FUNCTIONS OF BLOOD 99 



In the higher vertebrates including most reptiles, birds, 

 and mammals, but excluding the diving forms, the respiratory 

 environment is also on the whole fairly constant, although 

 some exceptions will be dealt with below, but differs signifi- 

 cantly from that of the fishes discussed above. The air in the 

 lungs contains normally about 15% oxygen, corresponding 

 to a pressure of 100-110 mm and 5-6% C0 2 or about 40 mm 

 pressure. In these the C0 2 effect on the 2 dissociation 

 curve of the blood is on the whole smaller, and very defi- 

 nitely reduced or abolished (Scott, 1938) at the normal loading 

 tensions. 



There are distinct differences in the dissociation curves 

 between different species and also within the species, differ- 

 ences which cannot so far be correlated with oecological 

 factors. The birds so far examined (Wastl und Leiner, 1931 ; 

 Christensen and Dill, 1935) have definitely lower loading and 

 unloading tensions. This is probably correlated with the 

 very high rates of metabolism during flight, but the high 

 body temperature of birds (42°C) may also have something 

 to do with it. 



Of the exceptions, alluded to above, from the general rule 

 of a fairly constant respiratory environment, one is concerned 

 with a definite period in the life of each individual, viz. the 

 foetal life, while the other concerns those individuals which 

 habitually or occasionally visit high altitudes. It will be con- 

 venient to deal with these in this place. 



The oxygen transport in foetal blood. During development 

 of the foetus both in mammals (J. Barcroft and others, 1935) 

 and in birds (F. G. Hall, 1935) the available amount of 

 oxygen appears to be a limiting factor for the rate of growth. 

 In the mammal, oxygen has to diffuse in the placental vessels 

 from the maternal to the foetal blood which involves in all 

 cases a definite fall in tension. In birds' eggs (and reptile 

 eggs), oxygen must diffuse through the shell and membranes 

 before reaching the blood, and this requires a considerable 

 pressure head. The fcetal blood shows a definitely steeper 



