COMPOSITION OF THE AIR. 135 



CHAPTER V. 



CHANGES WHICH THE AIR AND THE BLOOD UNDERGO IN RESPIRATION. 



Composition of the air Consumption of oxygen Exhalation of carbon dioxide Relations between the 

 quantity of oxygen consumed and the quantity of carbon dioxide exhaled Sources of carbon dioxide 

 in the expired air Exhalation of watery vapor Exhalation of ammonia Exhalation of organic matter 

 Exhalation of nitrogen Changes of the blood in respiration (haematosis) Difference in color between 

 arterial and venous blood Comparison of the gases in venous and arterial blood Analysis of the blood 

 for gases Nitrogen of the blood Condition of the gases in the blood Relations of respiration to nutri- 

 tion etc. The respiratory sense Sense of suffocation Respiratory efforts before birth Cutaneous 

 respiration Breathing in a confined space Asphyxia. 



FROM the allusions already made to the general process of respiration, it 

 is apparent that before the discovery of the nature of the gases which com- 

 pose the air and those which are exhaled from the lungs, it was impossible 

 for physiologists to have any correct ideas of the nature of this important 

 function. It is also evident that no definite knowledge of the processes of 

 respiration could exist prior to the discovery of the circulation of the blood. 



The discovery of the properties of oxygen and carbon dioxide were simply 

 isolated facts and failed to develop any definite idea of the changes of the air 

 and blood in respiration. The application of these facts was made by La- 

 voisier, whose observations mark the beginning of an accurate knowledge of 

 the physiology of respiration. With the balance, Lavoisier showed the nature 

 of the oxides of the metals ; he discovered that carbon dioxide is formed by a 

 union of carbon and oxygen ; and noting the consumption of oxygen and the 

 production of carbon dioxide in respiration, he advanced, for the first time, 

 the view that the one was concerned in the production of the other. Although, 

 as would naturally be expected, the doctrines of Lavoisier have been modified 

 with the advances in science, he developed facts which have served as the 

 starting-point of definite knowledge on this subject. 



Composition of the Air. Pure atmospheric air is a mechanical mixture 

 of 79-19 parts of nitrogen with 20-81 parts of oxygen (Dumas and Boussin- 

 gault). It contains, in addition, a very small quantity of carbon dioxide, 

 about one part in two thousand. The air is never free from moisture, which 

 is very variable in quantity, being generally more abundant at a high than at 

 a low temperature. Floating in the atmosphere, are large numbers of minute 

 organic bodies ; and various odorous and other gaseous matters sometimes are 

 present as accidental constituents. 



In considering the processes of respiration, it is not necessary to take 

 account of any of the constituents of the atmosphere except oxygen and 

 nitrogen, the others being either inconstant or existing in excessively minute 

 quantity. It is necessary to the regular performance of respiration, that the 

 air should contain about four parts of nitrogen to one of oxygen, and have 

 about the density which exists on the general surface of the globe. When 

 the density is very much increased, as in mines, respiration is more or less 

 disturbed. By exposure to a rarefied atmosphere, as in the ascent of high 

 mountains or in aerial voyages, respiration may be very seriously interfered 

 with, from the fact that less oxygen than usual is presented to the respiratory 



