4 i 4 TEXT-BOOK OF PHYSIOLOGY 



turbidity which immediately follows is due to the formation of barium or 

 calcium carbonate, which can be due only to the presence of carbon dioxid. 

 That this turbidity is not due to the carbon diox d normally present in the 

 air is shown by the fact that the solution remains clear until the passage of 

 the atmospheric air has been maintained for some time. From the percent- 

 age loss of oxygen and gain in carbon dioxid, the total oxygen absorbed and 

 carbon dioxid exhaled may be approximately calculated. Thus, if the 

 volume of air breathed daily be accepted at either 8500 or 12960 liters, and 

 the percentage loss of oxygen be 4.78, the total oxygen absorbed may be 

 obtained by the rule of simple proportion, e.g. : 



ioo : 4.78 :: 8500 : # 406 liters or 580 grams 1 

 Or 



ioo : 4.78 :: 12960 : # 620 liters or 885 grams. 



By the same method the total carbon dioxid exhaled is found to be either 

 372 liters or 735 grams, or 567 liters or 1122 grams; volumes in both in- 

 stances which agree very well with volumes obtained by other methods. 

 From the fact that only 567 liters of carbon dioxid are exhaled as 

 compared with 620 liters of oxygen absorbed, it is evident that not all of 

 the oxygen unites with carbon to form carbon dioxid and that the re- 

 mainder of the oxygen must unite with some other element. As there is 

 usually an excess of water eliminated over that introduced into the body, it 

 is highly probable that the oxygen combines with free hydrogen to form 

 water. The relative amounts o{ the oxygen so utilized are not fixed but 

 variable, and depend on the quality and quantity of the foods, exercise, etc. 

 The ratio of the volume of the carbon dioxid exhaled to the volume of 

 oxygen absorbed is known as the respiratory quotient, and is usually repre- 



CO 



sen ted by the symbol ~"-. Thus in the foregoing analysis the respiratory 



quotient is 0.916 which indicates that 0.916 parts of the oxygen absorbed 

 is utilized in oxidizing carbon and the remainder, 0.084, m oxidizing free 

 hydrogen to form water. 



The gain in nitrogen is a variable factor, ranging from zero to 0.9 per 

 cent. This gain is probably of accidental occurrence, due to absorption 

 from the large intestine, in which decomposition of nitrogen-holding com- 

 pounds is taking place. It is generally believed that free nitrogen plays no 

 part in any phenomenon of combination or decomposition within the body. 



The gain in watery vapor will depend on the amount previously present 

 in the air. This is conditioned by the temperature. With a rise in tempera- 

 ture the percentage of water increases; with a fall, it decreases. By breath- 

 ing into a vessel containing pumice stone saturated with sulphuric acid, the 

 vapor may be collected. The difference observed between the weight before 

 and after breathing is an indication of the amount by weight of water exhaled 

 during the time of breathing. It has been calculated that the amount of 

 water exhaled daily varies between 300 and 500 grams. Though invisible 

 at ordinary temperatures, it becomes visible at low temperature as soon as 

 it emerges from the respiratory tract. The loss of heat is followed by a con- 

 densation of the vapor, which appears at once as a cloudy precipitate. 



The gain in organic matter is also variable. The amount present is not 



1 i liter of oxygen weighs 1.4298 grams; i liter of carbon dioxid weighs 1.977 grams. 



