274 HANDBOOK OF PHYSIOLOGY. 



the carbon dioxide exhaled, divided by the oxygen absorbed, gives what 

 is known as the respiratory quotient ; thus 



C0 2 exhaled 

 2 absorbed 



Normally in man on a mixed diet the respiratory quotient is 



*--*- 5 = 0.8-0.9. 

 5 



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

 through variation in diet. On a carbohydrate diet the respiratory quo- 

 tient may rise above 0.9, since carbohydrate contain enough oxygen to 

 oxidize the carbon in their molecule. On a diet containing much fat it 

 is lowest, since oxygen is needed to completely oxidize it. And the same 

 is true, but to a less degree, in the case of proteids. Muscular exertion 

 raises the respiratory quotient, because in its performance carbohydrates 

 are used up. 



5. The watery vapor 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 circum- 

 stances, (1), by the quantity of air respired; for the greater this is, the 

 greater also will be the quantity of moisture exhaled; (2), by the quan- 

 tity 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 re- 

 quired 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 scarce ] y 

 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 twenty-four hours 

 ranges (according to the various modifying circumstances already men- 

 tioned) from about to 27 ounces, the ordinary quantity being about 9 

 or 10 ounces. Some of this is probably formed by the chemical com- 

 bination of oxygen with hydrogen in the system; but the far larger 

 proportion of it is water which has been absorbed, as such, into the 

 blood from the alimentary canal, and which is exhaled from the surface 

 of the air-passages and cells, as it is from the free surfaces of all moist 

 animal membranes, particularly at the high temperature of warm-blooded 

 animals. 



6. A small quantity of ammonia is added to the ordinary constitu- 

 ents of expired air, It seems probable, however, both from the fact that 



