The unloading of oxygen from the blood 161 



however, the absolute pressure in the case of haemoglobin solutions 

 containing unaggregated molecules is so small that a considerable 

 proportional increase does not produce much absolute rise ; with 

 blood, however, the opposite is the case. This is most apparent near 

 the bottom of the curves. In the absence of C0 2 at 20 % saturation 

 the corresponding pressure of oxygen would be 2 mm. in the case of 

 the haemoglobin solution and 6 in the case of the blood ; suppose 

 enough C0 2 to be present to double each of these, the pressure of 

 oxygen in the case of the unaggregated haemoglobin solution be- 

 comes 4 and of the blood 12, the blood gains greatly in the matter 

 of millimetres pressure. The effect of the salts in steadying the degree 

 of aggregation of colloid molecules is probably very great, for in the 

 few observed cases where the change of reaction has been sufficient 

 to produce a measurable change in aggregation, in spite of the salts 

 present, the person observed has been in a very abnormal condition, 

 either very much distressed as the result of exercise at high altitudes, 

 or practically moribund as some of the cases referred to in the last 

 chapter of this book. 



We have discussed the physiological significance of temperature, 

 acids and salts in their general aspect, but in regard to any particular 

 organ they have a special as well as a general aspect. The blood is 

 not the same when it leaves the organ as when it enters it ; in the 

 first place it is warmer ; in the second case it has acquired carbonic 

 acid and perhaps other acids ; in the third its saline content may have 

 been altered ; each of these will have its effect upon the pressure at 

 which the blood holds its oxygen. Of the salts we know nothing in 

 this connection, nor have we data which enable us to make any 

 allowance for the rise of temperature in the blood. It may be an 

 important factor in the case of actively contracting muscle. We 

 must therefore confine ourselves to the consideration of the effect 

 of the acids thrown into the blood in raising the head of oxygen 

 pressure and thus promoting rapid diffusion. 



It remains for some physiologist in the future systematically to 

 determine the influence of acid and other factors which we have 

 enumerated in the various tissues of the body. He would then be 

 able to present a statement of the pressure of oxygen in the vein 

 leading from each of the organs which he had studied. 



At present a single example must suffice to illustrate at once the 

 principles which are involved, and the order of oxygen pressure 

 which exists in the capillaries. The actual example forthcoming is 



B. R. P. 11 



