88 VERTEBRATE RESPIRATION 



adaptation to life under these conditions. In water the insulation 

 properties of fur are markedly reduced and in aquatic mammals 

 are replaced functionally by blubber beneath the skin. In seals 

 the skin temperature is only slightly above that of the surround- 

 ing water but the visceral temperature is maintained at 37° C. 

 Circulatory adaptations are important in aquatic homoio- 

 therms, for the temperature of the flippers and legs may be ten 

 degrees below the temperature of the main body. This is main- 

 tained by heat transfer across a counter-current exchange 

 system between the afferent and efferent blood flows into the 

 distal extremities (fig. 25). In this way the extremities continue 

 to receive their oxygen but get less heat from the body as it 

 would be dissipated at their surfaces. 



Centimetres 

 5 



R. Caudal 



(a) 



Lagenorhynchus acutus 



<\^ 



(b) 



10° 20° 30° AO' 



'i \ \ ras 



_91J9° 29° 39^ |z 





v > -l^y'^ JJJJ^^ J^J^ >^.>_ 



Extremity 





Trunk 



Fig. 25. 



Diagrams to show (a) the juxtaposition of the afferent and efferent 

 blood vessels to the fins and flukes of a porpoise. Each artery is sur- 

 rounded by a multiple venous channel, (b) The counter-current 

 mechanism ensures a conservation of heat (after Scholander, 1958). 



