266 RESPIRATION 



the loss being due to the fact that a portion of the oxygen absorbed is not 

 returned in the form of carbon dioxide. Since the oxygen of a given volume 

 of carbon dioxide would have the same volume as the carbon dioxide 

 itself at a given temperature and pressure, a portion of the oxygen absorbed 

 must be used for other purposes than the formation of carbon dioxide. 

 In fact, some of it is used in the formation of urea, some in the formation 

 of water, etc. The oxygen in the carbon dioxide exhaled, divided by the 

 oxygen absorbed, gives what is known as the respiratory quotient ; thus 



CO 3 exhaled 

 O a absorbed 



Normally in man on a mixed diet the respiratory quotient is 



4.0 to 4.5 



= 0.8 to 9.9. 



But it is subject to variation through several causes. For example, through 

 variation in diet. On a pure carbohydrate diet the respiratory quotient 

 will rise above 0.9, i.e., to i.o, since carbohydrates contain enough oxygen 

 to oxidize the hydrogen in their molecule. On a diet containing much fat 

 it is lowest, since relatively more oxygen is needed completely to oxidize fat. 

 The theoretical respiratory quotient for fats is 0.7. The same is true, but to 

 a less degree, in the case of proteids which also require much oxygen for their 

 complete oxidation. Muscular exertion raises the respiratory quotient, because 

 in its performance carbohydrates are used up in relatively greater quantity. 



The Watery Vapor in Respired Air is Increased. The quantity emitted 

 is, as a general rule, sufficient to saturate the expired air, or very nearly so. 

 Its absolute amount is, therefore, influenced by the following circumstances: 

 i, By the quantity of air respired; for the greater this is, the greater also 

 will be the quantity of moisture exhaled; 2, By the quantity of watery 

 vapor contained in the air previous to its being inspired; because the greater 

 this is, the less will be the amount to complete the saturation of the air; 3, 

 By the temperature of the expired air; for the higher this is, the greater will 

 be the quantity of watery vapor required to saturate the air; 4, By the length 

 of time which each volume of inspired air is allowed to remain in the lungs; 

 for although, during ordinary respiration, the expired air is always saturated 

 with watery vapor, yet, when respiration is performed very rapidly, the air 

 has scarcely time to be raised to the highest temperature or be fully charged 

 with moisture ere it is expelled. 



The quantity of water exhaled from the lungs in 24 hours ranges (accord- 

 ing to the various modifying circumstances already mentioned) from about 

 200 to 800 c.c., the ordinary quantity being about 400 to 500 c.c. Some of 

 this is probably formed by the chemical combination of oxygen with hydro- 

 gen in the system; but the far larger proportion of it is water which has been 



