CHAPTER XX 

 BREATHING 



Respiration has been defined earlier in this book as 

 the oxidation process which accompanies life and serves 

 to release the potential energy of the physiologic fuels. 

 In this, intimate sense respiration is best studied as a 

 part of metabolism, the chemistry of the living matter. 

 But while the facts of the circulation are fresh in mind 



FIG. 67.' A microscopic unit (air-cell) from the lung tissue. The 

 capillaries are wrapped about a sac the form of which may be likened 

 to that of a blackberry. In the drawing this sac is represented as 

 partly laid open and the minute bronchial tube leading to it is slit. 

 The rent shows the concavities of the interior surface. 



we shall do well to enter upon a treatment of breathing 

 and the carriage of gases in the blood. Breathing is 

 not respiration, in the best use of that word, but it is 

 preliminary to it and also subsequent to it; it introduces 

 the oxygen into the blood and provides for the escape 

 of the carbon dioxid into the air. 



The Respiratory Apparatus. It has been said already 

 that the lungs are organs in which the practical con- 

 tact of blood and air can be maintained. The pul- 



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