IV 

 RESPIRATION IN WATER 



Although the respiratory conditions in air are the more 

 favourable, there can be no doubt that life in water is the 

 more primitive type and we find there the simplest respiratory 

 arrangements. 



Respiration without respiratory organs and without circulation. 

 In small organisms oxygen can diffuse in through the surface 

 and reach every point within the body. Assuming a homo- 

 geneous spherical body in which oxygen is used up at a con- 

 stant rate, the same throughout, and assuming further that 

 the oxygen tension at the centre is maintained at 0, E. N. 

 Harvey (1928) gives the following equation 



A* 



C ° " 6D 



in which C is the concentration at the surface, expressed in 

 atmospheres, A is the respiratory exchange in ml/g/min, r the. 

 radius of the sphere in cm, and D the diffusion coefficient 

 atm. /cm/cm 2 . With constant metabolism the necessary O2 

 tension difference is seen to be proportional to the square of 

 the radius. 



Taking as an example a spherical organism with 1 cm 

 radius having a metabolism of 100 ml /kg/hour or 1/600 ml/g/ 

 min and a diffusion rate the same as connective tissue 

 (0.000011) we have the necessary oxygen pressure outside 



C ° = 600-6-0.000011 = 25 atm 



showing that an organism of this size cannot have the metab- 

 olism postulated if depending upon diffusion alone. Retain- 

 ing the same metabolism per unit weight, the tension necessary 

 for an organism with 1 mm radius would be 0.25 atm. or 



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