70 



RESPIRATORY MECHANISMS 



metabolism in not less than 40 minutes. Oxygen debts of 5-6 

 liters are easily reached. In several cases it was found that 

 breathing of 50% oxygen which increases the supply to the 

 working muscles would increase the amount of oxygen debt 

 which could be built up (Hill, Long and Lupton, 1924). At 

 a low oxygen pressure (high altitude) the maximum oxygen 

 debt is definitely reduced (H. T. Edwards, 1936). 



A comparatively small part of the oxygen debt is caused by 

 depletion of oxygen stores in the blood and in the muscle 

 haemoglobin. All the rest was ascribed by Hill and his co- 

 workers to lactic acid produced in the working muscles, but 

 there is now a general agreement that a large fraction of the 

 debt (perhaps about 1/2) is "alactacid": due to other and so 

 far unknown split products. 



During rest man cannot contract an oxygen debt, except 

 in experiments in which the blood supply to some part of the 

 body (e.g., the legs) is cut off. In experiments of this type 

 a complete cessation of blood-flow to a limb can be tolerated 



for more than 20 minutes, 

 and the muscles concerned 

 will absorb afterwards an 

 extra amount of oxygen 

 amounting to at least 1 

 liter. 



As we shall see presently, 

 certain animals are able to 

 build up an oxygen debt 

 even during rest and utilize 

 this power in prolonged 

 diving. 



The lungs and respiration of 

 birds. The respiratory mech- 

 anisms of birds are definitely 

 adapted to the function of 



Fig 42. Metal cast of lungs and air ffi , a§ evidenced b the 

 sacs ol pigeon. Note recurrent bronchi ° ' J 



from lower air-sacs. (Scharnke.) fact that birds which do not 



