Vk)° 'Respiration 



WITHOUT oxygen the cells of the body 

 deteriorate quite rapidly. Certain parts of 

 the brain, indeed, may be damaged irrepa- 

 rably by less than five minutes of complete 

 asphyxiation. This probably represents an 

 extreme of vulnerability, but the fact re- 

 mains that the cells of most animals need a 

 fairly constant supply of oxygen. Also, the 

 carbon dioxide produced by the body must 

 constantly be removed if the cells are to 

 maintain their living structure and activity. 



The problem of respiration is not very 

 acute among small aquatic animals, like Para- 

 mecium or Hydra. The respiratory gases have 

 relatively high diffusion rates. Oxygen from 

 the surrounding water enters the protoplasm 

 of such small organisms as fast as it is con- 

 sumed, and carbon dioxide passes out to the 

 environment before it accumulates unduly 

 in the protoplasm. Therefore no specialized 

 respiratory structures are necessary among 

 such small aquatic forms. 



With larger animals, the deeper-lying tis- 

 sues would soon be asphyxiated if they de- 

 pended solely upon diffusion in obtaining 

 their oxygen and disposing of their carbon 

 dioxide. Oxygen from the environment would 

 be used up by the superficial cells before it 

 could reach the deeper ones; and the super- 



ficial tissues would be called upon to elimi- 

 nate not only their own carbon dioxide, but 

 also that produced by the deeper cells. Ac- 

 cordingly, the problem of respiration becomes 

 more acute as the mass of the animal body 

 becomes greater, and in all larger animals 

 the circulatory system takes over the func- 

 tion of transporting the respiratory gases 

 between the deep-lying tissues and a respir- 

 atory surface. 



With the intervention of the circulatory 

 system, the processes of respiration become 

 more complicated. In fact, the respiration of 

 higher animals involves three processes: (1) 

 external respiration — the exchange ot oxy- 

 gen and carbon dioxide between the blood 

 and the environment; (2) the transportation 

 of oxygen and carbon dioxide in the blood 

 stream to and from the various tissues; and 

 (3) internal respiration — the exchange of 

 oxygen and carbon dioxide between the 

 blood and the tissues of the body. 



EXTERNAL RESPIRATION 



Each animal is equipped with some sort of 

 moist surface at which the blood undergoes 

 aeration. Typically the respiratory surface 

 comes into direct contact with the environ- 



357 



