INTRODUCTION 7 



decreases very markedly. Consequently the volume of water 

 necessary to supply sufficient oxygen to the fish increases about 

 46 times notwithstanding the assumption that it can extract 75 % 

 of the total volume of oxygen contained in the respiratory 

 current. This high utilisation is a notable feature of the fish 

 respiratory system but, even so, it cannot be maintained at very 

 high volumes of pumping. Lowering of the utilisation aggravates 

 the situation still further and it is easy to see that at a certain 

 level all the extra oxygen that a fish is able to extract from the 

 water will be used by the respiratory muscles themselves. Above 

 this level the animal cannot possibly satisfy its oxygen require- 

 ments. 



Although fish face many problems relating to their oxygen 

 supply, and the gills may be the site of ionic exchanges, at 

 least they are generally free from the dangers of desiccation 

 which may be considerable on land. The thin film of moisture 

 at the respiratory surface easily evaporates and may cause 

 significant loss of water. This factor is important when the 

 basic construction of these organs is considered. A very large 

 surface is necessary but loss of water must be reduced. In terres- 

 trial animals the increased surface results from the folding of an 

 internal sac which communicates with the exterior through a 

 relatively small opening. The cavity of this sac or lung can there- 

 fore remain almost saturated with water and loss through the 

 respiratory opening is relatively slight. Aquatic forms do not 

 have this problem because the respiratory surface usually 

 develops externally as a much divided gill. Furthermore, this 

 structure is well-supported by the aquatic medium and is not 

 in danger of collapsing and the respiratory epithelium becoming 

 occluded because of surface tension forces. 



Despite the marked diff'erences in the physical properties of 

 air and water the fundamental processes of gaseous exchange 

 at a gill and in an alveolus must be the same. This has been 

 emphasized by the recent discovery that it is possible to maintain 

 alive a mammal breathing liquid, provided that the partial 

 pressure of oxygen in it is increased so that the diff'usion gradient 

 is adequate to meet the demands of the animal. 



