BASES OF ADAPTATION 325 



The amount of oxygen delivered between these two pressures is a 

 measure of the biological efficiency of the pigment. 



The efficiency of the adaptation of the pigment involves, in addi- 

 tion, kinetic considerations. Both must be considered in relation to 

 the macroenvironment of the pigment, which itself is determined by 

 the physiology of the organism and the environment in which the 

 organism is found. 



For the present purpose the most important factor in the environ- 

 ment of the aerobic organism is the partial pressure of oxygen; this 

 may vary from that found in the intestines of mammals, where 

 Ascaris lives, to that found in the free atmosphere at the surface of 

 the earth. It is to the partial pressure of oxygen at the surface of 

 the organism to which the loading tension of the first pigment in the 

 oxygen-carrying chain must be adapted. We call this the initial 

 loading tension. It may be subject to cyclic variation due to factors 

 the organism cannot influence. This is seen in the case of Arenicola, 

 the lugworm, which lives in a burrow in the sand just above the water 

 mark, so that for the most of the time it is in well-oxygenated sea 

 water but when the tide falls is cut off from external sources of oxygen 

 (119). On the other hand, the organism may be a free-living form 

 and be able to select an environment with a suitable initial loading 

 tension. 



The other partial pressure of great importance for aerobiosis is 

 that required for the maximal action of the respiratory ferment. So 

 long as the oxygen pressure at the surface of the oxidase is not a 

 limiting factor for the metabolism of the animal, the functional 

 adaptation at this point will be satisfactory. The oxygen carrier 

 which delivers oxygen at the point must be capable of yielding it at 

 a pressure, the final unloading tension, which satisfies this require- 

 ment. In some organisms we find only one oxygen carrier between 

 the environmental oxygen and the respiratory ferment, while in others 

 we may find two or even three, e.g., maternal Hb — * fetal Hb —> fetal 

 ]\IHb -^ oxidase in fetal muscle. Where more than one pigment is 

 present, we must consider their mutual loading and unloading ten- 

 sions, and how well these are adapted to the initial loading tension 

 and to the final unloading tension of the system. 



Adaptation does not depend, however, solely on the thermody- 

 namics of the pigment. Equally important is the rate at which oxygen 

 is supplied to the tissues. This must be sufficient to enable the 

 organism to withstand stresses caused by its own activity and by the 



