62 VERTEBRATE RESPIRATION 



minutes is a reasonable estimate, but both this and the time sug- 

 gested for a seal are optimistic, because there is Uttle doubt 

 that the metabolic rate of an animal swimming under water 

 must be greater than during rest at the surface. Allowance has 

 been made for this difference in work on the guillemot where, 

 assuming a resting metabolic rate, the oxygen store was sufficient 

 for 3 minutes but if the animal was swimming at the not un- 

 reasonable speed of 2 metres per second, the oxygen would only 

 be sufficient for a 20-second dive. 



Many poikilotherms such as the alligator and toad show a 

 reduction in their metabolic rate when they dive and in this way 

 are able to extend the time before their oxygen stores become 

 exhausted. In some cases amphibians hibernate beneath the 

 surface and their oxygen consumption is reduced almost to zero. 

 Some diving turtles also have reduced metabolic rates but 

 others seem to depend almost entirely on anaerobic metabolism. 



Although the economic use of their Og stores is important, 

 the duration of the dives of some mammals is of a different order 

 from that of the dives of animals which can be sustained solely 

 by this means. The seal is a convenient animal for experimental 

 purposes and Scholander made the following observations when 

 an animal was submerged in the laboratory : 



(i) Air was expired from the lung, which facilitated the dive 

 by reducing the animal's buoyancy and also decreased the 

 danger of Caisson sickness which may result if too much 

 nitrogen is forced into solution in the blood (see p. 64). 

 (ii) Almost immediately the heart rate dropped from 150 to 

 10 beats per minute. Blood samples showed that the 

 oxygen in the arterial blood dropped from full saturation 

 (29 vols, per cent) to about 2 or 3 vols. % which is the 

 danger level, 

 (iii) The lactic acid concentration in the arterial blood in- 

 creased steadily as did the CO2 tension. 



On returning the animal to the surface : 

 (i) Air was immediately taken into the lungs ; 



