KESPIEATION 321 



processes the respiratory interchange is at once increased. 

 That the increase is dependent upon the increased func- 

 tional activity of the digestive organs is shown by the fact 

 that it is produced by the taking of substances which 

 cannot be absorbed and used in the metabolic processes of 

 the body. 



But while the immediate increase in the respiratory inter- 

 change following the taking of food is due to the increased 

 activity of the digestive structures, there is also an increase due 

 to the utilisation in the body of the food taken. Whether 

 proteins, fats, or carbohydrates, or all of these form the diet, 

 more oxygen is consumed and more carbon dioxide is given off 

 than during starvation. The proportion between the oxygen 

 taken and the carbon dioxide excreted is not, however, the 

 same with all these food-stuffs. If the food is rich in carbon 

 and poor in oxygen, a greater quantity of oxygen must be 

 taken to oxidise it than if it is rich in oxygen. 



The chief constituents of the food contain the following per- 

 centage amounts of oxygen and of carbon : 



Oxygen. Carbon. 



Carbohydrates ' . 53 40 



Proteins . . ' . . . 22 52 



Fats . ... 12 76 



Hence on a carbohydrate diet, such as that of the horse, 

 the respiratory quotient (p. 314) ^- 2 is high, about 0*9 to 1, 

 while on a fatty or protein diet it is low, 07 to-0'8. 



3. Temperature. If the temperature of the body be in- 

 creased, the metabolic processes become more active and 

 the respiratory interchanges are increased. But if the tem- 

 perature of the air round the body be elevated, the metabolic 

 processes may be diminished in activity, and the respiratory 

 exchange decreased.- 



4. Light. It has been shown that light increases the 

 metabolic changes and therefore the respiratory activity. 



5. Sleep. Since in sleep the animal is in a condition of 

 muscular rest, since light is excluded from the eyes and since 

 food is not taken, the respiratory exchanges are less active 

 during sleep than during the waking hours. Similarly, in 



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