The reduction of O2 tension 

 is more effective at higher temperatures. From Fry and Hart.'"" 



sumed.-'' The rate of increase in total heat production with body weight varies 

 for different species. 



The ability of animals to withstand temperature extremes and their metab- 

 olic acclimatization to extreme heat and cold constitute an interesting biological 

 story but one on which there is as yet only scattered information. Some 

 organisms live in environments in which the temperature never exceeds 0° 

 C."^^ Irving^ '**•'• cites the case of certain arctic animals, notably the ground 

 squirrel, CiteUiis, which can withstand environmental temperatures down to 

 minus 30° C. for six hours, under which conditions the respiratory exchange 

 increases, but the R.Q. decreases, indicating fat utilization. The arctic fox, 

 when exposed to this temperature, shows no metabolic increase (Scholander, 

 personal communication). The gas exchange of poikilothermic animals at 

 higher temperatures increases until a point is reached (at about 35° C. body 

 temperature) when certain deleterious effects appear, causing the oxygen 

 uptake to drop suddenly. Fry and Hart*-- have recently shown that the 



