10 



Conclusions 



From this survey of the respiration of vertebrates several general 

 conclusions can be drawn. 



1. We cannot help but be impressed by the variety of mechan- 

 isms that are used to maintain the supply of oxygen to the tissues 

 of vertebrates. They possess many adaptations which enable the 

 physiological requirements of individual species to be met under 

 very different conditions. These adaptations extend from gross 

 differences in the basic structure of a gill and a lung to finer 

 modifications of these organs to various ecological requirements. 



2. The mechanism for the renewal of the medium at the 

 respiratory surface is another feature which has been modified 

 a great deal (Table 12). Throughout the vertebrates the respir- 

 atory current is maintained by two basic types of pump — suction 

 pumps which draw the medium into contact with the respiratory 

 epithelium and pressure pumps which force it across or into 

 contact with these surfaces. Both mechanisms have advantages 

 and the most efficient ventilation is achieved by fishes and birds 

 which combine both systems and so maintain an almost un- 

 interrupted flow of the respiratory medium. 



3. In spite of paying detailed attention to such mechanisms we 

 have also become aware of the necessity to consider the physi- 

 ology of other systems. For example, we have needed to discuss 

 the structure and physiology of the heart and circulatory system, 

 the mechanisms of nervous co-ordination, the importance of 

 water conservation and ionic regulation at the thin respiratory 

 surfaces, and also nutrition which provides the different respira- 

 tory substrates and helps to maintain constant body conditions. 

 The impossibility of considering one organ system in complete 

 isolation is generally accepted in physiology, and for a com- 

 parative physiologist the principle is constantly before him 



