VOL. 4 (1950) BODY SIZE AND TISSUE RESPIRATION 263 



According to Table VIII, media I and III give approximately the same Q02 values. 

 The considerable differences in the concentration of bicarbonate, CO2 and phosphate in 

 these two media have thus no major effect on the Qq^ under the conditions tested. Since 

 medium I resembles the physiological environment more closely than the other media, 

 Qop, values obtained with this medium might be regarded as approximating more closely 

 to the physiological value than higher values found for brain, and the liver of the larger 

 animals in medium II. The latter are not likely to be standard Q02 values but no definite 

 statement can be made on this point because reliable data on the O2 consumption of 

 tissue in vivo are too scanty. In experiments of Noell and Schneider^^ the Og con- 

 sumption of dog brain cortex in vivo was 4.5 ml per minute per 100 g fresh weight, and 

 on the assumption that the dry weight of dog brain cortex is 21% of the fresh weight^°° 

 Q02 i^ vivo was -12.9. This value is in good agreement with the figure of -14.8 found 

 in medium III (Table VII) and favours the view that the values found for brain in the 

 Ca-free medium II are abnormally high. 



Effects of calcium in the Q02 of slices and homogenates have been described before 

 and have recently been reviewed by Cutting and McCance^". Elliott and Libet^ 

 found that Ca depresses the initial rate of respiration of brain homogenates, but delays 

 the falling off at the later stages of incubation, thus steadying the rate of respiration. 

 It does not seem to have been noted before that the effect of Ca on tissue slices is greater 

 in brain than in other tissues. 



Whilst there is some uncertainty as to which of the values obtained in the different 

 media constitute the 'basal' Qq^, it should be stated that the conclusions drawn in the 

 following sections are not affected by this uncertainty. 



2. Absolute level of Qq^ 



The Q02 values in all 3 media tend to be considerably higher than the values re- 

 ported in the literature for saline media^"^, especially in the case of brain, liver and 

 kidney. However, no strict comparison is possible because different substrates were used 

 in previous measurements. The combination of substrates added in the present experi- 

 ments give, in general, higher values than the substrates added in most previous work 

 (glucose or lactate). The Q02 values observed in the new media are of the same order 

 as the highest values recorded for serum. Thus the intention to include in the new media 

 the substances in serum which stimulate respiration^^ seems to have been accomplished. 



3. Qq^ and body size 



General survey. The data given in Tables IV, V and VII show that the Q02 values 

 of the tissues of the larger species are, in general, somewhat lower than the homologous 

 values of the smaller species. But there are many exceptions to this general rule. No 

 strict parallelism exists between the Q02 values of the homologous tissues and the basal 

 heat p-odaction per unit body weight of the intact animal. The Q02 values for brain, 

 kidney, spleen, and lung change much less, and those for liver slightly less, with the 

 body weight than the rate of basal heat production. Neither is there a simple correlation 

 between body size and Q02 within the same species. The body weights of the 7 mice 

 listed in Table IV varied between 9 and 35 g and that of the 5 mice listed in Table VI 

 between 9 and 45 g. There were variations between 36 and 72 kg in the body weight of 

 the 6 sheep of the first series. These differences of the body weight within one species 

 are not reflected by differences in the Q02 values, with the doubtful exception of the 

 References p. 267-269. 



