18 



RESPIRATORY MECHANISMS 



(Koch, 1936) oxygen has been observed to move from the 

 blood (or tracheae) towards the medium when the oxygen 

 pressure in the medium is made lower than that in the blood 

 (or tracheae). This argument would not have much force if 

 the epithelium was irreversibly damaged in the process, but 

 in the cases examined it could function normally afterwards, 

 and Koch even showed that the rate of diffusion in both 

 directions through the dragon fly gills was the same. 



A study of the quantitative aspect of the exchange by 

 diffusion requires determinations of diffusion rates, distances 

 through which diffusion has to take place, and quantities 

 actually transported. 



Air. 

 /^Utilization 

 of oxygen 

 VJl'ecLjat constant 

 '/iW///A rate 



/// f"e^brane,G 

 yAntlne or tl 



Atm.eir 



saturated 



water. 

 Well 

 mixed 



Gela- 

 ssue 



Air or fluid 



Units < 



or 



or 



Utilization 

 of oxygen at 

 constant rate 

 P 

 thickness 



area 



cjh length 



2 



cm cross-section 



Time in minutes 



Pressure difference in atmospheres (- 760 mm) 



Diffusion coefficients at 20° C. 



in air - 11 in water 0.000034 



gelatine 0.000028 



connective tissue 0.000011 



Diagram illustrating a steady state diffusion of oxygen through an air 

 space and through a membrane. 



Determinations of diffusion rates can be made with least 

 difficulty if a steady state can be established. Suppose that 

 in Fig. 2 (left) we have a tube opening at one end to the 

 atmosphere and at the other to a small space in which O2 is 

 used up at a known constant rate. In this space a certain 

 2 deficit will be established and maintained, and when this 

 is determined the rate of diffusion can be calculated. This 

 rate is proportional to the area of cross-section and inversely 



