4 VERTEBRATE RESPIRATION 



Chapter 5). This is partly because they are warm-blooded and 

 therefore the resting metabolism given is at a higher temperature 

 than for the others. But even at the same body temperature, 

 poikilothermic animals of the same size show a metabolic rate 

 which is markedly lower than that of birds and mammals. This 

 is also true when the oxygen consumption of mammals is 

 measured with their body temperature lowered (e.g., a man 

 under hypothermia). Despite these variations and those due to 

 body size, it is clear that the range of resting metabolic rates 

 found among vertebrates is not as great as might be imagined 

 when one considers the wide range in their structure and mode 

 of life. There are, of course, wide extremes such as humming 

 birds which require more than 10,000 ccs of oxygen per kilo- 

 gram per hour, but a man, for example, needs only 2 to 3 times 

 the volume of oxygen per kilogram per hour required by a bony 

 fish. The problems faced by a mammal and a fish in obtaining 

 these comparable volumes of oxygen are entirely different and, 

 as will be seen later, the performance of the fish is perhaps the 

 more notable. 



(b) A COMPARISON BETWEEN AIR AND WATER AS 

 RESPIRATORY MEDIA 



Some of the differences in physical properties of air and water, 

 from the respiratory point of view, are tabulated in Table 2. It 

 is immediately apparent that the volume of oxygen contained in 

 unit volume of air is very much greater than the maximum 

 possible for natural waters. Consequently, to obtain Ice of 

 oxygen a fish must pass a far greater volume of the medium 

 across its respiratory surface than a terrestrial vertebrate. 

 Another aspect of this difference is that a litre of air contains 

 sufficient oxygen to support a given animal about 25 times longer 

 than a litre of water saturated with air. Moreover, water is far 

 denser than air at the same temperature and its viscosity is much 

 greater. There is also a very marked diff'erence in the diff'usion 

 rates for oxygen and ventilation of the respiratory surface is 

 therefore very necessary. This applies both to aquatic and 

 terrestrial forms because even in a lung there is a thin film of 



