RESPIRATION 209 



The Chemistry of Respiration. 



1. Pulmonary Respiration. 



The general principles of the chemical side of respira- 

 tion are easily understood, though the details are in 

 many points still involved in obscurity. The essence 

 of the process consists in the conveyance of oxygen to 

 the tissues and the removal of carbonic acid from them. 

 The lungs thus play a double part. They absorb oxygen 

 from the air just as the stomach and intestine absorb 

 nutritive constituents from the food, and they excrete 

 carbonic acid just as the kidneys excrete urea. Disease 

 may result from a disorganization of either of these 

 functions : on the one hand from failure of the lungs 

 to absorb sufficient oxygen, and on the other hand from 

 an inability on their part to excrete carbonic acid. 



We may now look at some of the practical bearings 

 of this interchange between the lungs and the air in 

 greater detail. 



If one compares the composition of the air as it 

 enters and leaves the lungs, one gets such a result as 



the following : 



Inspired Air. Expired Air. 



Oxygen ... 20'96 per cent. 16'03 per cent. 

 Nitrogen ... 79 per cent. 79 per cent. 



CO 2 ... 0*04 per cent. 4*4 per cent.* 



In addition, the expired air is saturated with water 



* Variations in ' respiratory exchange ' i.e., the total consump- 

 tion of oxygen and excretion of C0 2 usually discussed under 

 1 Respiration,' are really the expression of variations in metabolism, 

 and are considered under that subject. 



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