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RESPIRATORY MECHANISMS 



Wigglesworth's discovery is that this fluid moves back and forth 

 in relation to the call for oxygen of the tissue as illustrated in 

 the diagram (Fig. 68). By the drawing back of the fluid in 

 the tracheoles, oxygen can reach the active cells by diffusion 

 through air instead of through fluid, and there is reason to 

 believe that the supply can exceed even that which is possible 

 by means of circulating blood in warm-blooded animals. By 

 far the largest oxygen absorptions have been recorded for 

 flying insects. 



Fig. 68. Tracheoles running to a muscle fibre; semischematic. A, muscle 

 at rest; terminal parts of tracheoles contain fluid. B, muscle fatigued; air 

 extends far into tracheoles. (Wiggles worth.) 



The mechanism of this regulation appears to be as follows: 

 The tracheole walls are permeable for water, and it should be 

 borne in mind that a high degree of permeability for oxygen 

 is, as far as we know, not physically possible without permea- 

 bility for water. The force of capillarity in tubes of the di- 

 mensions of tracheoles is very large, being about 10 atmos- 

 pheres in a wet tube of 0.3 /x diameter. The osmotic pressure 

 of the tissue fluid is of the same order, and if we assume the 

 tracheoles to be "semi-permeable," that is, permeable to water 

 only, equilibrium should be attained with the meniscus at some 

 point in the tracheoles, while water should be unable to enter 

 the wider tracheae. Activity brings about an increase in the 

 osmotic concentration in the active tissue, and this should 

 cause the water to recede towards the narrower part of the 

 tracheole. In a number of most beautiful experiments 



