70 VERTEBRATE RESPIRATION 



The factors of heat loss and heart size, however, cannot 

 be the full explanation of the relationship between metabolic 

 rate and (body weight) ^•'^^ for similar relationships hold in 

 poikilotherms. This is true not only of vertebrates but also of a 

 very wide range of invertebrates ; even single egg cells obey a 

 similar relationship. Zeuthen found that the power of the body 

 weight to which the metabolism is related varies in different 

 groups. The situation is extremely complex and cannot be 

 adequately interpreted in terms of a single relationship. He 

 pointed out that animals can either be small, grow rapidly, and 

 live intensely, or they can be large, grow slowly, and metabolise 

 at a low rate. Between these two, however, many intermediates 

 are possible. 



(b) METABOLIC RATE AND TEMPERATURE 



Most chemical reactions are influenced by temperature in 

 such a way that the velocity increases with rising temperature. 

 In living organisms most reactions are catalysed by enzymes and 

 over a certain range they behave in a similar way. However, 

 above a certain temperature the enzyme, being largely protein 

 in nature, becomes inactivated and consequently the rate of 

 reaction declines. The result of these two opposing effects leads 

 to the existence of an optimum temperature at which the rate of 

 a reaction is most rapid. For most enzymes it lies between 30° 

 and 40° C, and because this is also the range of body tempera- 

 ture found in mammals and birds, it has been suggested that 

 their temperature is that at which the enzymes function most 

 efficiently. The range of body temperatures over which animals 

 can survive is roughly from 0° to 50° C, which again coincides 

 with the range of activity of most enzymes. 



Most chemical processes which take place in living organisms 

 follow an exponential curve such that the rate of the reaction is 

 doubled or trebled for a rise of 10° C, i.e., Q^q = 2 — 3. The effect 

 of temperature on the metabolic rate of poikilotherms also tends 

 to show a similar relationship. Most terrestrial animals have an 

 upper lethal temperature of about 45° C. Such temperatures can 

 be survived very briefly so that if the time of exposure is taken 



