SIGNIFICANCE OF GAS EXCHANGE 9 



The! Measurement of Total Catabolism. Direct and Indirect 



Calorimetry. 



The adequate term for expressing total metabolism is the calorie, and 

 the standard method of measuring it is the biocalorimetric. Biocalori- 

 metry alone will give perfectly reliable determinations of the sum total 

 of the energy transformation independently of the nature of the 

 metabolic processes. 



The biocalorimetric method has until recently been rather difficult 

 to manipulate, when accurate results were aimed at. It has required 

 and does generally require costly and complicated apparatus, and 

 there has been therefore and still is a wide field for indirect calorimetry 

 by means of measurements of the respiratory exchange. With the 

 recent advances in calorimetric methods due to Atwater and Benedict, 

 Rubner [1911], and especially A. V. Hill, there is every reason to think 

 that direct determinations of the total metabolism will be preferred to 

 indirect in many cases, and for all classes of animals, as it is undoubtedly 

 preferable theoretically. 



On certain points, however, the determination of the respiratory 

 exchange possesses advantages over the direct calorimetry which 

 should secure its application. 



The determination of absorbed oxygen is distinctly more sensitive 

 than the determination of heat produced by organisms of very small 

 size. 2 cub. m.m. of oxygen absorbed in ten hours is at present 

 about the limit at which fairly accurate determinations can be made. 

 This corresponds to 10 mg. calories in the same time, or I per hour. 

 The limit of accuracy in the calorimetric measurements of Bohr and 

 Hasselbalch [1903] on the embryo of the fowl was about 100 mg. 

 calories per hour. 



In experiments on warm-blooded animals the determination of the 

 respiratory exchange possesses this advantage over the direct calori- 

 metry that it is not affected by changes in body temperature which 

 may cause considerable errors in results obtained calorimetrically over 

 short periods. This has been well shown by Williams, Riche and Lusk 

 [1912] who found that in the second and third hour after a large meal 

 of meat there was in the dog a discrepancy between the calorimetric 

 results obtained directly and indirectly. During this period the body 

 temperature and the increase in heat stored by the organism could 

 not be accurately estimated from the increase in temperature in the 

 rectum, because the temperature increment was not the same in all 



