CHAPTER VII 

 RESPIRATION 



Introduction 



Respiration is the exchange of oxygen and carbonic acid 

 between the organism and its surroundings. In evolu- 

 tion a special mechanism for the transport of these gases 

 makes its appearance as soon as any of the tissues are 

 excluded from direct contact with the medium in which 

 the animal lives. A separate tissue, the blood, is developed 

 principally, though not exclusively, for this function ; 

 the blood serving to carry oxygen from the external cells 

 which can supply it to the internal cells which need it, and 

 to drain the internal cells of the COg which is constantly 

 being formed within them. 



With the appearance of land animals the process becomes 

 comphcated, owing to the fact that gaseous exchange now 

 involves a change of state. Oxygen taken from the air 

 has to be brought into solution, and COg has to pass from 

 solution into the free state. Moreover, the exchange of 

 oxygen and carbonic acid between the animal and its 

 environment occurs no longer on the surface of the body 

 but in its interior — ^in the lungs. There are therefore no 

 less than four stages in the process of assimilating oxygen. 

 In the first, oxygen passes from the atmosphere to the air 

 in the lung ; in the second it passes into the blood ; in 

 the third it is transported in the blood to the whole body ; 

 in the fourth it passes from the blood to the tissues. Four 

 corresponding stages occur in the removal of carbonic acid. 

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