BIOCHEMISTRY AND TAXONOMY 321 



been confirmed many times. As these authors pointed out, a strong affinity 

 for oxygen allows a fish hving in badly aerated water to charge its blood 

 with oxygen more easily than could a fish possessing a haemoglobin of low 

 affinity. Moreover, a high affinity, accentuated by a low temperature, would 

 maintain the curve in the left-hand region between the axes, i.e. in the low- 

 pressure region, and consequently the oxygen would be released to the 

 tissues at a low pressure, unless the Bohr effect was to interfere and displace 

 the curve to the right. Several workers have confirmed that in fresh-water 

 fish the Bohr effect is more marked than in those living in deep cold waters 

 and that the blood of these same fish also has a lower affinity for oxygen at 

 low />CQ^ values. These two characteristics, feeble affinity and a relatively 

 powerful Bohr effect, counterbalance the properties imposed on the blood 

 by the low temperature. Various observations reveal the more or less 

 marked "pumping action" due to the Bohr effect, in deep-water fish, in 

 bringing about the liberation of oxygen in the swim-bladder. But if the 

 Bohr effect is more marked in fresh-water Teleosts than in mammals, it is 

 still more pronounced in the marine Teleosts which fits in with a character- 

 istic property of the marine Teleosts, that of having a low arterial ^co,» 

 corresponding to that of sea water. Here, the Bohr effect appears to be an 

 effective correction of the almost hyperbolic character of the dissociation 

 curve, without decreasing the efficiency of the charge mechanism as would 

 be the case in a medium rich in COg. 



Another example of a relation between the affinity for oxygen of the 

 haemoglobin in the arterial blood and the oxygen level of the environment is 

 provided by mammals living at high altitudes, such as the lama {Lama 

 huanachus), the vicuna [Lama vicugfia) and the viscacha [Lagostoma sp.), 

 the haemoglobins of which have a higher affinity for oxygen than those of 

 other mammals. 



When the venous blood leaves the tissues on its way to the heart, the 

 partial pressure of oxygen is 40mm Hg for man, 38mm in the horse, 56mm 

 in the goose, 37mm in the duck, 15mm in the turtle Chelydra serpentina, 

 14mm in the ray Raia ocellata, and the corresponding degrees of saturation 

 are 60%, 70%, 38%, 35% and 33% respectively. Thus, on the one hand a 

 sufficient ox}^gen concentration gradient is assured, this being an important 

 factor in the supply of this element to the cells, and on the other hand, the 

 venous blood carries a reserve of oxygen which can be called upon if the 

 rate of metabolism is increased, or if the supply of oxygen is curtailed. 



The part played by this reserve is prominent in the seal which, during a 

 dive, uses almost the whole of the oxygen in its blood. The almost total 

 reduction of the venous blood has been reported to occur in other diving 

 animals such as the duck and the musk-rat. 



The use of haemoglobin to provide a reserve of oxygen can be observed 

 duringperiodsof suspension of the breathing mechanism. In an Invertebrate 



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