INTRODUCTION 



The phenomena covered by the word "respiration" are very 

 diverse. When a person is seen to breathe, what is observed 

 is a movement of the chest and abdomen by which air is 

 alternately drawn into his lungs and again expelled. This 

 constitutes the mechanical aspect of respiration. In man the 

 mechanical breathing is essential to life, and it is one of the 

 old tests for death to see whether these movements have 

 ceased completely. In very cold weather it can be directly 

 seen that the air expired contains a considerable amount of 

 moisture above that present in the inspired air, and gas 

 analysis reveals the fact that it contains less oxygen and more 

 carbon dioxide. The absorption of oxygen into the lungs 

 and blood, and the elimination of a similar amount of carbon 

 dioxide constitute the physico-chemical aspect of respiration. 

 A comparison of the venous blood entering the lungs with the 

 arterial blood leaving them combined with a determination 

 of the rate of flow shows that the oxygen removed from the air 

 in the lungs is transferred to the blood and distributed to the 

 tissues where some of it disappears and is replaced by CO2 

 and H 2 0. This "internal respiration" is the essential process, 

 liberating energy like the burning up outside the organism of 

 coal or any suitable organic material, but with the essential 

 difference that while combustion processes outside an organ- 

 ism are generally limited by the supply of combustible ma- 

 terial and oxygen they are within the body regulated, so as to 

 supply the needs of the organism, and largely independent 

 both of the supply of material and of the quantity of oxygen 

 offered, provided this quantity is sufficient to meet the demand 

 (Pfliiger, 1872). 



Within each cell the demand varies with the activity of that 

 cell, and the demand of the organism taken as a whole also 



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