64 VERTEBRATE RESPIRATION 



without its harmful effects on the other parts of the animal. On 

 surfacing, the oxygen debt set up during the dive is paid off 

 rapidly, partly because of the efficient respiratory exchange 

 mechanisms but also because of the high oxygen-carrying 

 capacity of the blood. The lung volume is slightly larger in some 

 diving mammals (7% in dolphin, 5% in man and seals) but 

 among whales it is relatively small (2-5 %t). The tidal volume is 

 considerably greater in most divers, being as high as 80% of the 

 lung volume in the porpoise (20 % in man). 



Whether changes similar to those described for seals occur 

 during the diving of whales is not known. There is evidence 

 for bradycardia* but it has not been fully estabhshed. One 

 point of difference is that whales fill their lungs before they 

 dive. A mechanism for taking in oxygen through the single 

 dorsally placed nostril is very efficient, for a projection from the 

 larynx can communicate directly with the internal nostril and 

 the air need not enter the buccal cavity itself. The external 

 nostril is guarded by elaborate valves so that during the dive 

 the aperture is closed. Many whales dive very deeply and appear 

 to incur dangers well known to human divers such as 'Bends' 

 or Caisson sickness. During a deep dive any air taken down by 

 the animal is compressed and partly forced into solution in the 

 blood. On rising the air is decompressed and the gases tend to 

 come out of solution. Any oxygen coming out of solution is 

 readily utilised in the tissues but nitrogen presents a serious 

 problem. Bubbles of this gas appearing in the tissue fluids or 

 blood vessels can have very serious consequences and may lead 

 to death. Whales differ from human divers supplied with air 

 from the surface as this is continuously renewed, but in a whale 

 a given volume of air is taken down which is initially at atmo- 

 spheric pressure. Whales in fact take relatively small amounts 

 because their lung capacities are usually between 1 and 3 per cent 

 of the body weight whereas in man the lungs form at least 5 per 

 cent of the weight. Those whales which make longer and deeper 

 dives have smaller lungs. Evidently, then, they take down rela- 

 tively little air and furthermore the volume of the lung taken up 



* Slowing of the heart. t Litres/ 100 kgm. 



