262 COMPARISONS OF RESPIRATORY EXCHANGE. 



possible differences as high as 15 per cent which are not apparently 

 due to muscular movement, and says that no absolute predictions can 

 be made as to the total metabolism of an individual. Benedict 1 has 

 recently made an extensive study of the variations in the daily resting 

 metabolism of 35 normal individuals over periods varying from 5 days 

 to 4 years and 5 months. He found that the total metabolism as 

 measured b}^ the oxygen intake may show variations from 3.5 per cent 

 with an individual over a period of 12 days to 31.3 per cent with another 

 individual over a period of 8 months. The average extent of variation 

 was about 14 per cent. 



The respiratory quotient should not vary to any great degree, cer- 

 tainly not more than 0.03 or 0.04. From our experience with resting 

 men in the post-absorptive condition, i. e., without food for 12 hours 

 or more, it may be stated that the value for the respiratory quotient 

 is fairly constant for a considerable length of time, certainly 2 or 3 

 hours, and consequently large variations in the respiratory quotient 

 would not be expected during this period. For example, if a series of 

 quotients were obtained of 0.77, 0.70, and 0.77, the second quotient 

 would be looked upon with suspicion, and a search would be made for 

 the source of the possible error in the manipulation of the apparatus. 

 The low quotient may be due to two causes: (1) too low a carbon- 

 dioxide elimination, (2) an error in the measurement of the oxygen 

 consumption, or possibly a combination of these errors. 



The low carbon-dioxide elimination may be due to a perfectly natural 

 cause, such as under- ventilation in apnoea. If a graphic record of the 

 respiration has been obtained, either by means of a pneumograph or 

 a spirometer, and this shows clearly that apncea occurred, the cause 

 of the low value for the carbon-dioxide elimination is known absolutely. 

 If, then, the results are used, it will be with a clear understanding 

 that the respiratory quotient 0.70 does not indicate the true character 

 of the katabolism for that period. 



Since the respiratory quotient is the relation between the volume of 

 carbon dioxide produced and the volume of oxygen consumed, it may be 

 calculated directly from the increase in the carbon dioxide and the deficit 

 of the oxygen in the expired air.' 2 Analyses of expired air, such as are 

 made with the open-circuit method, give the volumetric content of 

 carbon dioxide and oxygen, and this ratio is in no way affected by 

 variations in barometric pressure, temperature, or even slight muscular 

 activity, but is dependent solely upon the character of the respiration 

 and (if this is normal) upon the character of the katabolism taking- 

 place in the body. 



'Benedict, Journ. Biol. Chem., 1915, 20, p. 291. 



2 Correction must be made, of course, for the carbon dioxide in inspired air and the change in 

 percentage of the nitrogen in inspired and expired air. 



