288 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 volume of the carbon dioxide exhaled, divided by the 

 volume of the oxygen absorbed, gives what is known as the respiratory quo- 

 tient; thus 



CO 2 exhaled 

 O 2 absorbed 

 Normally in man on a mixed diet the respiratory quotient is 



4.0 to 4.5 



= 0.8 to 0.9. 



But it is subject to variation through several causes; for example, through 

 variation in the composition of the 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 the molecule. On a diet 

 containing much fat the quotient 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 proteins 

 which also require much oxygen for their complete oxidation. Muscular 

 exertion raises the respiratory quotient, because in its performance carbo- 

 hydrates are used up in relatively greater quantity. 



The Watery Vapor in Respired Air. The quantity of water vapor 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 the volume of air, the 

 greater also will be the quantity of moisture exhaled; 2. by the quantity of 

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

 greater the moisture inhaled, the less will be the amount to complete the 

 saturation of the air; 3. by the temperature of the expired air; for the higher 

 the temperature the greater will be the quantity of water 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 water 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- 



