516 RESPIRATION 



over. On a mixed diet, however, their relationship remains practically unaltered. 

 The ingestion of different foodstuffs changes matters considerably. Thus, the 

 quotient rises to 1, if the muscular work is performed exclusively at the expense 

 of the carbohydrate material. This is rarely the case, although muscular work 

 depends chiefly upon the carbohydrates, because these bodies are more immediately 

 available and may also be slowly replenished from the proteins. The fats may also 

 be drawn upon, but since there is no evidence at hand to show that these substances 

 are first converted into carbohydrates, it must be concluded that the muscles 

 are capable of utilizing them as such. Obviously, therefore, the respiratory quo- 

 tient serves as a rieliable index of the oxidations only if the determinations establish- 

 ing its value have been extended over a long period of time. Short experiments 

 may lead to absolutely erroneous results on account of the occurrence of accidental 

 variations, such as occasional muscular contractions and voluntary changes in the 

 depth of the respiratory movements. ^ Even the mere ingestion of food may 

 increase the gaseous exchange, because it augments the mechanical and secretory 

 activities of the alimentary canal. ^ None of these influences possess a permanent 

 metabolic value. 



Sex and age influence the quotient through the general metabolism. In males 

 the average CO2 output is greater than in females, but this general difference 

 between the sexes is not in evidence if persons of the same body-weight are com- 

 pared. The influence of age manifests itself by the low value of the quotient in 

 children as compared with that of adults. Not only is the gaseous interchange in 

 proportion to the weight greater in the former, but more O is being absorbed by 

 them in comparison with the CO2 given off. Obviously, therefore, the child 

 possesses a more intense metabolism, presumably on account of the fact that its 

 surface in proportion to its weight is larger than that of the adult, thereby entailing 

 a greater loss of heat. Aside from this factor, age also influences the respiratory 

 interchange because the tissues gradually become less active. For the same reason, 

 the respiratory activity is greater in the robust than in the weak or sick. 



Increases in the external temperature tend to heighten the gaseous interchange 

 and hence, to increase the quotient. In cold-blooded animals, in particular, the 

 CO2 output decreases as the temperature of the medium falls and increases as the 

 latter rises. In warm-blooded animals, on the other hand, cold within physiolog- 

 ical limits has a tendency to stimulate the consumption of O as well as the produc- 

 tion of CO2. Involuntary muscular tremors (shivering) increase the respiratory 

 activity, the oxygen intake as well as the CO2 output becoming greater. ^ When 

 the body temperature rises, as in fever, the respiratory quotient remains at first 

 practically the same, although the volumes of O absorbed and CO2 produced are 

 increased. 



The rate and depth of the respiratory movements do not appreciably change the 

 relationship of the O and CO2, although, to begin with, the more ample ventilation 

 of the lungs tends to heighten the CO2 output. If the respiratory amplitude re- 

 mains the same while its rate is increased, the volume of air respired, as well as the 

 absolute quantity of CO2 discharged, is increased, but the amount of CO2 in com- 

 parison with the total volume of air becomes less. Very similar results are 

 obtained it the depth of respiration is increased while the frequency is permitted 

 to remain the same. Slow and deep respirations, of course, give rise to a greater 

 discharge of CO2. 



The composition of the air may be changed considerably before the gaseous 

 interchange is markedly altered, because a variation in the partial pressure of the 

 two principal gases is generally compensated for by changes in the activity of the 

 body as well as in the gas content of the blood. This compensation, however, has 

 its limits, so that any extraordinary alteration in the partial pressures of the gases 



^ Benedict and Cathcart, Muse, work, etc., Carnegie Institution of Washington, 

 1913. 



2 Zuntz and Mehring, Pfliiger's Archiv, xxxii, 1883. 173. 



3 Speck, Deutsch. Arch, fur klin. Med., xxxiii, 1889, 375. 



