Introduction 



The structure and organisation of vertebrates are extremely 

 complex whether considered at the gross level of the mechanics 

 of the skeletal and muscular systems or at the cellular and 

 molecular levels. Such complexity requires the expenditure of 

 energy if it is to be maintained. Furthermore, energy is re- 

 quired for the performance of the normal activities of the 

 organism. The energy for both of these processes is provided by 

 the degradation of large molecules and ultimately depends on 

 oxidative reactions in the cells. The provision of the oxygen and 

 hence the energy for the vital processes of the organism is the 

 fundamental contribution which respiration makes to the 

 physiology of all living organisms. Although these cellular 

 oxidative processes are of basic importance, they cannot take 

 place in animals the size of vertebrates in the absence of mechan- 

 isms which enable oxygen to be transferred from the external 

 medium to the cells. The first of these, involving the intake of 

 oxygen to the organism and the removal of carbon dioxide, is 

 referred to as external respiration. Among vertebrates, many 

 adaptations are found in the respiratory organs which enable 

 this exchange to take place efficiently. These are partly due to 

 the extent (Table 1) and nature of the surfaces which allow rapid 

 diffusion to take place, but also include mechanisms for chang- 

 ing the medium at the epithelial surface in order to ensure a 

 constant supply of oxygen. Diffusion processes are sufficient 

 to account for the exchange at all respiratory surfaces although 

 some early workers believed that oxygen was actively secreted 

 into the blood in some respiratory organs. 

 Above a certain average diameter, diffusion is insufficiently 



