Respiration and Metabolism 



237 



ment of metabolic activity as a function of the heat produced (direct calori- 

 metry) is less accurate and involves more care than the determination of the 

 oxygen consumption from which the metabolic rate may be calculated (indirect 

 calorimctry). 1 he o.xygcn required and the caloric yield depend on the type 

 and amount of organic food oxidized (Table 40). The respiratory quotient 



TABLE 40. OXYGEN REQUIRED AND CALORIES PRODUCED IN THE 

 UTlUZAriON OF DIFFERENT FOODS 



(R. Q.), the ratio of volume of carbon dioxide released to oxygen consumed, 

 is also shown in the table. 



The rapidity with which foods can be metabolized in different organisms 

 is known to vary, even for similar substances in closely related animals. The 

 respiratory exchange for the hexose sugars, fructose, glucose, and galactose, 

 has been studied in several mammals and speed of utilization determined. ^"^ 

 The results for the different animals are indicated in decreasing order as 

 follows: 



Man fructose, galactose, glucose 



Monkey glucose, fructose, galactose 



Goat fructose, glucose, galactose 



Cow glucose, galactose, fructose 



In both adult and larval Drosophila, fructose appears to be the most readily 

 utilizable sugar tested.^"*'' 



Oxygen Consumption in Relation to Activity. Every change in activity 

 made possible by aerobic respiratory processes can result in an alteration of 

 oxygen consumption, and it is assumed, in dealing with comparisons of other 

 factors on oxygen uptake, that activity remains constant. Thus "basal" metab- 

 olic rates are determined with actixity at a minimum."' Such a condition as a 

 standard of comparison is not always easily attainable, and some investigators 

 have resorted to the use of anaesthetics. Curare' -°- •'^"' ""'*** and urethane"**- 

 2iii, 2!>2 }^ayg been used to suppress general activity with no known specific 

 influence on the respiratory exchange. Extreme doses must be avoided, owing 

 to suppression of muscular tone. 



Oxygen consumption increases proportionally with activity to the limits 

 imposed by the ventilation capacity of the respiratory mechanism and the 

 ability of the transport system to carry the respiratory gases. The maximum 

 metabolic exchange in mammals is some 15 to 20 times greater than the resting 

 rate, although the oxygen consumed during the period of excessive activity 



* Not, however, under conditions of sleep, hibernation, or estivation. For an account 

 of the latter in the lunglish see Smith.'^"^ 



