VERTEBRATE RESPIRATION 97 



great. The steep part of the curve usually coincides with the 

 normal working range in the animal. Some indication of the 

 position of the curves is given by reference to the unloading 

 tension {Tu) i.e., the tension of oxygen at which the haemo- 

 globin is 50 per cent saturated, and the loading tension (Tl) 

 which is the oxygen tension at which it becomes 95 per cent 

 saturated. It is in the interests of the animal that the unloading 

 tension should be high, as this roughly defines the upper limit 

 of the tissue oxygen tension. The loading tension must be 

 related to the normal environmental oxygen tension, for if the 

 latter never reaches the level required for full saturation, then 

 clearly the haemoglobin will function inefficiently. In most 

 cases there is a distinct 'safety factor', because the loading tension 

 is usually less than the normal lower limit of the environ- 

 mental oxygen tensions. Consequently there are some fairly 

 definite correlations between 71, 7u, and the environmental 

 oxygen tension. Those amphibians and turtles which spend 

 much of their time on land have their dissociation curves to the 

 right of those for aquatic species. Fish which live in regions of 

 high oxygen have their curves shifted to the right relative to 

 those which inhabit stagnant waters. Mammals which live at 

 high altitudes have lower values for 7u and 71. 



The haemoglobins of different species vary not only from one 

 to another but also during the hfe cycle. A particularly notable 

 example is found among mammals, where the dissociation 

 curve for the foetal haemoglobin is quite distinct from that of 

 the maternal haemoglobin. The curves for sheep blood, for 

 example, lie to the left of the adult, i.e. the foetal haemoglobin 

 becomes oxygenated at much lower pressures (fig. 27c). Similar 

 differences between foetus and mother are found in snakes and 

 fishes which have viviparous habits. These make it possible for 

 oxygen to be transferred by diffusion from one system to the 

 other. As in the fish gill, the exchange is aided by the counter- 

 flow of the two media in the placenta. In viviparous animals, 

 therefore, oxygen is transferred from air to maternal blood to 

 foetal blood and finally to foetal tissue. 



A further feature of haemoglobins which assists considerably 



