BIOLOGICAL SIGNIFICANCE OF HEMOGLOBIN 195 



cannot be higher than that of the oxygen in atmospheric air — unless, 

 indeed, some secretion process is at work in the lung; and of course 

 there are reasons why it should be lower. In the lung of the warm- 

 blooded animal there must be upwards of 50 mm. pressure of aqueous 

 vapour and a considerable amount of COg , and the fact that the lung 

 is an almost closed bag from which oxygen is always being abstracted 

 means a considerable reduction in the pressure of the oxygen to which 

 the blood is exposed. Thus the tension of loading comes to be in the 

 vicinity of 100 mm. of mercury. That of unloading is about 40 mm. 

 Obviously it is desirable that the tension of unloading should be 

 sufficiently high in order to provide a pressure head capable of 

 effecting the passage of considerable quantities of gas from the 

 capillary to the tissue at the moments when the metabolism of the 

 tissue is most intense. 



In human blood the pressures of oxygen in the blood of the right and 

 left sides of the heart respectively are approximately 40 and 100 mm., 

 the saturations about 66 and 96 per cent. That is to say the blood 

 can lose about 30 per cent, of its oxygen, at so high a pressure as 

 40 mm., which pressure is the driving force available for the expulsion 

 of the oxygen from the capillaries into the tissues. 



If the dissociation curve were a hyperbola how much oxygen would 

 haemoglobin part with between the pressures of 100 and 40 mm. ? 

 To answer the question we may consider two hyperbolae, one of which, 

 like arterial blood, is 96 per cent, saturated at 100 mm. pressure, the 

 other, like venous blood, is 66 per cent, saturated at 40 mm. pressure. 



The hyperbola which indicates 96 per cent, saturation at 100 mm. 

 would indicate about 91 per cent, saturation at 40 mm., and so the 

 haemoglobin would only have lost 5-6 per cent, of its oxygen in the 

 transition. On the other hand, the hyperbola which yields 66 per cent, 

 saturation at 40 mm. will only indicate 83 per cent, saturation at 

 100 mm. so that there will only be a gain or loss of 17 per cent, in 

 the circuit. In either case the system would be very inefficient : in 

 the latter five-sixths of the haemoglobin is being carried round the 

 body to no purpose, in the former nine teen- twentieths. Haemoglobin 

 depends for its biological efficiency upon the double inflection of its 

 ciltve; that property it shares with haemocyanin — a quite different 

 substance, containing not iron but copper, and containing no 

 porphyrin, but which is like haemoglobin a conjugated protein; but 

 haemoglobin does not share this double inflection with its neighbour 

 haemochromogen, from which it differs only in the matter of the 



