316 THE MAINTENANCE OF THE INDIVIDUAL 



to all inner jjarts of the body where they end blindly making possible 

 the delivery of oxygen directly to the cells. Here again external res- 

 piration takes place in the spiracular region, while internal respiration 

 centers about the diffusion of gases to and from the tracheae and the 

 cells. The efficacy of this system is suggested by the rapid and sus- 

 tained metabolism common to many of the insects. 



Lungs. This type of respiratory system is found best developed 

 among the birds and mammals. The lungs of birds are specialized 

 for a high metabolic rate and for making lighter the load which must 

 be lifted in flight. Air sacs connected with the lungs are found 

 throughout the viscera, and even the bones are filled with air and so 

 are very light. The connection between these and the rest of the 

 respiratory system has been demonstrated by closing the trachea and 

 opening the air sac in an upper wdng bone. The fact that the bird 

 continues to breathe demonstrates this connection. 



The mammalian respiratory system is essentially the same regard- 

 less of the form studied. The most important part of the lungs are 

 the terminal air sacs called alveoli, in which the inspired air contacts 

 the many capillaries of the circulatory system found throughout 

 the moist mucous membranes. Oxygen and carbon dioxide diffuse 

 through the capillary walls surrounding the alveoli and so the 

 exchange of gases is effected. 



Internal Respiration 



It has been shown in the case of very simple animals, such as 

 Paramecium, that when oxidation of food takes place in the cell 

 energy results. In forms which possess complicated circulatory sys- 

 tems, external respiration must first take place, after which oxygen 

 is transported by the hemoglobin of the blood to the various parts 

 of the body where the actual work is to be done. Here real or in- 

 ternal respiration takes place, since cell activity depends upon food 

 and oxygen. 



As aerated blood passes through the capillaries these are bathed 

 in plasma in which the oxygen pressure is low. The oxyhemoglobin, 

 a compound of oxygen and hemoglobin, is stable only in an environ- 

 ment where oxygen pressure is comparatively high. Therefore the 

 hemoglobin delivers itself of the oxygen to the lymph, which in turn 

 transfers it to the cells. The pressure of carbon dioxide on the other 

 hand is higher in the cells thus facilitating its transfer to the lymph 

 and so to the blood stream proper. 



