2 VERTEBRATE RESPIRATION 



rapid to provide enough oxygen to meet the oxygen require- 

 ments of living organisms. All adult vertebrates come into this 

 range and the supply of oxygen to the tissues following its 

 absorption at the respiratory surface is maintained by a con- 

 vection process involving circulation of the blood. The circu- 

 latory system forms the transport mechanism of the body and 

 blood carries the oxygen partly in solution but mainly in com- 

 bination with a respiratory carrier (haemoglobin) contained in 

 the red blood corpuscles. There are one or two exceptions to 

 this generalisation, however, because haemoglobin is com- 

 pletely absent from the bloods of certain vertebrates. The 

 Leptocephalus larva of the eel is a well-known example which is 

 transparent and contains no red pigment. The supply of oxygen 

 is greatly facilitated by the thinness of these larvae and diffusion 

 through the relatively large body surface plays an important 

 part. Recently a whole group of fishes in the Antarctic has been 

 shown to be completely devoid of haemoglobin although they 

 live relatively normal lives. At the low temperatures of their 

 marine environments the water contains a large percentage of 

 oxygen and their metabolic rate is probably low. In addition to 

 these 'ice-fish' several examples have been described in verte- 

 brates of abnormal specimens containing no haemoglobin (e.g., 

 the clawed toad Xenopus). All these animals without haemo- 

 globin appear to lead fairly normal lives although they do not 

 become highly active. It is probable that the presence of haemo- 

 globin is essential for very active organisms but even when it is 

 destroyed by combination with carbon monoxide many fishes, 

 for example goldfish, are able to live for reasonable lengths of 

 time so long as they are kept at low temperatures and remain 

 inactive. 



(a) THE OXYGEN REQUIREMENTS OF VERTEBRATES 



The amount of oxygen used by animals varies according to 

 many factors. Most important are their size, activity, and the 

 temperature. The figures in Table 1 are representative of verte- 

 brates when at rest. It is clear that the metabolic rate of birds 

 and mammals is greater than that of lower vertebrates (see 



