CHAPTER XIX. 



THE CHEMISTRY OF RESPIRATION. 



The simplest way to investigate the study of the gas inter- 

 change that takes place in the lungs between the air and the 

 blood is to compare the composition of the expired air with that 

 of the atmosphere, and from the alteration found to have taken 

 place in the tidal stream we can arrive at the changes which the 

 air undergoes during its journey in and out of the air passages, 

 and we can then examine the venous and arterial blood in order 

 to ascertain the change the blood undergoes in becoming arterial. 



The atmosphere is made up of a mixture of nitrogen and oxy- 

 gen, with a variable amount of moisture and a minute proportion 

 of carbonic acid. 



The following table gives the volume* of the gases in dried 

 air: 



Oxygen, 20.96 per cent., or about 21 per cent. 



Nitrogen, 79.02 " *' 79 " 



Carbonic dioxide, 0.02-0.06 " * 4 parts in 10,000. 



The amount of moisture contained in the air is very variable, 

 and depends in a great measure upon the temperature and the 

 direction of the wind. The dampness of the air depends upon 

 the temperature, so that air containing the same absolute amount 

 of moisture may be relatively dry or damp, according as the 

 temperature rises or falls. As a general rule, the air is relatively 

 dry, that is to say, it does not contain so much moisture as it is 

 capable of taking up in the form of aqueous vapor at its ordinary 

 temperature. At certain times of the day the air may be satu- 

 rated, owing to a sudden fall of temperature. 



The temperature of the air which we breathe, of course, varies 



* On account of the difference in the atomic weights, the atmosphere 

 being only a mechanical mixture of the gases, the proportion by weight 

 is 'slightly different, being about, Oxygen 23 per cent., Nitrogen 77 per 

 cent. 



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