VOL. 4 (1950) BODY SIZE AND TISSUE RESPIRATION 265 



accepted that the muscles play a leading part in the regulation of heat production. 

 Evidence in support of this conception is the increased muscular activity on exposure 

 to cold, manifesting itself by increased tension and shivering, and the failure of the 

 curarized animal to maintain the physiological temperature level on exposure to cold. 

 It has not been directly demonstrated that the basal respiration of the musculature 

 varies with body size in the postulated fashion, and no satisfactory experimental proce- 

 dure has as yet been devised to carry out the necessary measurements. Data obtained 

 on isolated muscles bear no simple relation to the basal respiratory rate of the muscle 

 in situ because the Qq^ of muscle depends more than that of any other tissue on the state 

 of activity of the tissue. Activity may cause a thirty-fold rise of the resting rate of 

 respiration (Barcroft^"^, Meyerhof^"'^). As the state of activity is controlled by the 

 higher nervous centres detachment from the nervous system is bound to affect the rate 

 of respiration. 



5. Factors determining the level of tissue respiration 



If body size is not a major factor determining the O02 of the 5 tissues tested it 

 remains to be examined which other factors control the level of respiration of these 

 tissues. As the respiration of living tissues primarily serves to supply energy, the level 

 of tissue respiration is expected to be determined by the energy requirements. A variety 

 of factors contribute to the requirements. They may be classed in three groups : 



1. Energy is required when tissues perform mechanical, osmotic, chemical, or other 

 kinds of external work. 



2. Energy is required to maintain structures which are thermodynamically 

 unstable. An example is the maintenance of concentration gradients between tissue and 

 blood plasma of readily diffusable substances, such as inorganic ions, amino acids, 

 coenzymes. 



3. Energy is required to maintain the body temperature. 



Energy generated for the first two purposes always yields heat as a by-product and 

 in homeotherms this heat is partly, or wholly, utilised to maintain the body temperature. 

 In an organism performing some physical exercise, and living at a temperature not far 

 removed from that of the body temperature, the heat arising as a by-product may be 

 enough for the upkeep of the body temperature. In a cold environment the heat arising 

 as a by-product in a resting organism ma}^ no longer be sufficient to maintain the body 

 temperature, and extra heat has to be formed. It is reasonable to assume that the highly 

 differentiated cells whose task it is to carry out specialised functions, as do those of 

 brain or the glands, are designed to deal solely with these specialized functions rather 

 than to act as heat generators in the case of exceptional loss of heat. The extra source 

 of heat might be expected to be the muscle tissue which, for other reasons, has the 

 capacity of varying the rate of heat production. If this assumption is correct, in other 

 words, if the level of respiration of highly specialized tissues is determined by the energy 

 requirement falling under categories (i) and (2), it is to be expected that the rate of 

 energy production of the highly differentiated cells is not dependent on the size of the 

 animal, because the energy needed for the performance of a given piece of work is inde- 

 pendent of the size of the body. 



However, somewhat different from the question of energy requirements of the 

 highly differentiated cells is the problem of the energy requirements of organs as a whole. 

 Homologous organs of different species have by no means identical structures. For 

 References p. 26y-26g. 



