C H AND DISSOCIATION 253 



For example, let us try to determine what desaturation would 

 arise from raising the temperature from 36 C. to 39 C. 



Here 2\=273 +36=309 absolute, 



T 2 =273 + 39=312 

 and ^=28,000 cals., 



If # 36 be 30 per cent., then #39 is equal to 30<r' 4356 =19-4, 

 we find that haemoglobin which was 30 per cent, saturated at 

 36 becomes only 19-4 per cent, saturated at 39. 



An increase in 3 C. between the values of 36 and 39 causes the 

 HbO 2 to lose oxygen to the extent of about 10 per cent, of full 

 saturation. 



A physico-chemical factor, however, is much more potent than 

 temperature in producing desaturation. Active tissues tend to 

 become acid. In dealing with muscle, we have seen how lactic 

 acid is set free as the result of activity and how oxygen is required 

 before it can be replaced in the muscle complex. This free lactic 

 acid performs another service. Either directly by partially 

 diffusing into the surrounding lymph or indirectly by producing 

 alterations in the Helmholtz (polarising) electric charge, it causes 

 a potential alteration in the hydrogen ion concentration of the 

 tissue fluid. By a series of changes which we have already briefly 

 considered, and to which we shall return, the net effect is to 

 increase the tension of CO 2 in the capillaries. The molecules of 

 CO 2 are to be the new passengers on the erythrocytes, and because 

 of their acidity they cause an aggregation of the colloidal particles 

 of haemoglobin and a marked desaturation. Carbon-dioxide 

 acts as if the tension of oxygen in the tissues were reduced to 

 10/24ths of its real value. That is, haemoglobin parts with as 

 much oxygen at a tissue tension, of oxygen of 24 mm. Hg. as if 

 the tension of oxygen were only 10 mm. For example, blood 

 which, in the absence of CO 2 would be 30 per cent, desaturated is 

 actually desaturated to the extent of 78 per cent, by the presence 

 of a CO 2 tension of 40 mm. Hg. The blood becomes now a carrier 

 of CO 2 from the tissues to the lungs (Fig. 56, dash line curve). 



Handling of Oxygen. Consider an inland village supplied by 

 a canal coming as close as possible to the community. Internal 

 communication is effected by waterways fed from the canal. The 

 by-products of manufacture (sent elsewhere for elaboration) are 

 transported along these waterways to the canal and the same 



